 Chapter 25 Part 4 of Principles of Geology Principles of Geology by Charles Lyle Chapter 25 Aetna Part 4 Antiquity of the Cone of Aetna It was before remarked that confined notions in regard to the quantity of past time have tended, more than any other prepossessions, to retard the progress of sound theoretical views in geology. The inadequacy of our conceptions of the earth's antiquity, having cramped the freedom of our speculations in this science, very much in the same way as a belief in the existence of a vaulted firmament once retarded the progress of astronomy. It was not until Descartes assumed the indefinite extent of the celestial spaces and removed the supposed boundaries of the universe that just opinions began to be entertained of the relative distances of the heavenly bodies, and until we habituate ourselves to contemplate the possibility of an indefinite lapse of ages having been comprised within each of the modern periods of the earth's history, we shall be in danger of forming most erroneous and partial views in geology. If history had bequeathed to us a faithful record of the eruptions of Aetna and a hundred other of the principal active volcanoes of the globe during the last three thousand years, if we had an exact account of the volume of lava and matter ejected during that period and the times of their production, we might perhaps be able to form a correct estimate of the average rate of the growth of a volcanic cone, for we might obtain a mean result from the comparison of the eruptions of so great a number of vents, however irregular might be the development of the igneous action in any one of them, if contemplated singly during a brief period. It would be necessary to balance protracted periods of inaction against the occasional outburst of paroxymal explosions. Sometimes we should have evidence of a repose of seventeenth centuries, like that which was interposed in Ishkia between the end of the fourth century B.C. and the beginning of the fourteenth century of our era. Occasionally, a tremendous eruption, like that of Horurio, would be recorded, giving rise at once to a considerable mountain. If we desire to approximate to the age of a cone such as Aetna, we ought first to obtain some data in regard to the thickness of matter which has been added during the historical era, and then endeavour to estimate the time required for the accumulation of such alternating lavas and beds of sand and scurry, as are superimposed upon each other in the Val de Beauvais. Afterwards we should try to deduce, from observations on other volcanoes, the more or less rapid increase of burning mountains in all the different stages of their growth. There is a considerable analogy between the mode of increase of a volcanic cone and that of trees of exogenous growth. These trees augment, both in height and diameter, by the successive application externally of cone upon cone of new lignious matter, so that if we'd make a transverse section near the base of the trunk, we intersect a much greater number of layers than nearer to the summit. When branches occasionally shoot out from the trunk, they first pierce the bark, and then after growing to a certain size, if they chance to be broken off, they may become enclosed in the body of the tree, as it augments in size, forming knots in the wood, which are themselves composed of layers of lignious matter, cone within cone. In like manner, a volcanic mountain, as we have seen, consists of a succession of conical masses enveloping others, while lateral cones having a similar internal structure often project, in the first instance, like branches from the surface of the main cone, and then becoming buried again are hidden like the knots of a tree. We can ascertain the age of an oak or pine by counting the number of concentric rings of annual growth seen in a transverse section near the base, so that we may know the date at which the seedling began to vegetate. The Baal-Bab tree of Senegal, Adensonia digitata, is supposed to exceed almost any other in jevity. Adenson inferred that one which he measured, and found to be thirty feet in diameter, had attained the age of five thousand one hundred and fifty years. Having made an incision to a certain depth, he first counted three hundred rings of annual growth, and observed what thickness the tree had gained in that period. The average rate of growth of younger trees of the same species was then ascertained, and the calculation made according to a supposed mean rate of increase. Dicandol considers it not improbable that the celebrated taxodium of Chaplotepec in Mexico Cupresis distitia, Lin, which is a hundred and seventeen feet in circumference, may be still more aged. It is, however, impossible until more data are collected respecting the average intensity of the volcanic action to make anything like an approximation to the age of a cone like Etna, because in this case the successive envelopes of lava and scurrie are not continuous, like the layers of wood in a tree, and afford us no definite measure of time. Each conical envelope is made up of a great number of distinct lava currents and showers of sand and scurrie differing in quantity, in which may have been accumulated in unequal periods of time. Yet we cannot fail to form the most exalted conception of the antiquity of this mountain, when we consider that its base is about ninety miles in circumference, so that it would require ninety flows of lava, each a mile in breadth at their termination, to raise the present foot of the volcano as much as the average height of one lava current. There are no records within the historical error which lead to the opinion that the altitude of Etna has materially varied within the last two thousand years. Of the eighty most conspicuous minor cones which adorn its flanks, only one of the largest, Monte Rossi, has been produced within the times of authentic history. Even this hill, thrown up in the year 1669, although 450 feet in height, only ranks as a cone of second magnitude. Monte Menardo, near Bronte, rises even now to the height of 750 feet, although its base has been elevated by more modern lavas and ejections. The dimensions of these larger cones appear to bear testimony to paroxysms of volcanic activity, after which we may conclude, from analogy, that the fires of Etna remained dormant for many years, since nearly a century of rest has sometimes followed a violent eruption in the historical era. It must also be remembered that of the small number of eruptions which occur in a century, one only is estimated to issue from the summit of Etna for every two that proceed from the sides. Nor do all the lateral eruptions give rise to such cones as would be reckoned among the smallest of the eighty hills above enumerated. Some of them produce merely insignificant molecules, which are soon afterwards buried by showers of ashes. How many years then must we not suppose to have been expended in the formation of the eighty cones? It is difficult to imagine that a fourth part of them have originated during the last thirty centuries. But if we conjecture the whole of them to have been formed in 12,000 years, how inconsiderable an era would this portion of time constitute in the history of the volcano? If we could strip off from Etna all the lateral molecules now visible, together with the lavas and scourier that have been poured out from them, and from the highest crater during the period of their growth, the diminution of the entire mass would be extremely slight. Etna might lose, perhaps, several miles in diameter at its base and some hundreds of feet in elevation, but it would still be the loftiest of Sicilian mountains, studded with other cones, which would be recalled, as it were, into existence by the removal of the rocks under which they are now buried. There seems nothing in the deep sections of the Val de Beauvais to indicate that the lava currents of remote periods were greater in volume than those of modern times, and there are abundant proofs that the countless beds of solid rock and scourier were accumulated as now in succession. On the grounds, therefore, already explained, we must infer that a mass so many thousand feet in thickness must have required an immense series of ages anterior to our historical periods for its growth. Yet the whole must be regarded as the product of a modern portion of the tertiary epic. Such at least is the conclusion that seems to follow from geological data, which show that the oldest parts of the mountain, if not of posterior date to the marine strata around its base, were at least of co-evil origin. Some geologists contend that the sudden elevation of large continents from beneath the waters of the sea have again and again produced waves which have swept over vast regions of the earth. But it is clear that no devastating wave has passed over the far zone of Edna since any of the lateral cones before mentioned were thrown up. For none of these heaps of loose sand and scourier could have resisted for a moment the denuding action of a violent flood. To some, perhaps, it may appear that hills of such incoherent materials cannot be a very great antiquity because the mere action of the atmosphere must, in the course of several thousand years, have obliterated their original forms. But there is no weight in this objection, for the older hills are covered with trees and herbage which protect them from waste. And in regard to the newer ones, such as the porosity of their component materials, that the rain which falls upon them is instantly absorbed. And for the same reason that the rivers on Edna have a subterranean course, there are none descending the sides of the minor cones. No sensible alteration has been observed in the form of these cones since the earliest periods of which there are memorials. And there seems no reason for anticipating that in the course of the next 10,000 or 20,000 years, they will undergo any great alteration in their appearance unless they should be shattered by earthquakes or covered by volcanic ejections. In other parts of Europe, as in Auvergne and Velais in France, similar loose cones of scourier, probably of as high antiquity as the whole mass of Edna, stand uninjured at inferior elevations above the level of the sea. End of Chapter 25 Chapter 26 of Principles of Geology. This is a LibriVox recording. A LibriVox recordings are in the public domain. For more information or to volunteer, please visit LibriVox.org. Recording by Diongines, Salt Lake City, Utah. Principles of Geology by Charles Lyle. Chapter 26. Part one. Volcanic eruption in Iceland in 1783. New Island thrown up. Lava currents of Skoptar Jokel in same year. Their immense volume. Eruption of Gerulo in Mexico. Humboldt's theory of the convexity of the plane of Malpey. Eruption of Galangoon in Java. Submarine volcanoes. Graham Island formed in 1831. Volcanic Archipelagos. Submarine eruptions in Mid-Atlantic. The Canaries. Tenerife. Cones thrown up in Lansarote. 