 CHAPTER 30 ELEVATION AND SUBSIDENCE OF LAND WITHOUT EARTHQUAKES Here's in the relative level of land and sea in regions not volcanic. Opinion of Celsius that the waters of the Baltic Sea and northern ocean were sinking. Objections raised to his opinion. Proofs of the stability of the sea level in the Baltic. Playfairs' hypothesis that the land was rising in Sweden. Opinion of von Buch. Marks cut on the rocks. May of these in 1820. Facility of detecting slight alterations of level on coast of Sweden. Shores of the ocean also rising. Area upheaved. Shelly deposits of the Udovala. Of Stockholm containing fossil shells characteristic of the Baltic. Subsidence in south of Sweden. Fishing hut buried under marine strata. Evolunt Sweden not always in horizontal planes. Sinking of land in Greenland. Baring of these facts on geology. We have now considered the phenomena of volcanoes and earthquakes according to the division of the subject before proposed, and have next to turn our attention to those slow and insensible changes in the relative level of land and sea which take place in countries remote from volcanoes. And where no violent earthquakes have occurred within the period of human observation. Early in the last century the Swedish naturalist Celsius expressed his opinion that the waters both of the Baltic and northern ocean were gradually subsiding. From numerous observations he inferred that the rate of depression was about 50 Swedish inches in a century. In support of this position he alleged that there were many rocks both on the shores of the Baltic and the ocean known to have been once sunken reefs, and dangers to navigators, but which were in his time above water, that the waters of the Gulf of Bothnia had been gradually converted into land. Several ancient ports having been changed into inland cities, small islands joined to the continent and old fishing grounds deserted as being too shallow or entirely dried up. Celsius also maintained that the evidence of the change rested not only on modern observations but on the authority of the ancient geographers, who had stated that Scandinavia was formally an island. This island, he argued, must in the course of centuries, by the gradual retreat of the sea, have become connected with the continent, an event which he supposed to have happened after the time of Pliny and before the ninth century of our era. To this argument it was objected that the ancients were so ignorant of the geography of the most northern parts of Europe that their authority was entitled to no weight, and that the representation of Scandinavia as an island might with more propriety be adduced to prove the scantiness of their information than to confirm so bold in hypothesis. It was also remarked that if the land which connected Scandinavia with the main continent was late dry between the time of Pliny and the ninth century, to the extent to which it is known to have risen above the sea at the latter period, the rate of depression could not have been uniform, as was pretended, for it ought to have fallen much more rapidly between the ninth and eighteenth centuries. Many of the proofs relied on by Celsius and his followers were immediately controverted by several philosophers, who saw clearly that a fall of the sea in any one region could not take place without a general sinking of the waters over the whole globe. They denied that this was the fact, or that the depression was universal, even in the Baltic. In proof of the stability of the level of that sea, they appealed to the position of the island Saltholm, not far from Copenhagen. This island is so low that in autumn and winter it is permanently overflowed, and it is only dry in summer, when it serves for pastoring cattle. It appears from the documents of the year 1280 that Saltholm was then also in the same state and exactly on a level with the mean height of the sea, instead of having been about twenty feet underwater, as it ought to have been, according to the computation of Celsius. Several towns also on the shores of the Baltic, as Lubeck, Wismar, Rostock, Straussund, and others, after six and even eight hundred years, are as little elevated above the sea as at the era of their foundation, being now close to the water's edge. The lowest part of Danzig was no higher than the mean level of the sea in the year 1000, and after eight centuries its relative position remains exactly the same. Several of the examples of the gain of land and shallowing of the sea, pointed out by Celsius, and afterwards by Linnaeus, who embraced the same opinions, were ascribed by others to the deposition of sediment at points where rivers entered, and undoubtedly Celsius had not sufficiently distinguished between changes due to these causes and such as would arise if the waters of the ocean itself were diminishing. Many large rivers descending from a mountainous country, at the head of the Gulf of Bothnia, enter the sea charged with sand, mud, and pebbles, and it was said that in these places the low land had advanced rapidly, especially near Tornio. At Pitia also half a mile had been gained in forty-five years, at Lulio no less than a mile in twenty-eight years. Facts which might all be admitted consistently with the assumptions that the level of the Baltic had remained unchanged, like that of the Adriatic, during a period when the plains of the Poe and the Adija have greatly extended their area. It was also alleged that certain insular rocks once entirely covered with water had at length protruded themselves above the waves, and grown in the course of a century and a half to be eight feet high. The following attempt was made to explain away this phenomenon. In the Baltic, large erratic blocks as well as sand and smaller stones which lie on shoals are liable every year to be frozen into the ice where the sea freezes to the depth of five or six feet. On the melting of the snow and spring, when the sea rises about half a fathom, numerous ice islands float away, bearing up these rocky fragments so as to convey them to a distance, and if they are driven by the waves upon shoals they may convert them into islands by depositing the blocks. If stranded upon low islands they may considerably augment their height. Rualius, also and some other Swedish naturalists, affirmed that some islands were lower than formerly, and that by reference to this kind of evidence there was equally good reason for contending that the level of the Baltic was gradually rising. They also added another curious proof of the permanency of the water level at some points at least for many centuries. On the Finland coast were some large pines, growing close to the water's edge. These were cut down, and by counting the concentric rings of annual growth as seen in the transverse section of the trunk, it was demonstrated that they had stood there for four hundred years. Now according to the Celsius hypothesis the sea had sunk about fifteen feet during that period, in which case the germination and early growth of these pines must have been for many seasons below the level of the water. In like manner it was asserted that the lower walls of many ancient castles, such as those of Sonderberg and Abo, reached then to the water's edge, and must therefore, according to the theory of Celsius, have been originally constructed below the level of the sea. In reply to this last argument, Colonel Halström, a Swedish engineer, well acquainted with the Finland coast, assured me that the base of the walls of the castle of Abo is now ten feet above the water, so that there may have been a considerable rise of the land at that point, since the building was erected. Playfair in his illustrations of the Hatonian theory, in 1802, admitted the sufficiency of the proof seduced by Celsius, but attributed the change of level to the movement of the land, rather than to a diminution of the waters. He observed, quote, that in order to depress or elevate the absolute level of the sea by a given quantity in any one place, we must depress or elevate it by the same quantity over the whole surface of the earth, whereas no such necessity exists with respect to the elevation or depression of the land, unquote. The hypothesis of the rising of the land, he adds, agrees well with the Hatonian theory, which holds that our continents are subject to be acted upon by the expansive forces of the mineral regions, that by these forces they have been actually raised up and are sustained by them in their present situation. In the year 1807, Von Buch, after returning from Ritor and Scandinavia, announced his conviction that the whole country, from Frederikshal in Norway to Abo in Finland, and perhaps as far as St. Petersburg, was slowly and insensibly rising. He also suggested that Sweden may rise more than Norway, and the northern more than the southern part. He was led to these conclusions principally by information obtained from the inhabitants and pilots, and in part by the occurrence of marine shells of recent species, which he had found at several points on the coast of Norway above the level of the sea. He also mentions the marks set on the rocks. Von Buch, therefore, has the merit of being the first geologist who, after a personal examination of the evidence, declared in favor of the rise of land in Scandinavia. The attention excited by this subject in the early part of the last century induced many philosophers in Sweden to endeavor to determine, by accurate observations, whether the standard level of the Baltic was really subject to periodical variations, and under their direction, lines or grooves indicating the ordinary level of the water on a calm day, together with the date of the year, were chiseled out upon the rocks. In 1820 to 21, all the marks made before those years were examined by the officers of the pilotage establishment of Sweden. And in their report to the Royal Academy of Stockholm, they declared that on comparing the level of the sea at the time of their observations, with that indicated by the ancient marks, they found that the Baltic was lower, relatively to the land in certain places, but the amount of change during equal periods of time had not been everywhere the same. During their survey they cut new marks for the guidance of future observers, several of which I had an opportunity of examining 14 years after, in the summer of 1834, and in that interval, the land appeared to me to have risen at certain places north of Stockholm four or five inches. I also convinced myself, during my visit to Sweden, after conversing with many civil engineers, pilots, and fishermen, and after examining some of the ancient marks, that the evidence formally adduced in favor of the change of level, both on the coasts of Sweden and Finland, was full and satisfactory. The alteration of level evidently diminishes as we proceed from the northern parts of the Gulf of Bafnia towards the south, being very slight around Stockholm. Some riders have indeed represented the rate of depression of the waters at Stockholm as very considerable, because certain houses in that city, which are built on piles, have sunk down within the memory of persons still living, so as to be out of the perpendicular. And this, in consequence of the tops of the piles, giving way and decaying, owing to a fall of the waters which has exposed them to be alternately wet and dry. The houses alluded to are situated on the borders of Lake Mailer, a large lake, the outlet of which joins the Baltic. In the middle of Stockholm. This lake is certainly lower than formerly, but the principal cause of the change is not the elevation of the land, but the removal of two old bridges, built on piles, which formally obstructed the discharge of the fresh water into the sea. Another cause is the opening in the year 1819 of a new canal at Satuteljö, a place south of Stockholm, by means of which a new line of communication was formed between Lake Mailer and the Baltic. It will naturally be asked whether the mean level of a sea like the Baltic can ever be determined so exactly as to permit us to appreciate a variation of level amounting only two one or two feet. In reply, I may observe, that except near the catagate, there are no tides in the Baltic, and it is only when particular winds have prevailed for several days in succession, or at certain seasons, when there has been an unusually abundant influx of river water, or when these causes have combined, that this sea is made to rise two or three feet above its standard level. The fluctuations due to these causes are nearly the same from year to year, so that the pilots and fishermen believe and apparently with reason that they can mark a deviation, even of a few inches, from the ordinary or mean height of the waters. There are more over-peculiarities in the configuration of the shores of Norway and Sweden, which facilitate an remarkable degree the appreciation of slight changes in the relative level of land and water. It has often been said that there are two