 Chapter 6 Part 2 of Principles of Geology. In order to explain how so many animals can find support in this region, it is suggested that the underwood, of which their food chiefly consists, may contain much nutriment in a small bulk, and also that the vegetation has a rapid growth, for no sooner is a part consumed than its place, says Dr. Smith, is supplied by a fresh stock. Nevertheless, after making every allowance for this successive production and consumption, it is clear, from the facts above cited, that the quantity of food required by the larger herbivora is much less than we have usually imagined. Mr. Darwin conceives that the amount of vegetation supported at any one time by Great Britain may exceed, in a tenfold ratio, the quantity existing on an equal area in the interior parts of Southern Africa. It is remarked, moreover, in illustration of the small connection discoverable between abundance of food and the magnitude of indigenous mammalia, that while in the desert part of Southern Africa there are so many huge animals, in Brazil, where the splendor and exuberance of the vegetation are unrivaled, there is not a single wild quadruped of large size. It would doubtless be impossible for herds of mammoths and rhinoceroses to subsist at present throughout the year, even in the southern part of Siberia, covered as it is with snow during winter. But there is no difficulty in supposing a vegetation capable of nourishing these great quadrupeds to have once flourished between the latitudes forty degrees and sixty degrees north. Dr. Fleming has hinted that the kind of food which the existing species of elephant prefers will not enable us to determine, or even to offer a probable conjecture, concerning that of the extinct species. No one acquainted with the grominious character of the food of our fallow deer, stag, or roe would have assigned a lichen to the reindeer. Travelers mentioned that even now, when the climate of eastern Asia is so much colder than the same parallels of latitude farther west, there are woods not only of fur, but of birch, on the banks of the Lena, as far north is latitude sixty degrees. It has moreover been suggested, that as in our own times, the northern animal migrate, so the Siberian elephant and rhinoceros may have wandered towards the north in summer. The musk oxen annually desert their winter quarters in the south and cross the sea upon the ice to graze for four months, from May to September, on the rich pastureage of Melvin Island in latitude seventy-five degrees. The mammoths, without passing so far beyond the Arctic Circle, may nevertheless have made excursions, during the heat of a brief northern summer, from the central or temperate parts of Asia to the sixtieth parallel of latitude. Now, in this case, the preservation of their bones, or even occasionally of their entire carcasses in ice or frozen soil, may be accounted for, without resorting to speculations concerning certain revolutions in the former state and climate of the earth's surface. We are entitled to assume, that in the time of the extinct elephant and rhinoceros, the lowland of Siberia was less extensive towards the north than now, for we have seen, that the strata of this lowland, in which the fossil bones lie buried, were originally deposited beneath the sea. And we know, from the facts brought to light in Rangel's voyage, in the year 1821, 1822 and 1823, that a slow upheaval of the land along the borders of the ICC is now constantly taking place, similar to that experienced in part of Sweden. In the same manner, then, as the shores of the Gulf of Bosnia are extended, not only by the influx of sediment brought down by rivers, but also by the elevation and consequent drying up of the bed of the sea, so a like combination of causes may, in modern times, have been extending the low tract of land where marine shells and fossil bones occur in Siberia. Such a change in the physical geography of that region, implying a constant augmentation in the quantity of Arctic land, would, according to principles to be explained in the next chapter, tend to increase the severity of the winters. We may conclude, therefore, that before the land reached so far to the north, the temperature of the Siberian winter and summer was more nearly equalized, and a greater degree of winters cold may, even more than a general diminution of the mean annual temperature, have finally contributed to the extermination of the mammoth and its contemporaries. On referring to the map, the reader will see how all the great rivers of Siberia flow at present from south to north, from temperate to arctic regions, and they are all liable, like the Mackenzie in North America, to remarkable floods, in consequence of flowing in this direction. Where they are filled with running water in their upper or southern chorus, when completely frozen over for several hundred miles near their mouths, where they remain blocked up by ice for six months in every year. The descending waters, therefore, finding no open channel, rush over the ice, often changing their direction, and sweeping along forests and prodigious quantities of soil and gravel mixed with ice. Now the rivers of Siberia are among the largest in the world, and Yenisei having a course of 2,500, the Lena of 2,000 miles, so