1730 to 1736. Santorin and its contiguous aisles. Barren Island in the Bay of Bengal. Mud volcanoes. Mineral composition of volcanic products. Volcanic eruptions in Iceland. With the exception of Etna and Vesuvius. The most complete chronological records of a series of eruptions are those of Iceland. For their history reaches as far back as the ninth century of our era. And from the beginning of the 12th century, there is clear evidence that during the whole period, there has never been an interval of more than 40 and very rarely one of 20 years without either an eruption or a great earthquake. So intense is the energy of the volcanic action in this region that some eruptions of Haqla have lasted six years without ceasing. Earthquakes have often shaken the whole island at once, causing great changes in the interior, such as the sinking down of hills, the rending of mountains, the desertion by rivers of their channels, and the appearance of new lakes. New islands have often been thrown up near the coast, some of which still exist, while others have disappeared either by subsidence or the action of the waves. In the interval between eruptions, innumerable hot springs afford vent to subterranean heat, and sulfatars discharge copious streams of inflammable matter. The volcanoes in different parts of this island are observed, like those of the flagrian fields, to be in activity by turns, one vent often serving for a time as a safety valve to the rest. Many cones are often thrown up in one eruption. And in this case, they take a linear direction, running generally from northeast to southwest, from the northeastern part of the island, where the volcano Krabla lies to the promontory Reiki Anas, new island thrown up in 1783. The convulsions of the year 1783 appear to have been more tremendous than any recorded in the modern annals of Iceland. And the original Danish narrative of the catastrophe, drawn up in great detail, has since been substantiated by several English travelers, particularly in regard to the prodigious extent of country laid waste, and the volume of lava produced. About a month previous to the eruption on the mainland, a submarine volcano burst forth in the sea in latitude 63 degrees, 25 minutes north, longitude 23 degrees, 44 minutes west, at a distance of 30 miles in a southwest direction from Cape Reiki Anas, and ejected so much pumice that the ocean was covered with that substance to the distance of 150 miles. And ships were considerably impeded in their course. A new island was thrown up, consisting of high cliffs within which fire, smoke, and pumice were emitted from two or three different points. This island was claimed by his Danish Majesty, who denominated it Nio, or the new island. But before a year had elapsed, the sea resumed its ancient domain, and nothing was left but a reef of rocks from five to 30 fathoms underwater. Great eruption of Skoptar Jokul. Earthquakes, which had long been felt in Iceland, became violent on the 11th of June 1783, when Skoptar Jokul, distant nearly 200 miles from Nio, threw out a torrent of lava, which flowed down into the river Skopta, and completely dried it up. The channel of the river was between high rocks in many places from 400 to 600 feet in depth, and near 200 in breadth. Not only did the lava fill up this great defile to the brink, but it overflowed the adjacent fields to a considerable extent. The burning flood on issuing from the confined Rocky Gorge was then arrested for some time by a deep lake, which formerly existed in the course of the river between Skoptar Jokul and A, which it entirely filled. The current then advanced again, and reaching some ancient lava full of subterraneous caverns, penetrated and melted down part of it, and in some places where the steam could not gain vent, it blew up the rock, throwing fragments to the height of more than 150 feet. On the 18th of June, another ejection of liquid lava rushed from the volcano, which flowed down with amazing velocity over the surface of the first stream. By the damming up of the mouths of some of the tributaries of the Skoptar, many villages were completely overflowed with water, and thus great destruction of property was caused. The lava, after flowing for several days, was precipitated down a tremendous cataract called Stopofos, where it filled a profound abyss, which that great waterfall had been hollowing out for ages, and after this the fiery current again continued its course. On the 3rd of August, fresh floods of lava, still pouring from the volcano, a new branch was sent off in a different direction, for the channel of the Skoptar was now so entirely choked up, and every opening to the west and north so obstructed, that the melted matter was forced to take a new course, so that it ran in a southeast direction, and discharged itself into the bed of the river. Hairfist Flood, where a scene of destruction scarcely inferior to the former was occasioned. These Icelandic lavas, like the ancient streams, which are met with in Overn and other provinces of central France, are stated by Stevenson to have accumulated to a prodigious depth in narrow rocky gorges. But when they came to wide alluvial plains, they spread themselves out into broad burning lakes, sometimes from 12 to 15 miles wide, and 100 feet deep. When the fiery lake, which filled up the lower portion of the valley of the Skoptar, had been augmented by new supplies, the lava flowed up the course of the river to the foot of the hills, from whence the Skoptar takes its rise. This affords a parallel case to one which can be shown to have happened at a remote era in the volcanic region of the Viveret in France, where lava issued from the cone of Theots, and while one branch ran down, another more powerful stream flowed up the channel of the river Ardèche. The sides of the valley of the Skoptar present superb ranges of basaltic columns of older lava, resembling those which are laid open in the valleys descending from Montdour in Overn, where more modern lava currents on a scale very inferior in magnitude to those of Iceland have also usurped the beds of the existing rivers. The eruption of Skoptar Jockel did not entirely cease until the end of two years. And when Mr. Poulsen visited the tract eleven years afterwards, in 1794, he found columns of smoke still rising from parts of the lava and several rents filled with hot water. Although the population of Iceland was very much scattered and did not exceed fifty thousand, no less than twenty villages were destroyed, besides those inundated by water, and more than nine thousand human beings perished together with an immense number of cattle, partly by the depredations of the lava, partly by the noxious vapors which impregnated the air, and in part by the famine caused by showers of ashes throughout the island and the desertion of the coasts by the fish. Immense volume of the lava. But the extraordinary volume of melted matter produced in this eruption deserves the particular attention of the geologist, of the two branches which flowed in nearly opposite directions. The greatest was fifty and the lesser forty miles in length. The extreme breadth which the Skoptar branch attained in the low countries was from twelve to fifteen miles that of the other about seven. The ordinary height of both currents was one hundred feet, but in narrow defiles it sometimes amounted to six hundred. Professor Bischoff has calculated that the mass of lava brought up from the subterranean regions by this single eruption surpassed in magnitude the bulk of Mont Blanc. But a more distinct idea will be formed of the dimensions of the two streams if we consider how striking a feature they would now form in the geology of England had they been poured out on the bottom of the sea after the deposition and before the elevation of our secondary and tertiary rocks. The same causes which have excavated valleys through parts of our marine strata, once continuous, might have acted with equal force on the igneous rocks, leaving at the same time a sufficient portion undistroyed to enable us to discover their former extent. Let us then imagine the termination of the Skopta branch of lava to rest on the escarpment of the inferior and middle oolite where it commands the veil of Gloucester. The great platform might be one hundred feet thick and from 10 to 15 miles broad exceeding any which can be found in central France. We may also suppose great tabular masses to occur at intervals capping the summit of the Cotswold Hills between Gloucester and Oxford by Norleach, Burford and other towns. The wide valley of the Oxford clay would then occasion an interruption for many miles, but the same rocks might recur on the summit of Comnor and Shotover Hills and all the other oolitic eminences of that district. On the chalk of Berkshire, extensive plateaus six or seven miles wide would again be formed. And lastly, crowning the highest sands of Highgate and Hampstead, we might behold some remnants of the current five or six hundred feet in thickness causing those hills to rival or even to surpass in height Salisbury, Craigs and Arthur's Seat. The distance between the extreme points here indicated would not exceed 90 miles in a direct line, and we might then add at the distance of nearly 200 miles from London along the coast of Dorsetshire and Devonshire, for example, a great mass of igneous rocks to represent