coasts, the inner and an outer one, the inner being the shore of the main land. The outer one, a fringe of countless rocky islands of all dimensions called the skjär. Boats and small vessels make their coasting voyages within this skjär. For here they may sail in smooth water, even when the sea without is strongly agitated. But the navigation is very intricate and the pilot must possess a perfect acquaintance with the breadth and depth of every narrow channel in the composition of innumerable sunken rocks. If on such a coast the land rises one or two feet in the course of a half a century, the minute topography of the skjär is entirely altered. To a stranger indeed who revisits it after an interval of many years, its general aspect remains the same. But the inhabitant finds that he can no longer penetrate with his boat through channels which he formerly passed. He can tell of countless other changes in the late and breadth of isolated rocks now exposed, but once only seen through clear water. The rocks of Gneiss, Maikashist, and Quartz are usually very hard on this coast. Slow to decompose and when protected from the breakers remaining for ages unaltered in their form. Hence it is easy to mark the stages of their progressive emergence by the aid of natural and artificial marks imprinted on them. Besides the summits of fixed rocks there are numerous erratic blocks of vast size strewed over the shoals and islands in the skjär which have been probably drifted by the ice in the manner before suggested while these are observed to have increased in height and dimension with the last half century. Some which were formerly known as dangerous sunken rocks are now only hidden when the water is highest. On their first appearance they usually present a smooth bare, rounded protuberance a few feet or yards in diameter and a single seagull often appropriates to itself this resting place resorting there to devour its prey. Similar points in the meantime have grown to long reefs and are constantly whitened by a multitude of sea fowl while others have been changed from a reef annually submerged to a small islet on which a few leachens a fur seedling and a few blades of grass attest that the shoal has at length been fairly changed into dry land. Thousands of wooded islands around show the great alterations which time can work. In the course of centuries also the spaces intervening between the existing islands may be laid dry and become grassy plains encircled by heights well clothed with lofty furs. This last step of the process by which long fjords and narrow channels once separating wooded islands are now deserted by the sea has been exemplified within the memory of living witnesses on several parts of the coast. Had the apparent fall of the waters been observed in the Baltic only we might have endeavored to explain the phenomenon by local causes affecting that sea alone. For instance, the channel by which the Baltic discharges its surplus waters into the Atlantic might be supposed to have been gradually widened and deepened by the waves and currents in which case a fall of the water like the water alluded to in Lake Mailer might have occurred. But the lowering of level would in that case have been uniform and universal and the waters could not have sunk at Tornio while they retained their former level at Copenhagen. Such an explanation is also untenable on other grounds, for it is a fact as Celsius long ago affirmed that the alteration of level extends to the western shores of Sweden bordering the ocean the signs of elevation observed between Odevella and Gothenburg for as well established as those on the shores of the Bothean Gulf. Among the places where they may be studied are the islands of Marstrand and Gullholmen the last mentioned locality being one of those particularly pointed out by Celsius. The inhabitants there and elsewhere affirmed that the rate of the sinking of the sea or elevation of the land varies in different and adjoining districts being greatest the points where the land is low but in this they are deceived for they measure the amount of rise by the area gained which is most considerable where the land descends with a gentle slope into the sea. In the same manner some advocates of the Celcian theory formally appealed to the increase of lands near the mouths of rivers not sufficiently adverting to the fact that if the bed of the sea is rising the change will always be most sensible where the bottom has been previously rendered shallow whereas at a distance from these points where the scarped granitic cliffs plunge at once into deep water a much greater amount of elevation is necessary to produce an equally conspicuous change. As to the area in northern Europe which is subject to this slow, upheaving movement we have not as yet sufficient data for estimating it correctly. It seems probable however that it reaches from Gothenburg to Torneo and from thence to the North Cape the rate of elevation increasing always as we proceed farther northwards. The two extremities of this line are more than a thousand geographical miles distant from each other and as both terminate in the ocean we know not how much farther the motion may be prolonged under water. As to the breadth of the tract its limits are equally uncertain though it evidently extends across the widest parts of the Gulf of Bothnia and may probably stretch far into the interior both of Sweden and Finland. Now if the elevation continue a larger part of the Gulf of Bothnia will be turned into land as also more of the ocean off the west coast of Sweden between Gothenburg and Udavala and on the other hand if the change has been going on for thousands of years at the rate of several feet in a century large tracts of what is now land must have been submarine in periods comparatively modern. It is natural therefore to inquire whether there are any signs of the recent sojourn of the sea on districts now inland. The answer is most satisfactory. Near Udavala and the neighbouring coast land we find upraised deposits of shells belonging to species such as now live in the ocean while on the opposite or eastern side of Sweden near Stockholm Gefla and other places where the Bothnian Gulf there are analogous beds containing shells of species characteristic of the Baltic. Von Buch announced in 1807 that he had discovered in Norway and at Udavala in Sweden beds of shells of existing species at considerable heights above the sea. Since that time other naturalists have confirmed his observation and according to strong deposits occur in elevation of more than 400 feet above the sea in the northern part of Norway. Mr. Alex Brogniard when he visited Udavala ascertained that one of the principal masses of shells that of Capellbacken is raised more than 200 feet above the sea resting on rocks of gneiss while the species being identical with those now inhabiting the contiguous ocean. He also stated that on examining with care the surface of the gneiss immediately above the ancient shelly deposit he found barnacles bologna adhering to the rocks showing that the sea had remained there for a long time. It was fortunate enough to be able to verify this observation by finding in the summer of 1834 at Curid about two miles north of Udavala and at the height than 100 feet above the sea a surface of gneiss newly laid upon by the partial removal of a mass of shells used largely in the district for making line and repairing the roads. So firmly did these barnacles adhere to the gneiss that I broke off portions of the rock with the shells attached. The face of the gneiss was also encrusted with small zoophytes. It was a bit like a cello perra but had these or the barnacles been exposed in the atmosphere ever since the elevation of the rocks above the sea they would doubtless have decomposed and been obliterated. End of Section 73 CHAPTER 30 PART 2 OF PRINCIPALS OF GUIOLOGY This is LibriVox Recording All LibriVox recordings are in the public domain for more information or to volunteer The Town of Udavala stands at the head of a narrow creek overhung by steep and barren rocks of Gnice, of which all the adjacent country is composed, except in the low grounds and bottoms of valleys where strata of sand, clay, and marl frequently hide the fundamental rocks. To these newer and horizontal deposits, the fossil shells above mentioned belong, and similar marine remains are found at various heights above the sea on the opposite island of Urust. The extreme distance from the sea to which such fossils extend is as yet unknown, but they have been already found at Trollhattin in digging the canal there, and still farther inland on the northern borders of Lake Verner, fifty miles from the sea, at an elevation of two hundred feet near Lake Rogvarpin. To pass the Baltic, I observed near its shores, at Satertelje, sixteen miles southwest of Stockholm, strata of sand, clay, and marl more than a hundred feet high, and containing shells of species now inhabiting the Bothean Gulf. These consist partly of marine and partly of freshwater species, but they are few in number, the brackishness of the water appearing to be very unfavorable to the development of Testaesia. The most abundant species are the common cockle, and the common muscle and periwinkle of our shores, Cardiomeduli, Mytilus Edulus, and Litorina Litoria. Other with a small telina, T Baltica, and a few minute univalves, allied to Paludina Ulva. These live in the same water as Limnius, a Neurotina, in Fluveotilus, and some other freshwater shells. But the marine mollusks of the Baltic above mentioned, over very numerous in individuals, are dwarfish in size, scarcely ever retaining a third of the average dimensions which they acquire in the salter waters of the ocean. By this character alone, a geologist would generally be able to recognize an assemblage of Baltic fossils as distinguished from those derived from a deposit in the ocean. The absence also of oysters, barnacles, welks, scallops, limpets, austria, balanus, buccinum, pectin, patella, and many other forms abounding alike in the senior Udovala and in the fossiliferous deposits of modern date on that coast, supplies an additional negative character of the greatest value, distinguishing assemblages of Baltic from those of oceanic shells. Now the strata containing Baltic shells are found in many localities near Stockholm, Uppsala, and Geffle, and will probably be discovered elsewhere around the borders of the Bothnian Gulf, for I have seen similar remains brought from Finland, in Marl resembling that found near Stockholm. The utmost distance to which these deposits have yet been traced inland is on the southern shores of Lake Mailer, at a place seventy miles from the sea. Hence it appears from the distinct assemblage of fossil shells found on the eastern and western coasts of Sweden that the Baltic has been for a long period separated as now from the ocean, although the intervening tract of land was once much narrower, even after both seas had become inhabited by all the existing species of testacea. There's no accurate observations on the rise of the Swedish coast referred to periods more remote than a century and a half from the present time, and as traditional information, and that derived from ancient buildings on the coast do not enable the antiquary to trace back any monuments of change from more than five or six centuries, we cannot declare whether the rate of the upheaving force is uniform during very long periods. In those districts where the fossil shells are found at the height of more than two hundred feet above the ocean, as at Udavala, Orist, and Lake Rogvarpen, the present rate of rise seems less than four feet in a century. Even at that rate it would have required five thousand years to lift up those deposits, but as the movement is now very different in different places it may also have varied much in intensity at different eras. We have moreover yet to learn not only whether the motion proceeds always at the same rate, but also whether it has been uniformly in one direction. The level of the land may oscillate, and for centuries there may be a depression, and afterwards a re-elevation of the same district. Some phenomena in the neighborhood of Stockholm appear to me only explicable on the supposition of the alternate rising and sinking of the ground since the country was inhabited by man. In digging a canal in 1819 at Satutelje, about sixteen miles to the south of Stockholm, to Unite Lake Miller with the Baltic, marine strata containing fossil shells of Baltic species were passed through. At a depth of about sixty feet they came down upon what seems to have been a buried fishing hut, constructed of wood in a state of decomposition which soon crumbled away on exposure to the air. The lowest part, however, which has stood on a level with the sea, was in a more perfect state of preservation. On the floor of this hut was a rude fireplace consisting of a ring of stones, and within this there were cinders and