that we may easily conceive that the bodies of animals which fall into their waters may be transported to vast distances towards the arctic sea, and before arriving there, may be stranded upon and often frozen into thick ice. Afterwards, when the ice breaks up, they may be floated still farther towards the ocean, until at length they become buried in fluvitile and submarine deposits near the mouths of rivers. Humboldt remarks that near the mouths of the Lena a considerable thickness of frozen soil may be found at all seasons at the depth of a few feet, so that if a carcass be once embedded in mud and ice in such a region and in such a climate, its putrefaction may be arrested for indefinite ages. According to Professor von Beyer of St. Petersburg, the ground is now frozen permanently to the depth of 400 feet at the town of Jakutsk, on the western bank of the Lena, in latitude 62 degrees north, 600 miles distant from the polar sea. Mr. Haydenstrom tells us that throughout a wide area in Siberia, the boundary cliffs of the lakes and rivers consist of alternate layers of earthy materials and ice in horizontal stratification. And Mr. Mittendorf informed us in 1846 that in his tour there three years before, he had bored in Siberia to the depth of 70 feet, and after passing through much frozen soil mixed with ice, had come down upon a solid mass of pure, transparent ice, the thickness of which, after penetrating two or three yards, they did not ascertain. We may conceive, therefore, that even at the period of the mammoth, when the low land of Siberia was less extensive towards the north, and consequently the climate more temperate than now, the cold may still have been sufficiently intense to cause the rivers flowing in their present direction, to sweep down from south to north the bodies of drowned animals, and there bury them in drift ice and frozen mud. If it be true that the carcass of the mammoth was embedded in pure ice, there are two ways in which it may have been frozen in. We may suppose the animal to have been overwhelmed by drift snow. I have been informed by Dr. Richardson that, in the northern parts of America, comprising regions now inhabited by many herbivorous quadrupeds, the drift snow is often converted into permanent glaciers. It is commonly blown over the edges of steep cliffs, so as to form an inclined talus hundreds of feet high, and when a thaw commences, torrents rush from the land and throw down from the top of the cliff a louvial soil and gravel. This new soil soon becomes covered with vegetation and protects the foundation of snow from the rays of the sun. Water occasionally penetrates into the crevices and pores of the snow, but, as it soon freezes again, it serves the more rapidly to consolidate the mass into a compact iceberg. It may sometimes happen that cattle grazing in a valley at the base of such cliffs on the borders of a sea or river may be overwhelmed, and at length enclosed in solid ice, and then transported towards the polar regions. Or a herd of mammoths returning from their summer pastures in the north may have been surprised while crossing a stream by the sudden conglulation of the waters. The missionary Huck relates in his travels in Tibet in 1846 that, after many of his party had been frozen to death, they pitched their tents on the banks of the Mourois-ousson, which lower down becomes the famous Blue River, and saw from their encampment some black shapeless objects ranged in file across the stream. As they advanced nearer, no change either in form or distinctness was apparent, nor was it till they were quite close that they recognized in them a troop of the wild oxen called Yak by the Tibetans. There were more than fifty of them encrusted in the ice. No doubt they had tried to swim across at a moment of conglulation and had been unable to disengage themselves. Their beautiful heads, surmounted by huge horns, were still above the surface, but their bodies were held fast in the ice, which was so transparent that the position of the imprudent beasts was easily distinguishable. They looked as if still swimming, but the eagles and ravens had pecked out their eyes. The foregoing investigations, therefore, lead us to infer that the mammoth and some other extinct quadrupeds fitted to live in high latitudes, were inhabitants of northern Asia at a time when the geographical conditions and climate of that continent were different from the present. But the age of dysphona was comparatively modern in the earth's history. It appears that when the oldest or Eocene tertiary deposits were formed, a warm temperature pervaded the European seas and lands. Shells of the genus Nautilus and other forms characteristic of tropical latitudes, fossil reptiles such as the crocodile, turtle and tortoise, plants such as palms, some of them allied to the cocoonut, to screw pine, the custard apple and the acacia, all lead to this conclusion. This flora and fauna were followed by those of the myocene formation, in which indications of a southern but less tropical climate are detected. Finally the Pliocene deposits, which come next in succession, exhibit in their organic remains a much nearer approach to the state of things now prevailing in corresponding latitudes. It was towards the close of this period that the seas of the