those of contemporary origin, which were produced beneath the level of the sea, where the island of Nile rose up. Volume of ancient and modern flows of lava compared yet gigantic as must appear the scale of these modern volcanic operations, we must be content to regard them as perfectly insignificant in comparison to currents of the primeval ages. If we embrace the theoretical views of many geologists, which were not inaccurately expressed by the late Professor Alexander Braniart, when he declared that ox epochs, geonostiches, ancients, two less phenomenas, geologics, say Passoyant, Don Stiz, Dimensiones, Centuples, Decelles, Kiehl's, Precentent, Aux Jours, Wee. Had Schuptar Jokel, therefore, Benevolcano of the Olden Time, it would have poured forth lavas at a single eruption a hundred times more voluminous than those which were witnessed by the present generation in 1783. But it may, on the contrary, be affirmed that among the older formations, no igneous rock of such colossal magnitude has yet been met with. Nay, it would be most difficult to point out a mass of ancient date distinctly referable to a single eruption, which would even rival in volume the matter poured out from Schuptar Jokel in 1783. Eruption of Gerulo in 1759 as another example of the stupendous scale of modern volcanic eruptions, I may mention that of Gerulo in Mexico in 1759. The great region to which this mountain belongs has already been described. The plain of Malpey forms part of an elevated platform between two and three thousand feet above the level of the sea, and is bounded by hills composed of basalt, trachite, and volcanic tuff, clearly indicating that the country had previously, though probably at a remote period, been the theater of igneous action. From the era of the discovery of the new world to the middle of the last century, the district had remained undisturbed, and the space, now the site of the volcano, which is thirty six leagues distant from the nearest sea, was occupied by fertile fields of sugarcane and indigo, and watered by the two brooks, Kuitimba and San Pedro. In the month of June 1759, hollow sounds of an alarming nature were heard, and earthquakes succeeded each other for two months, until at the end of September, flames issued from the ground, and fragments of burning rocks were thrown to prodigious heights. Six volcanic cones, composed of scoriae and fragmentary lava, were formed on the line of a chasm, which ran in the direction from north northeast to south southwest. The least of these cones was three hundred feet in height, and Gerulo, the central volcano, was elevated sixteen hundred feet above the level of the plane. It sent forth great streams of basaltic lava, containing included fragments of granitic rocks, and its ejections did not cease till the month of February 1760. Humboldt visited the country more than forty years after this occurrence, and was informed by the Indians that when they returned, long after the catastrophe, to the plane, they found the ground uninhabitable from the excessive heat. When he himself visited the place, there appeared around the base of the cones, and spreading from them, as from a center, over an extent of four square miles, a mass of matter of a convex form, about five hundred fifty feet high, at its junction with the cones, and gradually sloping from them in all directions towards the plane. This mass was still in a heated state, the temperature in the fissures being on the decrease from year to year, but in 1780 it was still sufficient to light a cigar at the depth of a few inches. On this slightly convex protuberance, the slope of which must form an angle of about six degrees with the horizon, were thousands of flat-ish conical mounds from six to nine feet high, which as well as large fissures traversing the plane, acted as fumaroles giving out clouds of sulfurous acid and hot, aqueous vapor. The two small rivers before mentioned disappeared during the eruption, losing themselves below the eastern extremity of the plane, and reappearing as hot springs at its western limit. Cause of the convexity of the plane of Malpay. Humboldt attributed the convexity of the plane to inflation from below, supposing the ground four four square miles in extent to have risen up in the shape of a bladder to the elevation of five hundred fifty feet above the plane in the highest part. But Mr. Scrope has suggested that the phenomena may be accounted for far more naturally by supposing that lava flowing simultaneously from the different orifices and principally from Gerulo, united into a sort of pool or lake as they were poured forth on a surface previously flat, they would if their liquidity was not very great, remain thickest and deepest near their source and diminish in bulk from fence towards the limits of the space which they covered. Fresh supplies were probably emitted successively during the course of an eruption which lasted more than half a year and some of these resting on those first emitted might only spread to a small distance from the foot of the cone, where they would necessarily accumulate to a great height. The average slope of the great dome-shaped volcanoes of the Sandwich Islands formed almost exclusively of lava, with scarce any scoriae, is between six degrees thirty minutes and seven degrees forty six minutes, so that the inclination of the convex mass around Gerulo, if we adopt Mr. Scrope's explanation, is quite in accordance with the known laws which govern the flow of lava. The showers also of loose and pulverulent matter from the six craters and principally from Gerulo would be composed of heavier and more bulky particles near the cones and would raise the ground at their base where mixing with rain they might have given rise to the stratum of black clay which is described as covering the lava. The small conical mounds called hornitos or little ovens may resemble those five or six small hillocks which existed in 1823 on the Vesuvian lava and sent forth columns of vapor having been produced by the disengagement of elastic fluids heaping up small dome-shaped masses of lava. The fissures mentioned by Humboldt as of frequent occurrence are such as might naturally accompany the consolidation of a thick bed of lava contracting as it congeals and the disappearance of rivers is the usual result of the occupation of the lower part of a valley or plain by lava of which there are many beautiful examples in the old lava currents of Avern. The heat of the hornitos is stated to have diminished from the first and Mr. Bullock who visited the spot many years after Humboldt found the temperature of the hot spring very low, a fact which seems clearly to indicate the gradual congelation of a subjacent bed of lava which from its immense thickness may have been enabled to retain its heat for half a century. The reader may be reminded that when we thus suppose the lava near the volcano to have been together with the ejected ashes more than 500 feet in depth, we merely assign a thickness which the current of Skoptar Jokel attained in some places in 1783. Hollow sound of the plane when struck. Another argument adduced in support of the theory of inflation from below was the hollow sound made by the steps of a horse upon the plane which however proves nothing more than that the materials of which the convex mass is composed are light and porous. The sound called Rambambo by the Italians is very commonly returned by made ground when struck sharply and has been observed not only on the sides of Vesuvius and other volcanic cones where there is a cavity below but in such regions as the Campania di Roma composed in a great measure of tough and porous volcanic rocks. The reverberation, however, may perhaps be assisted by grottoes and caverns for these may be as numerous in the lavas of Gerulo as in many of those of Etna but their existence would lend no countenance to the hypothesis of a great arched cavity four square miles in extent and in the center 550 feet high. No recent eruptions of Gerulo. In a former addition I stated that I have been informed by Captain Vetch that in 1819 a tower at Guadalaxara was thrown down by an earthquake and that ashes supposed to have come from Gerulo fell at the same time at Guanoxuado a town situated 140 English miles from the volcano but Mr. Burkhart a German director of mines who examined Gerulo in 1827 ascertained that there had been no eruption there since Humboldt's visit in 1803. He went to the bottom of the crater and observed a slight evolution of sulfurous acid vapors but the Hornitos had entirely ceased to send forth steam. During the 24 years intervening between his visit and that of Humboldt vegetation had made great progress on the flanks of the New Hills. The rich soil of the surrounding country was once more covered with luxuriant crops of sugarcane and indigo and there was an abundant growth of natural underwood on all the uncultivated traps. End of chapter 26 part one. Chapter 26 of Principles of Geology. This is a LibriVox recording. A LibriVox recordings are in the public domain. For more information or to volunteer please visit LibriVox.org recording by Dion Giants, Salt Lake City, Utah. Principles of Geology by Charles Lyle. Chapter 26 part 2 Galangun Java 1822. The mountain of Galangun or Galungung was in 1822 covered by a dense forest and situated in a fruitful and thickly peopled part of Java. There was a circular hollow at its summit but no tradition existed of any former eruption. In July 1822 the waters of the river Kunir, one of those which flowed from its flanks, became for a time hot and turbid. On the 8th of October following a loud explosion was heard, the earth shook and immense columns of hot water and boiling mud mixed with burning brimstone, ashes, and lapili of the size of nuts were projected from the mountain like a water spout with such prodigious violence that large quantities fell beyond the river Tandoi which is 40 miles distant. Every valley within the range of this eruption became filled with a burning torrent and the rivers swollen with hot water and mud overflowed their banks and carried away great numbers of the people who were endeavoring to escape and the