charred wood. On the outside lay boughs of the fur, cut is with an axe, with the leaves or needles still attached. It seems very difficult to explain the position of this buried hut without imagining, as in the case of the Temple of Serapis, first a subsidence to the depth of more than sixty feet, then re-elevation. During that period of submergence the hut must have been covered over with gravel and shelly marl, under which not only the hut, but several vessels also were found of a very antique form, and having their timbers fastened together by wooden pegs instead of nails. Whether any of the land in Norway is now rising must be determined by future investigations. Marine fossil shells of recent species have been collected from inland places near Draunheim, but Mr. Everest in his travels through Norway informs us that the small island of Munkholm, which is an insulated rock in the harbor of Draunheim, affords conclusive evidence of the land having in that region remained stationary for the last eight centuries. The area of this isle does not exceed that of a small village, and by an official survey its highest point has been determined to be twenty-three feet above the mean high water mark, that is, the mean between meep and spring tides. Now a monastery was founded there by Knut the Great, A.D. 1028, and thirty-three years before that time it was in use as a common place of execution. According to the assumed average rate of rise in Sweden, about forty inches in a century, we should be obliged to suppose that this island had been three feet eight inches below high water mark when it was originally chosen as the site of the monastery. Professor Kjilhauer of Christiania, after collecting the observations of his predecessors respecting former changes of level in Norway and combining them with his own, has made the fact of a general change of level at a modern period, that is to say within the period of the actual Testatius fauna, very evident. He infers that the whole country from Cape Lindesnays to Cape North, and beyond that as far as a fortress of Vardhus, has been gradually upraised, and on the southeast coast the elevation has amounted to more than six hundred feet. The marks, which denote the ancient coastline, are so nearly horizontal that the deviation from horizontality, although the measurements have been made at a great number of points, is too small to be appreciated. More recently, 1844, however, it appears from the researchers of Miser Bravet, member of the French Scientific Commission of the North, that in the Gulf of Alten in Finmark, the most northerly part of Norway, there are two distinct lines of upraised ancient sea coast, one above the other, which are not parallel, and both of them imply that within a distance of fifty miles a considerable slope can be detected in such a direction as to show that the ancient shores have undergone a greater amount of upheaval in proportion as we advance inland. It has already been stated that in proceeding from the North Cape to Stockholm, the rate of upheaval diminishes from several feet to a few inches in a century. To the south of Stockholm, the upward movement seizes, and at length, in Skania, or the southernmost part of Sweden, it appears to give place to a movement in an opposite direction. In proof of this fact, Professor Milsen observes, in the first place, that there are no elevated beds of recent marine shells in Skania like those farther to the North. Finally, Linnaeus, with a view of ascertaining whether the waters of the Baltic were retiring from the Skanian shore, measured, in 1749, the distance between the sea and a large stone near Trelleborg. This same stone was, in 1836, a hundred feet nearer the water's edge than Linnaeus's time, or eighty-seven years before. Thirdly, there is also a submerged peat moss, consisting of land and freshwater plants, beneath the sea at a point to which no peat could have been drifted down by any river. Fourthly, and what is still more conclusive, it is found that in seaport towns, all along the coast of Skania, there are streets below the high water level of the Baltic, and in some cases below the level of the lowest tide. Thus, when the wind is high at Malmo, the water overflows one of the present streets, and some years ago some excavations showed an ancient street in the same place eight feet lower, and it was then seen that there had been an artificial raising of the ground, doubtless in consequence of that subsidence. There is also a street in Trelleborg, and another at Skanor, a few inches below high water mark, and a street in Stad, is exactly on a level with the sea at which it could not have been originally built. The inferences deduced from the foregoing facts are in perfect harmony with the proofs brought to light by two Danish investigators, Dr. Pingel and Captain Grah, of the sinking down of part of the west coast of Greenland for a space of more than six hundred miles from north to south. The observations of Captain Grah were made during a survey of Greenland in 1823 to 24, and afterwards in 1828 to 29. Those by Dr. Pingel were made in 1830 to 32. It appears from various signs and traditions that the coast has been subsiding for the last four centuries, from the Firth called Egaliko, in latitude sixty degrees forty-three minutes north, to Disco Bay, extending to nearly the sixty-ninth degree of north latitude. Ancient buildings on Lower Aki Islands, and on the shore of the mainland, have been gradually submerged, and experiences taught the aboriginal Greenlander never to build his hut near the water's edge. In one case, the Moravian settlers have been obliged, more than once, to move inland the poles upon which their large boats were set, and the old poles still remain beneath the water, as silent witnesses of the change. The probable cause of the movements above alluded to, whether of elevation or depression, will be more appropriately discussed in the following chapters, when the origin of subterranean heat is considered. But I may remark here that the rise of Scandinavia has naturally been regarded as a very singular and scarcely credible phenomenon, because no region on the globe is more free within the times of authentic history from violent earthquakes. In common, indeed, with our own island, and with almost every spot on the globe, some movements have been, at different periods, experienced, both in Norway and Sweden. Yet some of these, as for example during the Lisbon earthquake in 1755, may have been mere vibrations, or undulatory movements of the earth's crust prolonged from a great distance. Others, however, have been sufficiently local to indicate a source of disturbance immediately under the country itself. Notwithstanding these shocks, Scandinavia has upon the whole been as tranquil in modern times as free from subterranean convulsions as any region of equal extent on the globe. There is also another circumstance which has made the change of level in Sweden appear anomalous and has, for a long time, caused the proofs of the fact to be received with reluctance. Volcanic action, as we have seen, is usually intermittent, and the variations of level to which it has given rise have taken place by starts, not by a prolonged and insensible movement similar to that experienced in Sweden. Yet as we enlarge our experience of modern changes, we discover instances in which the volcanic eruption, the earthquake, and the permanent rise or fall of land, whether slow or sudden, are all connected. The union of these various circumstances was exemplified in the case of the Temple of Serapis, described in the last chapter, and we might derive other illustrations from the events of the present century in South America. Some writers indeed have imagined that there is geological evidence in Norway of the sudden upheaval of land to a considerable height at successive periods, since the arrow in the sea was inhabited by the living species of Testatia. They point in proof to certain horizontal lines of inland cliffs and sea beaches containing recent shells at various heights above the level of the sea. But these appearances, when truly interpreted, simply prove that there have been long pauses in the process of upheaval or subsidence. They mark eras at which the level of the sea has remained stationary for ages and during which new strata were deposited near the shore in some places, while in others the waves and currents had time to hollow out rocks, undermine cliffs, and throw up long ranges of shingle. They undoubtedly show that the movement has not been always uniform or continuous, but they do not establish the fact of any sudden alterations of level. When we are once assured of the reality of the gradual rise of a large region, it enables us to account for many geological appearances otherwise of very difficult explanation. There are large continental tracts and high table-lands where the strata are nearly horizontal, bearing no marks of having been thrown up by violent convulsions, nor by a series of movements such as those which occur in the Andes and cause the earth to be rent-open and raised or depressed from time to time while large masses are engulfed in subterranean cavities. The result of a series of such earthquakes might be to produce in great laps of ages a country of shattered, inclined, and perhaps vertical strata, but a movement like that of Scandinavia will cause the bed of the sea and although strata recently formed in it to be upheaved so gradually that it would merely seem as if the ocean had formally stood at a higher level and had slowly and tranquilly sunk down to its present bed. The fact also of a very gradual and insensible elevation of land may explain many geological movements of denudation on a grand scale, if, for example, instead of the hard granitic rocks of Norway and Sweden, a large part of the bed of the Atlantic, consisting chiefly of soft strata, should rise up century after century at the rate of about half an inch or an inch in a year, how easily might oceanic currents sweep away the thin film of matter thus brought up annually within the sphere of aqueous denudation. The tract, when it finally emerged, might present table-lands and ridges of horizontal strata with intervening valleys and vast plains where originally, and during its period of submergence, the surface was level and nearly uniform. These speculations relate to superficial changes, but others must be continually in progress in the subterranean regions. The foundations of the country, thus gradually uplifted in Sweden, must be undergoing important modifications, whether we ascribe these to the expansion of solid matter by continually increasing heat, or to the liquefaction of rock, or to the crystallization of a dense fluid, or the accumulation of pent-up gases. In whatever conjectures we indulge, we can never doubt for a moment that at some unknown depth beneath Sweden and the Baltic, the structure of the globe is, in our own times, becoming changed from day to day, throughout a space probably more than a thousand miles in length and several hundred in breadth. End of Chapter 30, End of Section 74. Chapter 31 of Principles of Geology This is a LibriVox recording. All LibriVox recordings are in the public domain. For more information or to volunteer, please visit LibriVox.org. Chapter 31. Causes of Earthquakes and Volcanoes It will hardly be questioned, after the description before given of the phenomena of earthquakes and volcanoes, that both of these agents have, to a certain extent, a common origin, and I may now therefore proceed to inquire into their probable causes, but first it may be well to recapitulate some of those points of relation and analogy which led naturally to the conclusion that they spring from a common source. The regions convulsed by violent earthquakes include within them the sight of all the active volcanoes. Earthquakes, sometimes local, sometimes extending over vast areas, often precede volcanic eruptions. The subterranean movement and the eruption return again and again, at irregular intervals of time and with unequal degrees of force, to the same spots. The action of either may continue for a few hours or for several consecutive years. Paroxymal convulsions are usually followed in both cases by long periods of tranquility. Thermal and mineral springs are abundant in countries of earthquakes and active volcanoes. Lastly, hot springs situated in districts considerably distant from volcanic vents have been observed to have their temperature suddenly raised and the volume of their water augmented by subterranean movements. All these appearances are evidently more or less connected with the passage of heat from the interior of the earth to the surface, and where there are active volcanoes, there must exist at some unknown depth below enormous masses of matter intensely heated and in many instances in a constant state of fusion. We have first then to inquire, whence is this heat derived? It has long been