northern hemisphere became more and more filled with floating icebergs, often charged with erratic blocks, so that the waters and the atmosphere were chilled by the melting ice, and an arctic fauna enabled for a time to invade the temperate latitudes both of North America and Europe. The extinction of a considerable number of land quadrupedes and aquatic molusca was gradually brought about by the increasing severity of the cold, but many species survived this revolution in climate, either by their capacity of living under a variety of conditions or by migrating for a time to more southern lands and seas. At length, by modifications in the physical geography of the northern regions and the cessation of floating ice on the eastern side of the Atlantic, the cold was moderated and a milder climate ensued, such as we now enjoy in Europe. A great interval of time appears to have elapsed between the formation of the secondary strata, which constitute the principal portion of the elevated land in Europe and the origin of the Eocene deposits. If we examine the rocks from the chalk to the new red sandstone inclusive, we find many distinct assemblages of fossils entombed in them, all of unknown species. And many of them referable to genera and families now most abundant between the tropics. Among the most remarkable are reptiles of gigantic size, some of them herbivorous, others carnivorous and far exceeding in size any now known even in the torrid zone. The genera are for the most part extinct, but some of them are for the most part extinct, but some of them, as the crocodile and monitor, have still representatives in the warmer parts of the earth. Coral reefs also were evidently numerous in the seas of the same periods, composed of species often belonging to genera now characteristics of a tropical climate. The number of large chambered shells also, including the nautilus, leads us to infer an elevated temperature, and the associated fossil plants, although imperfectly known, tend to the same conclusion, the psychardiae constituting the most numerous family. But it is from the more ancient coal deposits that the most extraordinary evidence has been supplied in proof of the former existence of a very different climate, a climate which seems to have been moist, warm, and extremely uniform, in those very latitudes which are now the colder, and in regard to temperature, the most variable regions of the globe. We learn from the researchers of Adolf Brognard, Göppert and other botanists, that in the flora of the carboniferous era there was a great predominance of ferns, some of which were arborescent, as for example colopteris, protopteris, and seronius, nor can this be accounted for, as some have supposed, by the greater power which ferns possess of resisting maceration in water. This prevalence of ferns indicates a moist, equitable, and temperate climate, and the absence of any severe cold, for such are the conditions which, at the present day, are found to be most favorable to that tribe of plants. It is only in the islands of the tropical oceans, and of the southern temperate zones such as Norfolk Island, Ota Hete, the Sandwich Islands, Tristan da Cunha, and New Zealand that we find any near approach to that remarkable preponderance of ferns which is characteristic of the carboniferous flora. It has been observed that tree ferns and other forms of vegetation, which flourished most luxuriously within the tropics, extend to a much greater distance from the equator in the southern hemisphere than in the northern, being found even as far as 46 degrees south latitude in New Zealand. There is little doubt that this is owing to the more uniform and moist climate occasioned by the greater proportional area of sea. Next to ferns and pines, the most abundant vegetable forms in the coal formation are the calomites, Lepidodendra, Sigillariae, and Stigmariae. These were formerly considered to be so closely allied to tropical genera, and to be so much greater in size than the corresponding tribes now inhabiting equatorial latitudes that they were thought to imply an extremely hot, as well as humid and equitable climate. But recent discoveries respecting the structure and relations of these fossil plants have shown that they deviated so widely from all existing types in the vegetable world that we have more reason to infer from this evidence a widely different climate in the Carboniferous era, as compared to that now prevailing, than a temperature extremely elevated. Palms, if not entirely wanting when the strata of the Carboniferous group were deposited, appear to have been exceedingly rare. The coniferae, on the other hand, so abundantly met within the coal, resemble Arocariae in structure, a family of the fur tribe, characteristic at present of the milder regions of the southern hemisphere, such as Chile, Brazil, New Holland, and Norfolk Island. In regard to the geographical extent of the ancient vegetation, it was not confined, C'est Monsieur Bronniard, to a small space, as to Europe, for example, for the same forms are met with again at great distances. Thus the coal plants of North America are, for the most part, identical with those of Europe, and all belong to the same genera. Some specimens also from Greenland are referable to ferns, analogous to