bodies of cattle, wildbeats, and birds. A space of 24 miles between the mountain and the river Tandoi was covered to such a depth with bluish mud that people were buried in their houses and not a trace of the numerous villages and plantations throughout that extent was visible. Within this space the bodies of those who perished were buried in mud and concealed, but near the limits of the volcanic action they were exposed and strewed over the ground in great numbers partly boiled and partly burnt. It was remarked that the boiling mud and cinders were projected with such violence from the mountain that while many remote villages were utterly destroyed and buried others much nearer the volcano were scarcely injured. The first eruption lasted nearly five hours and on the following days the rain fell in torrents and the rivers densely charged with mud deluged the country far and wide. At the end of four days, October 12th, a second eruption occurred more violent than the first in which hot water and mud were again vomited and great blocks of the salt were thrown to the distance of seven miles from the volcano. There was at the same time a violent earthquake and in one account it is stated that the face of the mountain was utterly changed its summits broken down and one side which had been covered with trees became an enormous gulf in the form of a semi-circle. This cavity was about midway between the summit and the plain and surrounded by steep rocks said to be newly heaped up during the eruption. New hills and valleys are said to have been formed and the rivers Banjaring and Wulan changed their course and in one night October 12th 2,000 persons were killed. The first intimation which the inhabitants of Bangdong received of this calamity on the 8th of October was the news that the river Wulna was bearing down into the sea the dead bodies of men and the carcasses of stags rhinoceroses tigers and other animals the Dutch painter Payen determined to travel from fence to the volcano and he found that the quantity of the ashes diminished as he approached the base of the mountain. He alludes to the altered form of the mountain after the 12th but does not describe the new semi-circular gulf on its side. The official accounts state that 114 villages were destroyed and above 4,000 persons killed. Submarine volcanoes although we have every reason to believe that volcanic eruptions as well as earthquakes are common in the bed of the sea it was not to be expected that many opportunities would occur to scientific observers of witnessing the phenomena the crews of vessels have sometimes reported that they have seen in different places sulfurous smoke flame jets of water and steam rising up from the sea or they have observed the waters greatly discolored and in a state of violent agitation as if boiling new shoals have also been encountered or a reef of rocks just emerging above the surface where previously there was always supposed to have been deep water. On some few occasions the gradual formation of an island by a submarine eruption has been observed as that of Sabrina in the year 1811 off St. Michael's in the Azores the throwing up of ashes in that case and the formation of a cone about 300 feet in height with a crater in the center closely resembled the phenomena usually accompanying a volcanic eruption on land Sabrina was soon washed away by the waves previous eruptions in the same part of the sea were recorded to have happened in 1691 and 1720 the rise of Nioe also a small island off the coast of Iceland in 1783 has already been alluded to and another volcanic isle was produced by an eruption near Reykjavík on the same coast in June 1830 Graham island 1831 we have still more recent and minute information respecting the appearance in 1831 of a new volcanic island in the Mediterranean between the southwest coast of Sicily and that projecting part of the African coast where ancient Carthage stood the site of the island was not any part of the Great Shoal or bank called Narita as was first asserted but a spot where Captain W. H. Smith had found in his survey a few years before a depth of more than 100 fathoms water the position of the island latitude 37 degrees eight minutes 30 seconds north longitude 12 degrees 42 minutes 15 seconds east was about 30 miles southwest of Siaka in Sicily and 33 miles northeast of Pantolaria on the 28th of June about a fortnight before the eruption was visible Sir Pultany Malcolm in passing over the spot in his ship felt the shocks of an earthquake as if he had struck on a sandbank and the same shocks were felt on the west coast of Sicily in a direction from southwest to northeast about the 10th of July John Correo the captain of a Sicilian vessel reported that as he passed near the place he saw a column of water like a water spout 60 feet high and 800 yards in circumference rising from the sea and soon afterwards a dense steam in its place which ascended to the height of 1800 feet the same Correo on his return from Giurgenti on the 18th of July found a small island 12 feet high with a crater in its center ejecting volcanic matter and immense columns of vapor the sea around being covered with floating cinders and dead fish the scoriae were of a chocolate color and the water which boiled in the circular basin was of a dingy red the eruption continued with great violence to the end of the same month at which time the island was visited by several persons and among others by captain Swinburne R.N. and M. Hoffman the Prussian geologist it was then from 50 to 90 feet in height and three quarters of a mile in circumference by the 4th of august it became according to some accounts above 200 feet high and three miles in circumference after which it began to diminish in size by the action of the waves and it was only two miles round on the 25th of august and on the 3rd of september when it was carefully examined by captain Wodehouse only three-fifths of a mile in circumference its greatest height being then 107 feet at this time the crater was about 780 feet in circumference on the 29th of september when it was visited by mature sea provost its circumference was reduced to about 700 yards it was composed entirely of incoherent ejected matter scoriae pumice and lapili forming regular strata some of which are described as having been parallel to the steep inward slope of the crater while the rust were inclined outwards like those of the susius when the arrangement of the ejected materials has been determined by their falling continually on two steep slopes that of the external cone and that of the crater which is always a hollow inverted cone a transverse section would probably resemble that given in the annexed figure but when i visited visuvius in 1828 i saw no beds of scoriae inclined towards the axis of the cone such may have once existed but the explosions or subsidences or whatever causes produced the great crater of 1822 had possibly destroyed them few of the pieces of stone thrown out from gram island exceeded a foot in diameter some fragments of dolomitic limestone were intermixed but these were the only non-volcanic substances during the month of august there occurred on the southwest side of the new island a violent abolition and agitation of the sea accompanied by the constant ascension of a column of dense white steam indicating the existence of a second vent at no great depth from the surface towards the close of october no vestige of the crater remained and the island was nearly leveled with the surface of the ocean with the exception at one point of a small montecule of sand and scoriae it was reported that at the commencement of the year following 1832 there was a depth of 150 feet where the island had been but this account was quite erroneous for in the early part of that year captain swinburne found a shoal and discolored water there and towards the end of 1833 a dangerous reef existed of an oval figure about three fifths of a mile in extent in the center was a black rock of the diameter of about 26 fathoms from nine to 11 feet underwater and round this rock are banks of black volcanic stones and loose sand at the distance of 60 fathoms from this central mass the depth increased rapidly there was also a second shoal at the distance of 450 feet southwest of the great reef with 15 feet water over it also composed of rock surrounded by deep sea we can scarcely doubt that the rock in the middle of the larger reef is solid lava which rose up in the principal crater and that the second shoal marks the site of the submarine eruption observed in august 1831 to the southwest of the island from the whole of the facts above detailed it appears that a hill 800 feet or more in height was formed by a submarine volcanic vent of which the upper part only about 200 feet high emerged above the waters so as to form an island this cone must have been equal in size to one of the largest of the lateral volcanoes on the flanks of etna and about half the height of the mountain gerulo in mexico which was formed in the course of nine months in 1759 in the center of the new volcano a large cavity was kept open by gaseous discharges which throughout scoriae and fluid level probably rose up in this cavity it is not uncommon for small subsidiary craters to open near the summit of a cone and one of these may have been formed in the case of gram island event perhaps connected with the main channel of discharge which gave passage in that direction to elastic fluids scoriae and melted lava it does not appear that either from this duct or from the principal vent there was any overflowing of lava but melted rock may have flowed from the flanks or base of the cone a common occurrence on land and may have spread in a broad sheet over the bottom of the sea the dotted lines in the annexed figure are an imaginary restoration of the upper part of the cone now removed by the waves the strong lines represent the part of the volcano which is still under water in the center is a great column or dike of solid lava two hundred feet in diameter supposed to fill the space by which the gaseous fluids rose and on each side of the dike is a stratified mass of scoriae and fragmentary lava the