a favorite conjecture that the whole of our planet was originally in a state of igneous fusion and that the central parts still retain a great portion of their primitive heat. Some have imagined, with the late Sir W. Herschel, that the elementary matter of the earth may have been first in a gaseous state resembling those nebulae which we behold in the heavens and which are of dimensions so vast that some of them would fill the orbits of the remotest planets of our system. The increased power of the telescope has of late years resolved the greater number of these nebulous appearances into clusters of stars, but so long as they were confidently supposed to consist of error form matter, it was a favorite conjecture that they might, if concentrated, form solid spheres, and it was also imagined that the evolution of heat, attendant on condensation, might retain the materials of the new globes in a state of igneous fusion. Without dwelling on such speculations, which can only have a distant bearing on geology, we may consider how far the spheroidal form of the earth affords sufficient ground for presuming that its primitive condition was one of universal fluidity. The discussion of this question would be superfluous where the doctrine of original fluidity less popular, for it may well be asked why the globes should be supposed to have had a pristine shape different from the present one. Why the terrestrial materials, when first called into existence or assembled together in one place, should not have been subject to rotation, so as to assume at once that form which alone could retain their several parts in a state of equilibrium. Let us, however, concede that the statical figure may be a modification of some other pre-existing form, and suppose the globe to have been at first a perfect and quiescent sphere covered with a uniform ocean. What would happen when it was made to turn round on its axis with its present velocity? This problem has been considered by playfair in his illustrations, and he has decided that if the surface of the earth, as laid out in Hutton's theory, has been repeatedly changed by the transportation of the detrius of the land to the bottom of the sea, the figure of the planet must in that case whatever it may have been originally be brought at length to coincide with the spheroid of equilibrium. Sir John Herschel, also in reference to the same hypothesis, observes, a centrifugal force would in that case be generated, whose general tendency would be to urge the water at every point of the surface to recede from the axis. A rotation might indeed be conceived so swift as to flip the whole ocean from the surface like water from a mop, but this would require a far greater velocity than what we now speak of. In the case supposed, the weight of the water would still keep it on the earth, and the tendency to recede from the axis could only be satisfied, therefore, by the water leaving the poles and flowing towards the equator. There, heaping itself up in a ridge and being retained in opposition to its weight or natural tendency towards the center by the pressure thus caused. This, however, could not take place without laying dry the polar regions so that perturbed land would appear at the poles and a zone of ocean be disposed around the equator. This would be the first or immediate effect. Let us now see what would afterwards happen if things were allowed to take their natural course. The sea is constantly beating on the land, grinding it down and scattering its worn off particles and fragments in the state of sand and pebbles over its bed. Geological facts afford abundant proof that the existing continents have all of them undergone this process even more than once and been entirely torn in fragments or reduced to powder and submerged and reconstructed. Land, in this view of the subject, loses its attribute of fixity. As a mass, it might hold together in opposition to forces which the water freely obeys, but in its state of successive or simultaneous degradation, when disseminated through the water, in the state of sand or mud, it is subject to all the impulses of that fluid. In the lapse of time then, the perturbed land would be destroyed and spread over the bottom of the ocean, filling up the lower parts and tending continually to remodel the surface of the solid nucleus in correspondence with the form of equilibrium. Thus, after a sufficient lapse of time, in the case of an earth in rotation, the polar protuberances would gradually be cut down and disappear, being transferred to the equator, as being then the deepest sea, till the earth would assume by degrees the form we observe it to have, that of a flattened or a blight ellipsoid. We are far from meaning here to trace the process by which the earth really assumed its actual form. All we intend is to show that this is the form to which, under a condition of a rotation on its axis, it must tend, and which it would attain even if originally and, so to speak, perversely constituted otherwise. In this passage, the author has contemplated the superficial effects of aqueous causes only, but neither he nor playfair seem to have followed out the same inquiry with reference to another part of Hutton's system, namely, that which assumes the successive fusion by heat of different parts of the solid earth. Yet the progress of geology has continually strengthened the evidence in favor of the doctrine that local variations of temperature have melted one part after another of the earth's crust, and this influence has perhaps extended downwards to the very center. If therefore, before the globe had assumed its present form, it was made to revolve on its axis, all matter to which freedom of motion was given by fusion, must, before consolidating, have been impelled towards the equatorial regions in obedience to the centrifugal force. Thus, lava flowing out in superficial streams would have its motion retarded when its direction was towards the pole, accelerated when towards the equator. Or if lakes and seas of lava existed beneath the earth's crust in equatorial regions, as probably now beneath the Peruvian Andes, the imprisoned fluid would force outwards and permanently upheave the overlying rocks. The statical figure, therefore, of the terrestrial spheroid, of which the longest diameter exceeds the shortest by about 25 miles, may have been the result of gradual and even of existing causes and not of a primitive, universal, and simultaneous fluidity. Experiments made with the pendulum and observations on the manner in which the earth attracts the moon have shown that our planet is not an empty sphere, but on the contrary, that its interior, whether solid or fluid, has a higher specific gravity than the exterior. It has also been inferred that there is a regular increase in density from the surface towards the center and that the equatorial perturbance is continued inwards. That is to say that layers of equal density are arranged elliptically and symmetrically from the exterior to the center. These conclusions, however, have been deduced rather as a consequence of the hypothesis of primitive and simultaneous fluidity than proved by experiment. The inequalities in the moon's motion by which some have endeavored to confirm them are so extremely slight that the opinion can be regarded as little more than a probable conjecture. The mean density of the earth has been computed by Laplace to be about five and a half or more than five times that of water. Now the specific gravity of many of our rocks is from two and a half to three and the greater part of the metals range between that density and 21. Hence, some have imagined that the terrestrial nucleus may be metallic, that it may correspond, for example, with the specific gravity of iron, which is about seven. But here a curious question arises in regard to the form which materials, whether fluid or solid, might assume if subjected to the enormous pressure which must obtain at the earth's center. Water, if it continued to decrease in volume according to the rate of compressibility deduced from experiment, would have its density doubled at the depth of 93 miles and be as heavy as mercury at the depth of 362 miles. Dr. Young computed that at the earth's center steel would be compressed into one fourth and stone into one eighth of its bulk. It is more than probable, however, that after a certain degree of condensation, the compressibility of bodies may be governed by laws altogether different from those which we can put to the test of experiment. But the limit is still undetermined, and the subject is involved in such obscurity that we cannot wonder at the variety of notions which have been entertained respecting the nature and conditions of the central nucleus. Some have conceived it to be fluid, other solid. Some have imagined it to have a cavernous structure and have endeavored to confirm this opinion by appealing to observed irregularities in the vibrations of the pendulum in certain countries. An attempt has recently been made by Mr. Hopkins to determine the least thickness which can be assigned to the solid crust of the globe if we assume the whole to have been once perfectly fluid and a certain portion of the exterior to have acquired solidity by gradual refrigeration. This result he has endeavored to obtain by a new solution of the delicate problem of the processional motion of the pull of the earth. It is well known that while the earth revolves around the sun, the direction of its axis remains very nearly the same, i.e. its different positions in space are all nearly parallel to each other. This parallelism, however, is not accurately preserved, so that the axis instead of coming exactly into the position which it occupied a year before becomes inclined to it at a very small angle, but always retaining very nearly the same inclination to the plane of the earth's orbit. This motion of the pole changes the position of the equinoxes by about 50 seconds annually and always in the same direction. Thus the pole star after a certain time will entirely lose its claim to that appellation until in the course of somewhat more than 25,000 years the earth's axis shall again occupy its present angular position and again point very nearly as now to the pole star. This motion of the axis is called precession. It is caused by the attraction of the sun and moon and principally the moon on the perturbant parts of the earth's equator and if these parts were solid to a great depth the motion thus produced would differ considerably from that which would exist if they were perfectly fluid and encrusted over with a thin shell only a few miles thick. In other words the disturbing action of the moon will not be the same upon a globe all solid and upon one nearly all fluid or it will not be the same upon a globe in which the solid shell forms one half of the mouse and another in which it forms only one tenth. Mr. Hopkins has therefore calculated the amount of precessional motion which would result if we assume the earth to be constituted as above stated i.e. fluid internally and enveloped by solid shell and he finds that the amount will not agree with the observed motion unless the crust of the earth be of a certain thickness. In calculating the exact amount some ambiguity arises in consequence of our ignorance of the effect of pressure in promoting the solidification of matter at high temperatures. The hypothesis least favorable for a great thickness is found to be that which assumes the pressure to produce no effect on the process of solidification. Even on this extreme assumption the thickness of the solid crust must be nearly 400 miles and this would lead to the remarkable result that the proportion of the solid to the fluid part would be as 49 to 51 or to speak in round numbers. There would be nearly as much solid as fluid matter in the globe. The conclusion however which Mr. Hopkins announces as that to which his researchers have finally conducted him is thus expressed. Upon the whole then we may venture to assert that the minimum thickness of the crust of the globe which can be deemed consistent with the observed amount of precession cannot be less than one fourth or one fifth of the earth's radius that is from 800 to 1000 miles. It will be remarked that this is a minimum and any still greater amount would be quite consistent with the actual phenomena. The calculation is not being opposed to the supposition of the general solidity of the entire globe. Nor do they preclude us from imagining that great lakes or seas of melted matter may be distributed through a shell 400 or 800 miles thick provided they be so enclosed as to move with it whatever motion of rotation may be communicated by the disturbing forces of the sun and moon. Central heat. The hypothesis of internal fluidity calls for the more attentive consideration as it has been found that the heat in mines augments in proportion as we descend. Observations have been made not only on the temperature