those of our European coal mines. The fossil plants brought from Melville Island, although in a very imperfect state, have been supposed to warrant similar conclusions, and assuming that they agree with those of Baffin's Bay, mentioned by Monsieur Bronniard, how shall we explain the manner in which such a vegetation lived through an arctic night of several months duration? It may seem premature to discuss this question until the true nature of the fossil flora of the arctic regions has been more accurately determined. Yet, as the question has attracted some attention, let us assume for a moment that the coal plants of Melville Island are strictly analogous to those of the strata of Northumberland. Would such a fact present an inexplicable enigma to the vegetable physiologist? Plants, it is affirmed, cannot remain in darkness, even for a week, without serious injury, unless in a torpid state, and if exposed to heat and moisture, they cannot remain torpid, but will grow, and must therefore perish. If then, in the latitude of Melville Island, seventy-five degrees north, a high temperature and consequent humidity prevailed at that period when we know the arctic seas were filled with corals and large multi-locular shells, how could plants of tropical forms have flourished? Is not the bright light of equatorial regions as indispensable a condition of their well-being as the sultry heat of the same countries? And how could they annually endure a night prolonged for three months? Now, in reply to this objection, we must bear in mind in the first place that, so far as experiments have been made, there is every reason to conclude that the range of intensity of light to which living plants can accommodate themselves is far wider than that of heat. No palms or tree ferns can live in our temperate latitudes without protection from the cold, but when placed in hot houses, they grow luxuriously, even under a cloudy sky, and where much light is intercepted by the glass and framework. At St. Petersburg, in latitude sixty degrees north, these plants have been successfully cultivated in hot houses, although there they must exchange the perpetual equinox of their native regions for days and nights which are alternately protracted to nineteen hours and shortened to five. How much farther towards the pole they might continue to live, provided a due quantity of heat and moisture was applied, has not yet been determined, but St. Petersburg is probably not the utmost limit, and we should expect that in latitude sixty-five degrees at least, where they would never remain twenty-four hours without enjoying the sun's light, they might still exist. It should also be borne in mind, in regard to tree ferns, that they grow in the gloomiest and darkest parts of the forest of warm and temperate regions, even extending to nearly the forty-sixth degree of south latitude in New Zealand. In equatorial countries, says Humboldt, they abound chiefly in the temperate, humid and shady parts of mountains. As we know therefore, that elevation often compensates for the effect of latitude in the geographical distribution of plants, we may easily understand that a class of vegetables, which grows at a certain height in the torrid zone, would flourish on the plains at greater distances from the equator, if the temperature, moisture and other necessary conditions were equally uniform throughout the year. Nor must we forget that in all the examples above alluded to, we have been speaking of living species, but the coal plants were of perfectly distinct species. Nay, few of them except the ferns and pines can be referred to genera or even families of the existing vegetable kingdom. Having a structure therefore, and often a form which appears to the botanist so anomalous, they may also have been endowed with a different constitution, enabling them to bear a greater variation of circumstances in regard to light. We find that particular species of plants and tree ferns require at present different degrees of heat, and that some species can thrive only in the immediate neighbourhood of the equator, others only a distance from it. In the same manner, the minimum of light, sufficient for the now existing species, cannot be taken as the standard for all analogous tribes that may ever have flourished on the globe. But granting that the extreme northern point to which a flora like that of the Carboniferous Era could ever reach, may be somewhere between the latitudes of 65 degrees and 70 degrees, we should still have to inquire whether the vegetable remains might not have been drifted from thence, by rivers and currents, to the parallel of Melville Island was still farther. In the northern hemisphere at present we see that the materials for future beds of lignite and coal are becoming amassed in high latitudes, far from the districts where the forests grew, and on shores where scarcely a stunted shrub can now exist. The Mackenzie and other rivers of North America carry pines with their roots attached for many hundred miles towards the north into the Arctic sea where they are embedded in deltas, and some of them drifted still farther by currents towards the pole. Before we can decide on this question of transportation we must know whether the fossil coal plants occurring in high latitudes bear