solid nucleus of the reef where the black rock is now found withstands the movements of the sea while the surrounding loose tufts are cut away to a somewhat lower level in this manner the lava which was the lowest part of the island or to speak more correctly which scarcely ever rose above the level of the sea when the island existed has now become the highest point in the reef no appearances observed either during the eruption or since the island disappeared gave the least support to the opinion promulgated by some writers that part of the ancient bed of the sea had been lifted up bodily the solid products says dr john davie whether they consisted of sand light cinders or vesicular lava differed more informed than in composition the lava contained augite and the specific gravity was two point oh seven and two point seven oh when the light spongy cinder which floated on the sea was reduced to fine powder by triteration and the greater part of the entangled air got rid of it was found to be of the specific gravity of two point six four and that of some of the sand which fell in the eruption was two point seven five so that the materials equaled ordinary granites in weight and solidity the only gas evolved in any considerable quantity was carbonic acid submarine eruptions in mid-atlantic in the nautical magazine for 1835 page 642 and for 1838 page 361 and in the comps rendus april 1838 accounts are given of a series of volcanic phenomena earthquakes troubled water floating scoriae and columns of smoke which have been observed at intervals since the middle of the last century in a space of open sea between longitudes 20 degrees and 22 degrees west about half a degree south of the equator these facts says mr darwin seemed to show that an island or an archipelago is in process of formation in the middle of the atlantic a line joining st helena and ascension would if prolonged intersect this slowly nascent focus of volcanic action should land be eventually formed here it will not be the first that has been produced by igneous action in this ocean since it was inhabited by the existing species of testesia at porto praia in st. yago one of the azores a horizontal calcarius stratum occurs containing shells of recent marine species covered by a great sheet of basalt 80 feet thick it would be difficult to estimate too highly the commercial and political importance which a group of islands might acquire if in the next two or three thousand years they should rise in mid ocean between st helena and ascension and of chapter 26 part two chapter 26 of principles of geology this is a liberovox recording a liberovox recordings are in the public domain for more information or to volunteer please visit liberovox.org recording by dion giants salt lake city utah principles of geology by charles liall chapter 26 part 3 canary islands eruption in lanserot 1730 to 1736 the effects of an eruption which happened in lanserot one of the canary islands between the years 1730 and 1736 were very remarkable and a detailed description has been published by van boek who had an opportunity when he visited that island in 1815 of comparing the accounts transmitted to us of the event with the present state and geological appearances of the country on the first of september 1730 the earth split open on a sudden two leagues from yara in one night a considerable hill of ejected matter was thrown out and a few days later another vent opened and gave out a lava stream which overran chin and feya and other villages it flowed first rapidly like water but became afterwards heavy and slow like honey on the seventh of september and immense rock was protruded from the bottom of the lava with a noise like thunder and the stream was forced to change its course from north to northwest so that st. catalina and other villages were overflowed whether this mass was protruded by an earthquake or was a mass of ancient lava blown up like that before mentioned in 1783 in iceland is not explained on the 11th of september more lava flowed out and covered the village of maso entirely and for the space of eight days precipitated itself with a horrible roar into the sea dead fish floated on the waters in indescribable multitudes or were thrown dying on the shore after a brief interval of repose three new openings broke forth immediately from the site of the consumed st. catalina and sent out an enormous quantity of lapilli sand and ashes on the 28th of october the cattle throughout the whole country dropped lifeless to the ground suffocated by putrid vapors which condensed and fell down in drops on the first of december a lava stream reached the sea and formed an island round which dead fish were strewed number of cones thrown up it is unnecessary here to give the details of the overwhelming of other places by fiery torrents or of a storm which was equally new and terrifying to the inhabitants as they had never known one in their country before on the 10th of january 1731 a high hill was thrown up which on the same day precipitated itself back again into its own crater fiery brooks of lava flowed from it to the sea on the 3rd of february a new cone arose others were thrown up in march and poured forth lava streams numerous other volcanic cones were subsequently formed in succession till at last their number amounted to about 30 in june 1731 during a renewal of the eruptions all the banks and shores in the western part of the island were covered with dying fish of different species some of which had never before been seen smoke and flame arose from the sea with loud detonations these dreadful commotions lasted without interruption for five successive years so that a great emigration of the inhabitants became necessary their linear direction as to the height of the new cones van buke was assured that the formerly great and flourishing saint katalina lay buried under hills 400 feet in height and he observes that the most elevated cone of the series rose 600 feet above its base and 1300 and 78 feet above the sea and that several others were nearly as high the new vents were all arranged in one line about two geographical miles long and in a direction nearly east and west if we admit the probability of von buke's conjecture that these vents opened along the line of a cleft it seems necessary to suppose that this subterranean fissure was only prolonged upwards to the surface by degrees and that the rent was narrow at first as is usually the case with fissures caused by earthquakes lava and elastic fluids might escape from some point on the rent where there was least resistance till the first aperture becoming obstructed by ejections and the consolidation of lava other orifices burst open in succession along the line of the original fissure von buke found that each crater was lowest on that side on which lava had issued but some craters were not breached and were without any lava streams in one of these were open fissures out of which hot vapors rose which in 1815 raised the thermometer to 145 degrees Fahrenheit and was probably at the boiling point lower down the exhalations seem to consist of aqueous vapor yet they could not be pure steam for the crevices were encrusted on either side by siliceous center and opal like hydrate of silica of a white color which extended almost to the middle this important fact attests the length of time during which chemical processes continue after eruptions and how open fissures may be filled up laterally by mineral matter sublimed from volcanic exhalations the lavas of this eruption covered nearly a third of the whole island often forming on slightly inclined planes great horizontal sheets several square leagues in area resembling very much the basaltic platforms of avern pretended distinction between ancient and modern lavas one of the new lavas was observed to contain masses of olivine of an olive green color resembling those which occur in one of the lavas of the vivarays von buke supposes the great crystals of olivine to have been derived from a previously existing basalt melted up by the new volcanoes but we have scarcely sufficient data to bear out such a conjecture the older rocks of the island consist in a great measure of that kind of basaltic lava called dolerite sometimes columnar and partly of common basalt and amygdaloid some recent lavas assumed on entering the sea a prismatic form and so much resembled the older lavas of the canaries that the only geological distinction which von buke appears to have been able to draw between them was that they did not alternate with conglomerates like the ancient basalts some modern writers have endeavored to discover in the abundance of these conglomerates a proof of the dissimilarity of the volcanic action in ancient and modern times but this character is more probably attributable to the difference between submarine operations and those on the land all the blocks and imperfectly rounded fragments of lava transported during the intervals of eruption by rivers and torrents into the adjoining sea or torn by the continued action of the waves from cliffs which are undermined must accumulate in stratified breccias and conglomerates and be covered again and again by other lavas this is now taking place on the shores of sicily between catania and trezza where the sea breaks down and covers the shore with blocks and pebbles of the modern lavas of etna and on parts of the coast of ischia where numerous currents of trachite are in like manner undermined in lofty precipices so often then as an island is raised in a volcanic archipelago by earthquakes from the deep the fundamental and relatively to all above the oldest lava will often be distinguishable from those formed by subsequent eruptions on dry land by their alternation with beds of sandstone and fragmentary rocks the supposed want of identity then between the volcanic phenomena of different epochs resolves itself partly at least into the marked difference between the operations simultaneously in progress above and below the waters such indeed is the source as was stated in the first book chapter five of many of our strongest theoretical prejudices in geology no sooner do we study and endeavor to explain