of the air in mines but on that of the rocks and on the water issuing from them. The mean rate of increase calculated from results obtained in six of the deepest coal mines in Durham and Northumberland is one degree Fahrenheit for a descent of 44 English feet. A series of observations made in several of the principal lead and silver mines in Saxony gave one degree Fahrenheit for every 65 feet. In this case the bulb of the thermometer was introduced into cavities purposely cut in the solid rocket depths varying from 200 to above 900 feet. But in other mines of the same country it was necessary to descend thrice as far for each degree of temperature. Athermometer was fixed in the rock of the Dulkoth mine in Cornwall by Mr. Fox at the great depth of 1,380 feet and frequently observed during 18 months. The mean temperature was 68 degrees Fahrenheit that of the surface being 50 degrees which gives one degree for every 75 feet. Kupfer after an extensive comparison of the results in different countries makes the increase one degree Fahrenheit for about every 37 English feet. M. Cordier announces as the result of his experiments and observations on the temperature of the interior of the earth that the heat increases rapidly with the depth but the increase does not follow the same law over the whole earth being twice or three times as much in one country as in another. And these differences are not in constant relation either with the latitudes or longitudes of places. He is of opinion however that the increase would not be overstated at one degree centigrade for every 25 meters or about one degree Fahrenheit for every 45 feet. The experimental well board at Grinnell near Paris gave about one degree Fahrenheit for every 60 English feet when they had reached a depth of 1,312 feet. Some riders have endeavored to refer to these phenomena which however discordant as to the ratio of increasing heat appear all 2.1 way to the condensation of air constantly descending from the surface into the mines. For the air under pressure would give out latent heat on the same principle as it becomes colder where rarefied in the higher regions of the atmosphere but besides that the quantity of heat is greater than could be supposed to flow from this source the argument has been answered in a satisfactory manner by Mr. Fox who has shown that in the mines of Cornwall the ascending have generally a higher temperature than the descending aerial currents. The difference between them was found to vary from 9 degrees to 17 degrees Fahrenheit a proof that instead of imparting heat these currents actually carry off a large quantity from the mines. If we adopt M. Cordia's estimate of one degree Fahrenheit for every 45 feet of depth as the mean result and assume with the advocates of central fluidity that the increasing temperatures continued downwards we should reach the ordinary boiling point of water at about two miles below the surface and at a depth of about 24 miles should arrive at the melting point of iron a heat sufficient to fuse almost every known substance. The temperature of melted iron was estimated at 21,000 degrees Fahrenheit by Wedgewood but his pyrometer gives as is now demonstrated very erroneous results. Professor Daniel ascertained that the point of fusion is 2786 degrees Fahrenheit. According to Mr. Daniel's scale we ought to encounter the internal melted matter before penetrating through a thickness represented by that of the outer circular line in the NX diagram figure 92. Whereas if the other or less correct scale be adopted we should meet with it at some point between the two circles the space between them together with the lines themselves representing a crust of 200 miles in depth. In either case we must be prepared to maintain that a temperature many times greater than that sufficient to melt the most refractory substances known to us is sustained at the center of the globe. While a comparatively thin crust resting upon the fluid remains unmelted or is even according to M. Cordier increasing in thickness by the continual addition of new internal layers solidified during the process of refrigeration. The mathematical calculations of Fourier on the passage of heat through conducting bodies have been since appealed to in support of these views for he has shown that it is compatible with theory that the present temperature of the surface might coexist with an intense heat at a certain depth below but his reasoning seems to be combined to the conduction of heat through solid bodies and the conditions of the problem are wholly altered when we reason about a fluid nucleus as we must do if it be assumed that the heat augments from the surface to the interior according to the rate observed in minds for when the heat of the lower portion of a fluid is increased a circulation begins throughout the mass by the ascent of hotter and the descent of colder currents and this circulation which is quite distinct from the mode in which heat is propagated through solid bodies must evidently occur in the supposed central ocean if the laws of fluid and of heat are the same there as upon the surface in Mr. Daniel's experiments for obtaining a measure of the heat of bodies at their point of fusion he invariably found that it was impossible to raise the heat of a large crucible of melted iron gold or silver a single degree beyond the melting point so long as a bar of the respective metals was kept immersed in the fluid portions so in regard to other substances however great the quantities fused their temperature could not be raised while any solid pieces immersed in them remained unmelted every a session of heat being instantly absorbed during their liquefaction these results are in fact no more than the extension of a principle previously established that so long as a fragment of ice remains in water we cannot raise the temperature of the water above 32 degrees fahrenheit if then the heat of the earth's center amount to 450 degrees fahrenheit as mcortier deems highly probable that is to say about 20 times the heat of melted iron even according to wedge wedges scale and upwards of 160 times according to the improved pyrometer it is clear that the upper parts of the fluid mass could not long have a temperature only just sufficient to melt rocks there must be a continual tendency towards a uniform heat and until this were accomplished by the interchange of portions of fluid of different densities the surface could not begin to consolidate nor on the hypothesis of primitive fluidity can we conceive any crust to have been formed until the whole planet had cooled down to about the temperature of incipient fusion it cannot be objected that hydrostatic pressure would prevent a tendency to equalization of temperature for as far as observations have yet been made it is found that the waters of deep lakes and seas are governed by the same laws as a shallow pool and no experiments indicate that solids resist fusion under high pressure the arguments indeed now controverted always proceed on the admission that the internal nucleus is in a state of fusion it may be said that we may stand upon the hardened surface of a lava current while it is still in motion nay mate descend into the crater of visuvius after any eruption and stand on the scoria while every crevice shows that the rock is red hot two or three feet below us and at a somewhat greater depth all is perhaps in a state of fusion may not then a much more intense heat be expected at the depth of several hundred yards or miles the answer is that until a great quantity of heat has been given off either by the emission of lava or in a latent form by the evolution of steam and gas the melted matter continues to boil in the crater of a volcano but ebullition ceases when there is no longer a sufficient supply of heat from below and then a crust of lava may form on the top and showers of scoria may then descend upon the surface and remain unmelted if the internal heat be raised again ebullition will recommence and soon fuse the superficial crust so in the case of the moving current we may safely assume that no part of the liquid beneath the hardened surface is much above the temperature sufficient to retain it in a state of fluidity it may assist us in forming a clearer view of the doctrine now controverted if we consider what would happen were a globe of homogeneous composition placed under circumstances analogous in regard to the distribution of heat to those above stated if the whole planet for example were composed of water covered with a spheroidal crust of ice 50 miles thick and with an interior ocean having a central heat about 200 times that of the melting point of ice or 6400 degrees fahrenheit and if between the surface and the center there was every intermediate degree of temperature between that of melting ice and that of the central nucleus could such a state of things last for a moment if it must be conceded in this case that the whole spheroid would be instantly in a state of violent ebullition that the ice instead of being strengthened annually by new internal layers would soon melt and form part of an atmosphere of steam on what principle can it be maintained that analogous effects would not follow in regard to the earth under the conditions assumed in the theory of central heat in courtier admits that there must be tides in the internal melted ocean but their effect he says has become feeble although originally when the fluidity of the globe was perfect the rise and fall of these ancient land tides could not have been less than from 13 to 16 feet now granting for a moment that these tides have become so feeble as to be incapable of causing the fissured shell of the earth to be first uplifted and then depressed every six hours still may we not ask whether during eruptions the lava which is supposed to communicate with a great central ocean would not rise and fall sensibly in a crater such as stromboli where there is always melted matter in a state of ebullition whether chemical changes may produce volcanic heat having now explained the reasons which have induced me to question the hypothesis of central heat as the primary source of volcanic action it remains to consider what has been termed the chemical theory of volcanoes it is well known that many perhaps all of the substances of which the earth is composed are continually undergoing chemical changes to what depth these processes may be continued downwards must in a great degree be matter of conjecture but there is no reason to suspect that if we could descend to a great distance from the surface we should find elementary substances differing essentially from those with which we are acquainted all the solid fluid and gaseous bodies known to us consist of a very small number of these elementary substances variously combined the total number of elements at present known is less than 60 and not half of these enter into the composition of the more abundant inorganic productions some portions of such compounds are daily undergoing decomposition and their constituent parts being set free are passing into new combinations these processes are by no means confined to minerals at the earth's surface and are very often accompanied by the evolution of heat which is intense in proportion to the rapidity of the combinations at the same time there is a development of electricity the spontaneous combustion of beds of bituminous shale and of refuse coal thrown out of mines is generally due to the decomposition of pyrites and it is the contact of air and water which brings about the change heat results from the oxidation of the sulfur and iron though on what principle heat is generated when two or more bodies having a strong affinity for each other unite suddenly is wholly unexplained electricity as a source of volcanic heat it has already been stated that chemical changes develop electricity which in its turn becomes a powerful disturbing cause as a chemical agent says davie it's silent and slow operation in the economy of nature is much more important than its grand and impressive operation in lightning and thunder it may be considered not only as directly producing an infinite variety of changes but as influencing almost all which take place it would seem indeed that chemical attraction itself is only a peculiar form of the exhibition of electrical attraction now that it has been demonstrated that magnetism and electricity are always associated and are perhaps only different conditions of the same power the phenomena of terrestrial magnetism have become of no ordinary interest to the geologist soon after the first great discovery is of Oster and electromagnetism ampere suggested that all the phenomena of the magnetic needle might be explained by supposing currents of electricity to circulate constantly in the shell of the globe in directions parallel to the magnetic equator this theory has acquired additional consistency the farther we have advanced in science and according to the experiments of mr fox on the electromagnetic properties of metalliferous veins some trace of electric current seems to have been detected in the interior of the earth some philosophers subscribe these currents to the chemical action going on in the superficial parts of the globe to which air and water have the readiest access while others refer them in part at least to thermal electricity excited by the solar rays on the surface of the earth during its rotation successive parts of the atmosphere land and sea being exposed to the influence of the sun and then cooled again in the night that this idea is not a mere speculation is proved by the correspondence of the diurnal variations of the magnet with the apparent motion of the sun and by the greater amount of variation in summer than in winter and during the day than in the night although conceding that such minor variations of the needle may be due to thermoelectricity contends that the general phenomena of terrestrial magnetism must be attributed to currents far more intense which though liable to secular fluctuations act with much greater constancy and regularity than the causes which produce the diurnal variations the remark seems just yet it is difficult to assign limits to the accumulated influence even of a very feeble force constantly acting on the whole surface of the earth the subject however must evidently remain obscure until we become acquainted with the causes which give a determinant direction to the supposed electric currents already the experiments of Faraday on the rotation of magnets have led him to speculate on the matter in which the earth when once it had become magnetic might produce electric currents within itself in consequence of its diurnal rotation we have seen also in a former chapter page 129 that the recent observations of Schwab 1852 have led Colonel Sabine to the discovery of a connection between certain periodical changes which take place in the spots on the sun and a certain cycle of variations in terrestrial magnetism these seem to point to the existence of a solar magnetic period and suggest the idea of the sun's magnetism exerting an influence on the mass of our planet in regard to thermoelectricity I may remark that it may be generated by great inequalities of temperature arising from a partial distribution of volcanic heat wherever for example masses of rock occur of great horizontal extent and of considerable depth which are at one point in a state of fusion as beneath some active volcano at another red hot and at a third comparatively cold strong thermoelectric action may be excited some perhaps may object that this is reasoning in a circle first to introduce electricity as one of the primary causes of volcanic heat and then to derive the same heat from thermoelectric currents but there must in truth be much reciprocal action between the agents now under consideration and it is very difficult to decide which should be regarded as the prime mover or to see where the train of changes once begun would terminate whether subterranean electric currents if once excited might sometimes possess the decomposing power of the voltaic pile is a question not perhaps easily answered in the present state of science but such a power if developed would at once supply us with a never failing source of chemical action from which volcanic heat might be derived recapitulation before entering in the next chapter still farther into the inquiry how far the phenomena of volcanoes and earthquakes accord with the hypothesis of a continued generation of heat by chemical action it may be desirable to recapitulate in a few words the conclusions already obtained first the primary causes of the volcano and the earthquake are to a great extent the same and must be connected with the passage of heat from the interior to the surface secondly this heat has been referred by many to a supposed state of igneous fusion of the central parts of the planet when it was first created of which a part still remains in the interior but is always diminishing in intensity thirdly the spheroidal figure of the earth adduced in support of this theory does not of necessity imply a universal and simultaneous fluidity in the beginning for supposing the original figure of our planet had been strictly spherical which however is a gratuitous assumption resting on no established analogy still the statical figure must have been assumed if sufficient time be allowed by the gradual operation of the centrifugal force acting on the materials brought successively within its action by aqueous and igneous causes fourthly it appears from experiment that the heat and mines increases progressively with their depth and if the ratio of increase be continued uniformly from the surface to the interior the whole globe with the exception of a small external shell must be fluid and the central parts must have a temperature many times higher than that of melted iron fifthly but the theory adopted by m courtier and others which maintains the actual existence of such a state of things seems wholly inconsistent with the laws which regulate the circulation of heat through fluid bodies four if the central heat were as intense as is represented there must be a circulation of currents tending to equalize the temperature of the resulting fluids and the solid crust itself would be melted sixthly instead of an original central heat we may perhaps refer the heat of the interior to chemical changes constantly going on in the earth's crust for the general effect of chemical combination is the evolution of heat and electricity which in their turn becomes sources of new chemical changes and of chapter thirty one recording by lisa s where chapter thirty two part one of principles of geology this is a LibriVox recording all LibriVox recordings are in the public domain for more information auto volunteer please visit LibriVox.org recording by sycamore Rockwell principles of geology by Charles Lyle causes of earthquakes and volcanoes continued review of the proofs of internal heat theory of an unoxidated metallic nucleus whether the decomposition of water may be a source of volcanic heat geysers of Iceland causes of earthquakes wave like motion expansive power of liquid gases connection between the state of the atmosphere and earthquakes permanent upheaval and subsidence of land expansion of rocks by heat the balance of dry land how preserved subsidence in excess conclusion when we reflect that the largest mountains are but insignificant protuberances upon the surface of the earth and that these mountains are nevertheless composed of different parts which have been formed in succession we may well feel surprised that the central fluidity of the planet should have been called into account for volcanic phenomena to suppose the entire globe to be in a state of igneous fusion with the exception of a solid shell no more than from 30 to 100 miles thick and to imagine that the central heat of this fluid spheroid exceeds by more than 200 times that of liquid lava is to introduce a force altogether disproportionate to the effects which it is required to explain the ordinary repose of the surface implies on the contrary an inertness in the internal mass which is truly wonderful when we consider the combustible nature of the elements of the earth so far as they are known to us the facility with which their compounds may be decomposed and made to enter into new combinations the quality of the heat which they evolved during these processes when we recollect the expansive power of steam and that water itself is composed of two gases which by their union produce intense heat when we call to mind the number of explosive and detonating compounds which have been already discovered we may be allowed to share the astonishment of Pliny that a single day should pass without a general conflagration. The signs of internal heat observable on the surface of the earth do not necessarily indicate the permanent existence of subterranean heated masses whether fluid or solid by any means so vast as our continents and seas yet how insignificant would these appear if distributed through an external shell of the globe one or two hundred miles in depth the principle facts and proof of the accumulation of heat below the surface may be summed up in a few words several volcanoes are constantly in eruption as Tromboli and Nicaragua others are known to have been active for periods of 60 or even 150 years as those of Sangae in Quito, Popocata Petal in Mexico and the volcano of the Isle of Bourbon many craters emit hot vapors in the intervals between eruptions and sulfur towers evolve incessantly the same gases as volcanoes steam of high temperatures has continued for more than 20 centuries to issue from stufas as the Italians call them thermal springs abound not only in regions of earthquakes but are found in almost all countries however distant from active vents and lastly the temperature in the minds of various parts of the world is found to increase in proportion as we descend the diagram figure 93 in the next page may convey some idea of the proportion which our continents and the ocean bear to the radius of the earth if all the land were about as high as the Himalaya mountains and the ocean everywhere as deep as the pacific the whole of both might be contained within a space expressed by the thickness of a line ab and masses of nearly equal volume might be placed in the space marked by the line cd in the interior seas of lava therefore of the size of the Mediterranean or even the Atlantic would be as nothing if distributed through such an outer shell of the globe as is represented by the shaded portion of the figure abcd if throughout that space we imagine electrochemical causes to be continually in operation even a very feeble power they might give rise to heat which if accumulated at certain points might melt or render red hot entire mountains or sustain the temperature of stufas and hot springs for ages theory of an unoxidated metallic nucleus when sir age davey first discovered the metallic basis of the earth's and alkalis he threw out the idea that those metals might abound in an unoxidated state in the subterranean regions to which water must occasionally penetrate whenever this happens gaseous matter would be set free the metals would combine with the oxygen of the water and sufficient heat might be evolved to melt the surrounding rocks this hypothesis although afterwards abandoned by its author was at first very favorably received both by the chemist and the geologist for silica lumina lime soda and oxide of iron substances of which lovers are principally composed would all result from the contact of the inflammable metals alluded to with water but whence this abundant store of unsaturated metals in the interior it was assumed that in the beginning of things the nucleus of the earth was mainly composed of inflammable metals and that oxidation went on with intense energy at first till at length when a superficial crust of oxides had been formed the chemical action became more and more languid this speculation like all others respecting the primitive state of the earth's nucleus rests unavoidably on arbitrary assumptions but we may fairly inquire whether any existing causes may have the power of deoxidating the earthly and alkaline compounds formed from time to time by the action of water upon the metallic bases if so and if the original crust or nucleus of the planet contained distributed through it here and there some partial stores of potassium sodium and other metallic bases these might be oxidated and again deoxidated so as to sustain for ages a permanent chemical action yet even then we should be unable to explain why such a continuous circle of operations after having been kept up for thousands of years in one district should entirely cease and why another region which had enjoyed a respite from volcanic action for one or many geological periods should become a theater for the development of subterranean heat it is well known to chemists that the metallization of oxides the most difficult to reduce may be affected by hydrogen brought into contact with them at a red heat and it is more than probable that the production of potassium itself in the common gun barrel process is due to the power of nascent hydrogen derived from the water which the hydrated oxide contains according to the recent experiments also of faraday it would appear that every case of metallic reduction by voltaic agency from saline solutions in which water is present is due to the secondary action of hydrogen upon the oxide both of these being determined to the negative pole and then reacting upon one another it is admitted that intense heat would be produced by the occasional contact of water with the metallic bases and it is certain that during the process of saturation vast volumes of hydrogen must be evolved the hydrogen thus generated might permeate the crust of the earth in different