submarine appearances then we feel to use a common expression out of our element and unwilling to concede that our extreme ignorance of processes now continually going on can be the cause of our perplexity we take refuge in a pre existent order of nature and unwilling to concede that our extreme ignorance of processes now continually going on can be the cause of our perplexity we take refuge in a pre-existent order of nature recent formation of olytic travertine in lanserote throughout a considerable part of lanserote the old lavas are covered by a thin stratum of limestone from an inch to two inches in thickness it is of a hard stalactitic nature sometimes olytic like the jura limestone and contains fragments of lava and terrestrial shells chiefly helices and spiral bulimmy it sometimes rises to the height of 800 feet above the level of the sea von buke imagines that this remarkable super stratum has been produced by the furious northwest storms which in winter drive the spray of the sea in clouds over the whole island from whence calcaria's particles may be deposited stalactitically mr darwin informs me that he found a limestone in st helena the harder parts of which correspond precisely to the stone of lanserote he attributes the origin of this rock in st helena not to the spray of the sea but to drifting by violent winds of the finer particles of shells from the sea beach some parts of this drift are subsequently dissolved by atmospheric moisture and redeposited so as to convert calcaria sand into oolite recent eruption in lanserote from the year 1736 to 1815 when von buke visited lanserote there had been no eruption but in august 1824 a crater opened near the port of recife informed by its ejections in the space of 24 hours a considerable hill violent earthquakes proceeded and accompanied this eruption tenorath the peak of tenorath is about 12 000 feet high and stands says von buke like a tower encircled by its fossa and bastion the bastion consists like the semicircular escarpment of soma turned towards vasuvius of precipitous cliffs composed of trackite basalt coarse conglomerates and tufts traversed by volcanic dykes mostly vertical and of basalt these cliffs vary in height from 1000 to 1800 feet and are supposed by von buke to have been heaved up into their present position by a force exerted from below in accordance with the theory proposed by the same author for the origin of the cones of vasuvius and etna according to the observations of emdeville in 1839 the trackites are often granitoid in their aspect and contain instead of glossy felspar the allied mineral called oligocloss which had been previously considered as characteristic of more ancient igneous rocks the same traveler supposes although he found no limestone or trace of fossils in any of the rocks of tenorath that the alternating trackites and trachetic conglomerates originated beneath the sea if this opinion be correct and it is at least very probable geologists may still speculate on two modes in which the mass of the island acquired its present form and elevation above the sea first the advocates of von buke's crater of elevation hypothesis may imagine that a succession of horizontally superimposed beds were upheaved by a sudden movement and tilted so as to dip in all directions outwards from the center of a new dome shaped eminence in the middle of which a large opening or bowl shaped cavity was produced secondly or according to the theory which to me appears preferable a submarine hill in the form of a flattened dome may have gradually accumulated partly below the waters and partly above by the continued outpourings of sheets of lava and the ejection of ashes from a central orifice in this case the dykes would represent the fissures which were filled during successive eruptions and the original inclination of the beds may have been increased by the distention and upheaval of the mass during reiterated convulsions acting most forcibly at or near the channel of discharge which would become partially sealed up with lava from time to time and then be burst open again during eruptions at length the whole island may have been raised bodily out of the sea by a gradual upward movement whatever theory we adopt we must always explain the abrupt termination of the dykes and layers of trochite and basalt in the steep walls of the escarpments surrounding the great crater by supposing the removal of part of the materials once prolonged farther inwards towards the center if according to the elevation crater hypothesis a series of sheets of lava and ashes originally spread over a level and even surface have been violently broken and uplifted why do not the opposite walls of the chasm correspond in such a manner as to imply by their present outline that they were formerly united it is evident that the precipices on opposite sides of the crater of form hollow would not fit if brought together there being no projecting masses in one wall to enter into indentations in the other as would happen with the sides of many mineral veins trap dykes and faults could we extract the intrusive manner now separating them and reunite the rocks which have been fractured and disjoint the highest crater of the peak has merely disengaged sulfurious vapors ever since it has been known to europeans but an eruption happened in june 1798 not far from the summit and others are recorded which poured out streams of lava from great heights besides many which have broken out near the level of the sea all these however seem to be dependent on one great center of eruption or on that open channel communicating between the interior of the earth and the atmosphere which terminates in the highest crater of the peak we may consider tenorath then as having been from a remote period the principal and habitual vent of the volcanic archipelago of the canaries the discharges which have taken place in the contiguous aisles of palma lancerote and the rest may be of a subsidiary kind and have probably been most frequent and violent when the greater crater has been partially sealed up just as the violent eruptions of ischia or that of mont novo coincided with the dormant state of visuvius end of chapter 26 part 3 chapter 26 of principles of geology this is a libravox recording a libravox recordings are in the public domain for more information or to volunteer please visit libravox.org recording by dianjain's salt lake city utah principles of geology by charles lile chapter 26 part 4 santoren the gulf of santoren in the grecian archipelago has been for 2000 years a scene of active volcanic operations the largest of the three outer islands of the group to which the general name of santoren is given is called thera or sometimes santoren and forms more than two-thirds of the circuit of the gulf the length of the exterior coastline of this and the other two islands named therasia and as spranisi taken together amounts to about 30 miles and that of the inner coastline of the same islands to about 18 miles in the middle of the gulf are three other islands called the little the new and the old kemenas or burnt islands the accompanying map has been reduced from a recent survey executed in 1848 by captain graves rn and shortly to be published by the admiralty plenty informs us that the year 186 bc gave birth to the old kemeni also called herea or the sacred aisle and in the year 19 of our era theia the divine made its appearance above water and was soon joined by subsequent eruptions to the older island from which it was only 250 paces distant the old kemeni also increased successively in size in 726 and in 1427 a century and a half later in 1573 another eruption produced the cone and crater called microkemeni or the small burnt island the next great event which we find recorded occurred in 1650 when a submarine outbreak violently agitated the sea at a point three and a half miles to the northeast of therea and which gave rise to a shoal carefully examined during the late survey in 1848 by captain graves and found to have 10 fathoms water over it the sea deepening around it in all directions this eruption lasted three months covering the sea with floating pumice at the same time an earthquake destroyed many houses in therea while the sea broke upon the coast and overthrew two churches exposing to view two villages one on each side of the mountain of st steven both of which must have been overwhelmed by showers of volcanic matter during some previous eruptions of unknown date the accompanying evolution of sulfur and hydrogen issuing from the sea killed more than 50 persons and above 1000 domestic animals a wave also 50 feet high broke upon the rocks of the isle of nea about four leagues distant and advanced 450 yards into the interior of the island of sequino lastly in 1707 and 1709 nea come any or the new burnt island was formed between the two others palaea and micro the old and little isles this isle was composed originally of two distinct parts the first which rose was called the white island composed of a mass of pumice extremely porous gorse the jesuit who was then in santoren says that the rock cut like bread and that when the inhabitants landed on it they found a multitude of full grown fresh oysters adhering to it which they ate this mass was afterwards covered in great part by the matter ejected from the crater of a twin island formed simultaneously and called black island consisting of brown trachite the trachitic lava which rose on this spot appears to have been a long time in an intumacent state for the new Kemeni was sometimes lowered on one side while it gained height on the other and rocks rose up in the sea at different distances from the shore and then disappeared again the eruption was renewed at intervals during the years 1711 and 1712 and at length a cone was piled up to the height of 330 feet above the level of the sea its exterior slope forming an angle of 33 degrees with the horizon and the crater on its summit being 80 yards in diameter in addition to the two points of sub aerial eruption on the new and little Kemenis two other cones indicating the sites of submarine outbursts of unknown date were discovered underwater near the Kemenis during the late survey in regard to the white island which was described and visited by Gorse in 1707 we are indebted to Mr. Edward Forbes for having in 1842 carefully investigated the layer of pymissius ash of which it is constituted he obtained from it many shells of marine genera pectunculus arca cardida trocas and others both univalve and bivalve all of recent Mediterranean species they were in a fine state of preservation the bivalves with the epidermis remaining and valves closed showing that they had been suddenly destroyed Mr. Forbes from his study of the habits of the moluska living at different depths in the Mediterranean was able to decide that such an assemblage of species could not have lived at a less depth than 220 feet so that a bodily upheaval of the mass to that amount must have taken place in order to bring up this bed of ashes and shells to the level of the sea and they now rise five or six feet above that level we may compare this partial elevation of solid matter to the rise of a hardened crust of scoriae such as is usually formed on the surface of lava currents even while they are in motion and which although stony and capable of supporting heavy weights may be upraised without bursting by the intumus sense of the melted matter below that the upheaval was merely local is proved by the fact that the neighboring Kemenis did not participate in the movement still less the three more distant or outer islands before mentioned the history therefore of the Kemenis shows that they have been the result of intermittent action and it lends no support to the hypothesis of the sudden distention of horizontal beds blown up like a bladder by a single paroxymal effort of expensive gases it will be seen by the accompanying map and sections that the Kemenis are arranged in a linear direction running northeast and southwest in a manner different from that represented in the older charts in their longest diameter they form at their base a ridge nearly bisecting the gulf or crater on considering these facts we are naturally led to compare the smaller and newer islands in the center of the gulf to the modern cone of Vesuvius surrounded by the older semicircular escarpment of Soma or to liken them to the peak of Tenerife before described as surrounded by its fossa and bastion this idea will appear to be still more fully confirmed when we study the soundings taken during the late hydrographical survey Thera which constitutes alone more than two-thirds of the outer circuit presents everywhere towards the gulf high and steep precipices composed of rocks of volcanic origin in all places near the base of its cliffs a depth of from 800 to 1000 feet of water was found and Lieutenant Leicester informs us that if the gulf which is six miles in diameter could be drained a bowl-shaped cavity would appear with walls 2449 feet high in some places and even on the southwest side where it is lowest nowhere less than 1200 feet high while the Kemenis would be seen to form in the center a huge mountain five and a half miles in circumference at its base with three principal summits the old the new and the little burnt islands rising severally to the heights of 1251 1629 and 1158 feet above the bottom of the abyss the rim of the great cauldron thus exposed would be observed to be in all parts perfect and unbroken except at one point where there is a deep and long chasm or channel known by mariners as the northern entrance between Thera and Therese and called by Lieutenant Leicester the door into the crater it is no less than 1170 feet deep and constitutes as will appear by the soundings a remarkable feature in the bed of the sea there is no corresponding channel passing out from the gulf into the Mediterranean at any other point in the circuit between the outer islands the greatest depth they are ranging from seven to 66 feet we may conceive therefore if at some former time the whole mass of Santoran stood at a higher level by 1200 feet that this single ravine or narrow valley now forming the northern entrance was the passage by which the sea entered a circular bay and swept out in the form of mud and pebbles the materials derived by denudation from wasting cliffs in this manner the original crater may have been slowly widened and deepened after which the whole archipelago may have been partially submerged to its present depth that such oscillations of level may in the course of ages have taken place will be the more readily admitted when we state that part of Thera has actually sunk down in modern times as for example during the great earthquake before alluded to which happened in 1650 the subsidence alluded to is proved not only by tradition but by the fact that a road which formerly led between two places on the east coast of Thera is now 12 fathoms underwater Missour's Bobley and Verlitt mentioned that the waves are constantly undermining and encroaching on the cliffs of Theraeja and Espronissi and shoals or submarine ledges were found during the late survey to occur around a great part of these islands attesting the recent progress of denudation Missour Verlitt also remarks in regard to the separation of the three islands forming the walls of the crater that the channels between them are all to the west and northwest the quarter most exposed to the waves and currents Mr. Darwin in his work on volcanic islands has shown that in the Maritius and in Santiago there is an external circle of basaltic rocks of vast diameter in the interior of which more modern eruptions have taken place the older rocks dipping away from the central space in every direction as in the outer islands of Santorin he refers the numerous breaches some of them very wide in the external ramparts of those islands to the denuding action of the sea every geologist therefore will be prepared to call in the aid of the same powerful cause to account for the removal of a large part of the rocks which must have once occupied the interior space in the same manner as they attribute the abstraction of matter from elliptical valleys of elevation such as those of Woolhope and the Weldon in England to the waves and currents of the sea Thera, Therese and Espronese are all composed of volcanic matter except the southern part of Thera where Mount Saint Elias rises to three times the height of the loftiest of the igneous rocks reaching an elevation of 1,887 feet above the sea this mountain is formed of granular limestone and argyllaceous schist and must have been originally a submarine eminence in the bed of the Mediterranean before the volcanic cone one side of the base of which now abuts against it was formed the inclination strike and fractures of the calcareous and agryllaceous strata of Saint Elias have no relation to the great cone but according to Mr. Borey Saint Vincent have the same direction as those of the other aisles of the Grecian archipelago namely from north northwest to south southeast each of the three islands Thera, Therese and Espronese is capped by an enormous mass of white two-facious conglomerate from 40 to 50 feet thick beneath which are beds of trachetic lava and tough having a gentle inclination of only three degrees or four degrees each bed is usually very narrow and discontinuous the successive layers being molded or dovetailed as Mr. Burlatt expresses it into the inequalities of the previously existing surface on which showers of cinders or streams of melted matter have been poured nothing therefore seems more evident than that we have in Santorin the basal remains of a great ruined cone or flattened dome and the absence of dikes in the cliffs surrounding the gulf would indicate that the eruptions took place originally as they have done in the last two thousand years not near the margin but in the center of the space now occupied by the gulf the central portions of the dome have since been removed by engulfment or denudation or by both these causes an important fact is adduced by Mr. Burlatt to show that the gentle dip of the lava streams in the three outer islands towards all points of the compass away from the center of the gulf has not been due to the upheaval of horizontal beds as conjectured by van Boet who had not visited Santorin the French geologist found that the vesicles or pores of the trachetic masses were lengthened out in the several directions in which they would have flowed if they had descended from the axis of a cone once occupying the center of the crater for it is well known that the bubbles of confined gas in a fluid in motion assume an oval form and the direction of their longer axis coincides always with that of the stream on a review therefore of all the facts now brought to light respecting Santorin I attribute the moderate slope of the beds in Thera and the other external islands to their having originally descended the inclined flanks of a large volcanic cone the principal orifice or vents of eruption having been always situated where they are now in or near the center of the space occupied by the gulf or crater in other words where the outburst of the Kemenis has been witnessed in historical times the single long and deep opening into the crater is a feature common to all those remnants of ancient volcanoes the central portions of which have been removed and is probably connected with aqueous denudation this denuding process has been the work of ages when the sea was admitted into an original crater and has taken place during the gradual emergence of the island from the sea or during various oscillations in its level the volcanic island of st paul in the midst of the indian ocean latitude 38 degrees 44 minutes south longitude 77 degrees 37 minutes east surveyed by captain blackwood in 1842 seems to exemplify the first stage in the formation of such an archipelago as that of santoren we have there a crater one mile in diameter surrounded by steep and lofty cliffs on every side save one where the sea enters by a single passage nearly dry at low water in the interior of the small circular bay or crater there is a depth of 30 fathoms or 180 feet the surface of the island slopes away on all sides from the crest of the rocks and circling the crater barren island there is great