directions and becomes stored up for ages in fissures and caverns sometimes in liquid form under the necessary pressure whenever at any subsequent period in consequence of the changes affected by earthquakes in the shell of the earth this gas happened to come in contact with metallic oxides at high temperature the reduction of these oxides might be the result no theory seems at first more startling than that which represents water as affording an inexhaustible supply of fuel to the volcanic fires yet is it by no means visionary it is a fact that must not be overlooked that while a great number of volcanoes are entirely submarine the remainder occur for the most part in islands or marine tracks there are a few exceptions but some of these observes dr delbeni are near island salt lakes as in central tartary while others form part of a train of volcanoes the extremities of which are near the sea surage devy suggested that when the sea is distant as in the case of some of the south african volcanoes they may still be supplied with water from subterranean lakes since according to humboldt large quantities of fish are often thrown out during eruptions mr daner also in his valuable and original observations of the volcanoes of the sandwhich islands reminds us of the prodigious volume of atmospheric water which must be absorbed into the interior of such large and lofty domes composed as they are entirely of porous lava to this source alone he refers to the production of the steam by which the melted matter is propelled upwards even to the summits of cones three miles in height when treating of springs and overflowing wells i have stated that porous rocks are percolated by fresh water to great depths and that seawater probably penetrates in the same manner through the rocks which form the bed of the ocean but besides this universal circulation in regions not far from the surface it must be supposed that wherever earthquakes prevail much larger bodies of water will be forced by the pressure of the ocean into fishers at great depths or swallowed up in chasms in the same manner as on the land towns houses cattle and trees are sometimes engulfed it will be remembered that these chasms often close again after houses have fallen into them and for the same reason when water has penetrated to a mass of melted lava the steam into which it is converted may often rush out at a different aperture from that by which the water entered the gases it is said exhaled from volcanoes together with the steam are such as would result from the decomposition of salt water and the fumes which escape from the visovian lava have been observed to deposit common salt the emission of free muriatic acid gas in great quantities is also thought by many to favor the theory of the decomposition of the salt contained in seawater it has been objected however that musubosango did not meet with this gas in his examination of the elastic fluids evolved from the volcanoes of equatorial america which only gave out aqueous vapor in very large quantity carbonic acid gas sulfurous acid gas and sometimes fumes of sulfur in reply dr. delbeni has remarked that muriatic acid may have ceased to be disengaged because that volcanic action has become languid in equatorial america and seawater may no longer obtain admission monsieur gaylosak while he avows his opinion that the decomposition of water contributes largely to volcanic action cold attention nevertheless to the supposed fact that hydrogen had not been detected in a separate form among the gaseous products of volcanoes nor can it he says be present for in that case it would be inflamed in the air by the red hot stones thrown out during an eruption dr. david in his account of graham island says i watched when the lightning was most vivid and the eruption of the greatest degree of violence to see if there was any inflammation occasioned by this natural electric spark any indication of the presence of inflammable gas but in vain may not the hydrogen gailusak inquires be combined with chlorine and produce muriatic acid for this gas has been observed to be evolved from visuvius and the chlorine may have been derived from sea salt which was in fact extracted by simple washing from the visuvius lava in 8022 in the proportion of nine percent but it was answered that sir age david's experiments had shown that hydrogen is not combustible when mixed with muriatic acid gas so that if muriatic gas was evolved in large quantities the hydrogen might be present without inflammation monsoor orbit on the other hand assures us that although it be true that vapor illuminated by incandescent lava has often been mistaken for flame yet he clearly detected in the eruption of visuvius in 1834 the flame of hydrogen monsoor gailusak in the memoir just alluded to expressed doubt as to the presence of sulfurous acid but the abundant disengagement of this gas during eruptions has been since a certain and thus all difficulty in regard to the general absence of hydrogen in an inflammable state is removed for as dr. daubany suggests the hydrogen of decomposed water may unite with sulfur to form sulfurated hydrogen gas and this gas will then be mingled with the sulfurous acid as it rises to the crater it is shown by experiment that these gases mutually decompose each other when mixed where steam is present the hydrogen of the one immediately uniting with the oxygen of the other to form water while the excess of sulfurous acid alone escapes into the atmosphere sulfur is at the same time precipitated this experiment is sufficient but it may also be observed that the flame of hydrogen would rarely be visible during an eruption as that gas when inflamed in a pure state burns with a very faint blue flame which even in the night could hardly be perceptible by the side of red hot and incandescent cinders its immediate conversion into water when inflamed in the atmosphere might also account for its not appearing in a separate form dr. daubany is of opinion that water containing atmospheric air may descend from the surface of the earth to the volcanic foci and that the same process of combustion by which water is decomposed may deprive such subterranean air of its oxygen in this manner he explains the great quantities of nitrogen evolved from volcanic vents in thermal waters and the fact that air disengaged from the earth in volcanic regions is either wholly or part deprived of oxygen sir h. davy in his memoir on the phenomena volcanoes remarks that there was every reason to suppose in visuvius the existence of a descending current of air and he imagined that subterranean cavities which throughout large volumes of steam during the eruption might afterwards in the quiet state of the volcano become filled with atmospheric air the presence of ammonia salts in volcanic emanations and of ammonia which is in part composed of nitrogen in lava favours greatly the notion of air as well as water being deoxidated in the interior of the earth it has been alleged by professor bischoff that the slight specific gravity of the metals of the alkalis is fatal to davy's hypothesis for if the mean density of the earth as determined by astronomers surpass that of all kinds of rocks these metals cannot exist at least not in great quantities in the interior of the earth but dr. daubany has shown that if we take the united specific gravity of potassium sodium silicon iron and all the materials which when united with oxygen constitute ordinary lava and then compare their weight with lava of equal bulk the difference is not very material the specific gravity of the lava only exceeding by about one fourth that of the unoxidized metals besides at great depths the metallic bases of the earths and alkalis may very probably be rendered heavier by pressure nor is it fair to embarrass the chemical theory of volcanoes with a doctrine so purely gratuitous as that which supposes the entire nucleus of the planet to have been at first composed of unoxidized metals professor bunson at marburg after analyzing the gases which escape from the volcanic fumaroles and sulfur tariffs of iceland and after calculating the quantity of hydrogen evolved between two eruptions affirms in contradiction of opinions previously entertained that the hydrogen bears a perfect relation in quantity to the magnitude of the streams of lava assuming the fusion of the last to have been the result of the heat evolved during the oxidation of alkaline and earthly metals and this to have been brought about by the decomposition of water yet after having thus succeeded in removing the principal objection once so triumphantly urged against davi's hypothesis bunson concludes by declaring that the hydrogen evolved in volcanic regions cannot have been generated by the decomposition of water coming in contact with alkaline and earthly metallic bases for says the professor this process presupposes the prevalence of a temperature in which carbonic acid cannot exist in contact with hydrogen without suffering a partial reduction to carbon oxide and not a trace of carbonic oxide is ever found in volcanic accelerations at the same time it will be seen by consulting the able memoirs of the marburg chemist that he supposes many energetic kinds of chemical action to be continually going on in the interior of the earth capable of causing the disengagement of hydrogen and there can be no doubt that this gas may be a source of innumerable new changes capable of producing the local development of internal heat cause of volcanic eruptions the most probable causes of a volcanic outburst at the surface have been in a great degree anticipated in the preceding speculations on the liquefaction of rocks and the generation of gases when a minute hole is bored in a tube filled with gas condensed into a liquid the hole becomes instantly airy form or as some writers have expressed it flashes into vapor and often burst the tube such an experiment may represent the mode in which gaseous matter may rush through a rent in the rocks and continue to escape for days or weeks through a small orifice with an explosive power sufficient to reduce every substance which opposes its passage into small fragments or even dust lava may be propelled upwards at the same time and ejected in the form of scoriae in some places where the fluid lava lies at the bottom of a deep fissure communicating on one hand with the surface and on the other with a cavern in which a considerable body of vapor has been formed there may be an efflux of lava followed by the escape of gas eruptions often commence and close with the discharge of vapor and when this is the case the next outburst may be expected to take place by the same vent for the concluding evolution of elastic fluids will keep open the duct and leave it unobstructed the breaking out of lava from the side or base of a lofty cone rather than from the summit may be attributed to the hydrostatic pressure to which the flanks of the mountain are exposed when the column of lava has risen to a great height or if before it has reached the top there should happen to be any stoppage in the main duct the upward pressure of the ascending column of gas and lava may burst a lateral opening in the case however of mount lower in the sandwich islands there appears to be a singular want of connection or sympathy between the eruptions of the central and the great lateral vent the great volcanic cone alluded to rises to the height of 13,760 feet above the level of the sea having a crater at its summit from which powerful streams of lava have flowed in recent times and having another still larger crater called Kilauea on its southeastern slope about 4000 feet above the sea this lateral cavity resembles a huge quarry cut in the mountain side being about 1000 feet deep when in its ordinary state it is seven miles and a half in circuit and scattered over its bottom at different levels are lakes and pools of lava always in a state of abolition the liquid in one of these will sometimes sink a hundred or a hundred and fifty feet while it is overflowing in another at a higher elevation there being it should seem no communication between them in like manner lava overflows in the summit crater of mount lower nearly 14 000 feet high while the great lateral cauldron just alluded to of Kilauea continues as tranquil as usual affording no relief to any part of the gases or melted matter which are forcing their way upwards in the center of the mountain how asks dr. Dana if there were any subterranean channel connecting the two great vents could this want of sympathy exist how according to the laws of hydrostatic pressure can a column of fluid stand 10 000 feet higher in one leg of the siphon than in the other the eruptions he observes are not proxysmal on the contrary the lava rises slowly and gradually to the summit on the lofty cone and then escapes there without any commotion manifesting itself in Kilauea a gulf always open on the flanks of the same mountain on conclusion he says is