analogy between the structure of barren island in the bay of ben gall latitude 12 degrees 15 minutes and that of santoren last described when seen from the ocean this island presents on almost all sides a surface of bare rocks rising with a moderate aclivity towards the interior but at one point there is a cleft by which we can penetrate into the center and there discover that it is occupied by a great circular basin filled by the waters of the sea and bordered all around by steep rocks in the midst of which rises a volcanic cone very frequently in eruption the summit of this cone is about 500 feet in height corresponding to that of the circular border which encloses the basin so that it can be seen from the sea only through the ravine it is most probable that the exterior enclosure of barren island is nothing more than the remains of a truncated cone a great portion of which has been removed by engulfment explosion or denudation which may have preceded the formation of the new interior cone end of chapter 26 part 4 chapter 26 of principles of geology this is a LibriVox recording a LibriVox recordings are in the public domain for more information or to volunteer please visit LibriVox.org recording by Dion John's Salt Lake City Utah principles of geology by Charles Lyle chapter 26 part 5 mud volcanoes Iceland Mr. R. Bunsen in his account of the pseudo volcanic phenomena of Iceland describes many valleys where sulfuris and aqueous vapors burst forth with a hissing sound from the hot soil formed a volcanic tough in such spots a pool of boiling water is seen in which a bluish black argyllaceous paste rises in huge bubbles these bubbles on bursting throw the boiling mud to a height of 15 feet and upwards accumulating it in ledges around the crater or basin of the spring Baku on the Caspian the formation of a new mud volcano was witnessed on the 27th of November 1827 at Takmali on the peninsula of Absharan east of Baku flames blazed up to an extraordinary height for a space of three hours and continued for 20 hours to rise about three feet above a crater from which mud was ejected at another point in the same district where flames issued fragments of rock of large size were hurled up into the air and scattered around Sicily at a place called Makaluba near Giurgenti in Sicily are several conical mounds from 10 to 30 feet in height with small craters at their summits from which cold water mixed with mud and bitumen is cast out bubbles of carbonic acid and carbureted hydrogen gas are also disengaged from these springs and at certain periods with such violence as to throw the mud to the height of 200 feet these air volcanoes as they are sometimes termed are known to have been in the same state of activity for the last 15 centuries and Dr. Dobbeni imagines that the gases which escape may be generated by the slow combustion of beds of sulfur which is actually in progress in the blue clay out of which the springs rise but as the gases are similar to those disengaged in volcanic eruptions and as they have continued to stream out for so long a period they may perhaps be derived from a more deep-seated source Baila in India in the district of Luce or Lus south of Baila about 120 miles northwest of Kutch and the mouths of the Indus numerous mud volcanoes are scattered over an area of probably not less than 1000 square miles some of these have been well described by Captain Hart and subsequently by Captain Robertson who has paid a visit to that region and made sketches of them which he has kindly placed at my disposal from one of these the annexed view has been selected these conical hills occur to the westward of the hara mountains and the river hub one of the cones is 400 feet high composed of light colored earth and having at its summit a crater 30 yards in diameter the liquid mud which fills the crater is continually disturbed by air bubbles and here and there is cast up in small jets mineral composition of volcanic products the mineral called felspar forms in general more than half of the mass of modern lavas when it is in great excess lavas are called trachitic they consist generally of a base of compact felspar in which crystals of glassy felspar are disseminated when augite or pyroxene predominates lavas are termed basaltic they contain about 50 percent of silica or much less than the trachites in which there is usually about 75 percent of that mineral they also contain about 11 percent of protoxide of iron and as much of lime both of which are wanting or only in insignificant quantities in the trachitic rocks but lavas occur of an intermediate composition between the trachitic and basaltic which from their color have been called graystones the abundance of quartz forming distinct crystals or concretions characterizes the granitic and other ancient rocks now generally considered by geologists as of igneous origin whereas that mineral is rarely exhibited in a separate form in recent lavas although silica enters so largely into their composition horn blend so common in hypogean rocks or those commonly called primary is rare in modern lava nor does it enter largely into rocks of any age in which agate abounds it should however be stated that the experiments of Mr. Gustav Rose have made it very questionable whether the minerals called horn blend and agate can be separated as distinct species as their different varieties seem to pass into each other whether we consider the characters derived from their angles of crystallization their chemical composition or their specific gravity the difference in form of the two substances may be explained by the different circumstances under which they have been produced the form of horn blend being the result of slower cooling crystals of agate have been met with in the scoriae of furnaces but never those of horn blend and crystals of agate have been obtained by melting horn blend in a platina crucible but horn blend itself has not been formed artificially mica occurs plentifully in some recent trachytes but is rarely present where agate is in excess frequency of eruptions and nature of subterranean igneous rocks when we speak of the igneous rocks of our own times we mean that small portion which in violent eruptions is forced up by elastic fluids to the surface of the earth the sand, scoriae and lava which cool in the open air but we cannot obtain access to that which is congealed far beneath the surface under great pressure equal to that of many hundred or many thousand atmospheres during the last century about fifty eruptions are recorded of the five european volcanic districts of vesuvius, etna, volcano, centaurin and island but many beneath the sea in the grecian archipelago and near island may doubtless have passed unnoticed if some of them produced nalava others on the contrary like that of skeptar jokel in 1783 poured out melted matter for five or six years consecutively which cases being reckoned as single eruptions will compensate for those of inferior strength now if we consider the active volcanoes of europe to constitute about a 40th part of those already known on the globe and calculate that one with another they are about equal in activity to the burning mountains in other districts we may then compute that there happen on the earth about 2000 eruptions in the course of a century or about 20 every year however inconsiderable therefore may be the superficial rocks which the operations of fire produce on the surface we must suppose the subterranean changes now constantly in progress to be on the grandest scale the loftiest volcanic cones must be as insignificant when contrasted to the products of fire in the nether regions as are the deposits formed in shallow estuaries when compared to submarine formations accumulating in the abysses of the ocean in regard to the characters of these volcanic rocks formed in our own times in the bowels of the earth whether in rents and caverns or by the cooling of lakes of melted lava we may safely infer that the rocks are heavier and less porous than ordinary lavas and more crystalline although composed of the same mineral ingredients as the hardest crystals produced artificially in the laboratory require the longest time for their formation so we must suppose that where the cooling down of melted matter takes place by insensible degrees in the course of ages a variety of minerals will be produced far harder than any formed by natural processes within the short period of human observation these subterranean volcanic rocks moreover cannot be stratified in the same manner as sedimentary deposits from water although it is evident that when great masses consolidate from a state of fusion they may separate into natural divisions for this is seen to be the case in many lava currents we may also expect that the rocks in question will often be rent by earthquakes since these are common in volcanic regions and the fissures will be often injected with similar matter so that dikes of crystalline rock will traverse masses of similar composition it is also clear that no organic remains can be included in such masses as also that these deep-seated igneous formations considered in mass must underlie all the strata containing organic remains because the heat proceeds from below upwards and the intensity required to reduce the mineral ingredients to a fluid state must destroy all organic bodies in rocks included in the myths of them if by a continued series of elevatory movements such masses shall hereafter be brought up to the surface in the same manner as sedimentary marine strata have in the course of ages been upheaved to the summit of the loftiest mountains it is not difficult to foresee what perplexing problems may be presented to the geologist he may then perhaps study in some mountain chain the very rocks produced at the depth of several miles beneath the Andes Iceland or Java in the time of libnates and draw from them the same conclusion which that philosopher derived from certain igneous products of high antiquity for he conceived our globe to have been for an indefinite period in the state of a comet without an ocean and uninhabitable alike by aquatic or terrestrial animals end of chapter 26 part 5