certain namely that volcanoes are no safety valves as they have been called for here two independent and apparently isolated centers of volcanic activity only 16 miles distant from each other are sustained in one and the same cone without pretending to solve this enigma i cannot refrain from remarking that the supposed independence of several orifices of eruption in one crater like Kilauea when induced in confirmation of the doctrine of two distinct sources of volcanic action underneath one mountain proves too much no one can doubt that the pools of lava in Kilauea have been derived from some common reservoir and have resulted from a combination of causes commonly called volcanic which are at work in the interior at some unknown distance below these causes have given rise in mount lower to eruptions from many points but principally from one center so that a vast doe of ejected matter has been piled up the subsidiary crater has evidently never given much relief to the imprisoned heated and liquefied matter for Kilauea does not form a lateral protuberance interfering with the general shape or uniform outline of mount lower end of chapter 32 part 1 recording by sycamore rockwell chapter 32 part 2 of principles of geology this is a LibriVox recording all LibriVox recordings are in the public domain for more information or to volunteer please visit LibriVox.org recording by sycamore rockwell principles of geology by Charles Lyle geysers of iceland as aqueous vapor constitutes the most abundant of the area form products of volcanoes in eruption it may be well to consider attentively a case in which steam is exclusively the moving power that of the geysers of iceland these intermittent hot springs occur in a district situated in the southwestern division of iceland where nearly one hundred of them are said to break out within a circle of two miles that the water is of atmospheric origin derived from rain and melted snow is proved says professor Bunson by the nitrogen which rises from them either pure or mixed with other gases the springs rise through a thick current of lava which may perhaps have flowed from mount heckler the summit of that volcano being seen from the spot at the distance of more than 30 miles in this district the rushing of water is sometimes heard in chasms beneath the surface for here as on etna rivers flow in subterranean channels through the porous and cavernous it has more than once happened after earthquakes that some of the boiling fountains have increased or diminished in violence and volume or entirely ceased or that new ones have made their appearance changes which may be explained by the opening of new rents and the closing of pre-existing fishes few of the geysers play longer than five or six minutes at a time although sometimes half an hour the intervals between their eruptions are for the most part very irregular the great geyser rises out of a spacious basin at the summit of a circular mound composed of siliceous incrustations deposited from the spray of its waters the diameter of this basin in one direction is 56 feet and 46 in another in the center is a pipe 78 feet in perpendicular depth and from 8 to 10 feet in diameter but gradually widening as it rises into the basin the inside of the basin is whitish consisting of a siliceous crust and perfectly smooth as are likewise two small channels on the sides of the mound down which the water escapes when the bowel is filled to the margin the circular basin is sometimes empty as represented in the following sketch but is usually filled with beautifully transparent water in the state of ebullition during the rise of the boiling water in the pipe especially when the ebullition is most violent and when the water is thrown up in jets subterranean noises are heard like the distant firing of a cannon and the earth is slightly shaken the sound then increases and the motion becomes more violent till at length a column of water is thrown up with loud explosions to the height of one or 200 feet after playing for a time like an artificial fountain and giving off great clouds of vapor the pipe or tube is emptied and a column of steam rushing up with amazing force and a thundering noise terminates the eruption if stones are thrown into the crater they are instantly ejected and such is the explosive force that very hard rocks are sometimes shivered into small pieces henderson found that by throwing a great quantity of large stones into the pipe of stoker one of the geysers he could bring on an eruption in a few minutes the fragments of stone as well as the boiling water were thrown in that case to a much greater height than usual after the water had been ejected a column of steam continued to rush up with a deafening roar for nearly an hour but the geyser as if exhausted by this effort did not send out a fresh eruption when its usual interval of rest had elapsed the account given by Sir George McKenzie of a geyser which he saw in eruption in 1810 agrees perfectly with the above description by henderson the steam and water rose for half an hour to the height of 70 feet and the white column remained perpendicular notwithstanding a brisk gale of wind which was blowing against it stones thrown into the pipe were projected to a greater height than the water to leeward of the vapor a heavy shower of rain was seen to fall among the different theories proposed to account for these phenomena i shall first mention one suggested by Sir Jay Herschel a limitation of these jets he says may be produced on a small scale by heating red hot the stem of a tobacco pipe filling the bowl with water and so inclining the pipe as to let the water run through the stem its escape instead of taking place in a continued stream is then performed by a succession of violent explosions at first of steam alone then of water mixed with steam and as the pipe cools almost wholly of water at every such proxiesmal escape of the water a portion is driven back accompanied with steam into the ball the intervals between the explosions depend on the heat length and inclination of the pipe their continuance on its thickness and conducting power the application of this experiment to the geysers merely requires that a subterranean stream flowing through the pores and crevices of lava should suddenly reach a fissure in which the rock is red hot or nearly so steam would immediately be formed which rushing up the fissure might force up water along with it to the surface while at the same time part of the steam might drive back the water of the supply for a certain distance towards its source and when after the space of some minutes the steam was all condensed the water would return and a repetition of the phenomena take place there is however another mode of explaining the action of the geyser perhaps more probable than the above described suppose water percolating from the surface of the earth to penetrate into the subterranean cavity ad by the fissures ff while at the same time steam at an extremely high temperature such as is commonly given out from the rinse of lava currents during congliation emanates from fissures c a portion of the steam is at first condensed into water while the temperature of the water is raised by the latent heat thus evolved till at last the lower part of the cavity is filled with boiling water and the upper with steam under high pressure the expansive force of the steam becomes at length so great that the water is forced up the fissure or pipe e b and runs over the rim of the basin when the pressure is thus diminished the steam in the upper part of the cavity a expands until the water d is driven into the pipe and when this happens the steam being the lighter of the two fluids rushes up through the water with great velocity if the pipe be choked up artificially even for a few minutes a great increase of heat must take place for it is prevented from escaping in a latent form in steam so that the water is made to boil more violently and this brings on an eruption professor bunson before sighted adopts this theory to account for the play of the little geyser but says it will not explain the phenomena of the great one he considers this like the others to be a thermospring having a narrow funnel shaped tube in the upper part of its course where the walls of the channel have become coated with silicious incrustations at the mouth of this tube the water has a temperature corresponding to the pressure of the atmosphere of about 212 degrees fahrenheit but at a certain depth below it is much hotter this the professor succeeded improving by experiment a thermometer suspended by a string in the pipe rising to 266 degrees fahrenheit or no less than 48 degrees above the boiling point after the column of water has been expelled what remains in the basin and pipe is found to be much cooled previously to these experiments of bunson and disclozaks made in iceland in 1846 it would scarcely have been supposed possible that the lower part of a free and open column of water could be raised so much in temperature without causing a circulation of ascending and descending currents followed by an almost immediate equalization of heat such circulation is no doubt impeded greatly by the sides of the wall not being vertical and by numerous contractions of its diameter but the phenomenon may be chiefly due to another cause according to recent experiments on the cohesion of liquids by mr donny of gent it appears that when water is freed from all admixture of air its temperature can be raised even under ordinary atmospheric pressure to 275 degrees fahrenheit so much does the cohesion of its molecules increase when they are not separated by particles of air as water long boiled becomes more and more deprived of air it is probably very free from such intermixture at the bottom of the geysers among other results of the experiments of bunson and his companion they convinced themselves that the column of fluid filling the tube is constantly receiving a sessions of hot water from below while it becomes cooler above by evaporation on the broad surface of the basin they also came to a conclusion of no small interest as bearing on the probable mechanism of ordinary volcanic eruptions namely that the tube itself is the main seat of focus of mechanical force this was proved by letting down stones suspended by strings to various depths those which were sunk to considerable distances from the surface were not cast up again whereas those nearer to the mouth of the tube were ejected to great heights other experiments also were made tending to demonstrate the singular fact that there is often scarce any motion below when a violent rush of steam and water is taking place above it seems that when a lofty column of water possesses a temperature increasing with depth any slight abolition or disturbance of equilibrium in the upper portion may first force up water into the basin and then cause it to flow over the edge a lower portion thus suddenly relieved of part of its pressure expands and is converted into vapor more rapidly than the first owning to its greater heat this allows the next sub-jacent stratum which is much hotter to rise and flash into a gaseous form and this process goes on till the evolution has descended from the middle to near the bottom of the funnel in speculating therefore on the mechanism of an ordinary volcanic eruption we may suppose that large subterranean cavities exist at the depth of some miles below the surface of the earth in which melted lava accumulates and when water containing the usual mixture of air penetrates into these the steam thus generated may press upon the lava and force it up the duct of a volcano in the same manner as a column of water is driven up the pipe of a geyser in other cases we may suppose a continuous column of liquid lava mixed with red hot water for water may exist in that state as professor Bunson reminds us under pressure and this column may have a temperature regularly increasing downwards a disturbance of equilibrium may first bring on an eruption near the surface by the expansion and conversion into gas of entangled water in other constituents of what we call lava so as to occasion a diminution of pressure more steam would then be liberated carrying up with it jets of melted rock which being hurled up into the air may fall in showers of ashes on the surrounding country and at length by the arrival of lava and water more and more heated at the orifice of the duct or the crater of the volcano expansive power may be acquired sufficient to expel a massive current of lava after the eruption has ceased a period of tranquility succeeds during which fresh accessions of heat are communicated from below and additional masses of rocks fused by degrees while at the same time atmospheric or seawater is descending from the surface at length the conditions required for a new outburst are obtained and another cycle of similar changes is renewed end of chapter 32 part 2 recording by sycamore rockwell