the marks of friction and of having decayed previously to fossilization. Many appearances in our English coal fields certainly prove that the plants were not floated from great distances, for the outline of the stems of succulent species preserve their sharp angles, and others have their surfaces marked with the most delicate lines and streaks. Long leaves also are attached in many instances to the trunks or branches, and leaves we know in general are soon destroyed when steeped in water, although ferns will retain their forms after an immersion of many months. It seems fair to presume that most of the coal plants grew upon the same land which supplied materials for the sandstones and conglomerates of the strata in which they are embedded. The coarseness of the particles of many of these rocks attest that they were not born from very remote localities, and there was land therefore in the vicinity wasting away by the action of moving waters. The progress also of modern discovery has led to the very general admission of the doctrine that beds of coal have, for the most part, been formed of the remains of trees and plants that grew on the spot where the coal now exists, the land having been successively submerged so that a covering of mud and sand was deposited upon accumulations of vegetable matter. That such has been the origin of some coal seams is proved by the upright position of fossil trees, both in Europe and America, in which the roots terminate downwards in beds of coal. To return therefore from this digression the flora of the coal appears to indicate a uniform and mild temperature in the air, while the fossils of the contemporaneous mountain limestone, comprising abundance of lamelliferous corals, large chambered cephalopods and crinoidea, naturally lead us to infer a considerable warmth in the waters of the northern sea of the Carboniferous period. So also in regard to strata older than the coal, they contain in high northern latitudes mountain masses of corals which must have lived and grown on the spot, and large chambered uni-valves, such as orthocerata and nautilus, all seeming to indicate, even in regions bordering on the arctic circle, the former prevalence of a temperature more elevated than that now prevailing. The warmth and humidity of the air and the uniformity of climate, both in the different seasons of the year and in different latitudes, appears to have been most remarkable when some of the oldest of the fossiliferous strata were formed. The approximation to a climate similar to that now enjoyed in these latitudes does not commence till the era of the formations termed tertiary, and while the different tertiary rocks were deposited in succession, from the eocene to the pyocene, the temperature seems to have been lowered and to have continued to diminish, even after the appearance upon the earth of a considerable number of the existing species. The cold reaching its maximum of intensity in European latitudes during the glacial epoch, or the epoch immediately antecedent to that in which all the species now contemporary with man were in being. Principles of Geology by Charles Lyall Father examination of the question as to the assumed discordance of the ancient and modern causes of change. On the causes of vicissitudes in climate, remarks on the present diffusion of heat over the globe. On the dependence of the mean temperature on the relative position of land and sea, isothermal lines, currents from equatorial regions, drifting of icebergs, different temperature of northern and southern hemispheres, combination of causes which might produce the extreme cold of which the earth's surface is susceptible, conditions necessary for the production of the extreme of heat and its probable effects on organic life. Causes of vicissitudes in climate As the proofs enumerated in the last chapter indicate that the earth's surface has experienced great changes of climate since the deposition of the older sedimentary strata, we have next to inquire how such vicissitudes can be reconciled with the existing order of nature. The cosmogenist has availed himself of this, as of every obscure problem in geology, to confirm his views concerning a period when the planet was in a nascent or half-formed state, or when the laws of the animate and inanimate world differed essentially from those now established, and he has in this, as in many other cases, succeeded so far as to divert attention from that class of facts which, if fully understood, might probably lead to an explanation of the phenomena. At first it was imagined that the earth's axis had been for ages perpendicular to the plane of the ecliptic, so that there was a perpetual equinox and uniformity of seasons throughout the year. That the planet enjoyed this paradisiacal state until the era of the Great Flood, but in that catastrophe, whether by the shock of a comet or some other convulsion, it lost its equal poise, and hence the obliquity of its axis, and with that the varied seasons of the temperate zone and the long nights and days of the polar circles. When the progress of astronomical science had exploded this theory, it was assumed that the earth at its creation was in a state of fluidity and red-hot, and that ever since that era it had been cooling down, contracting its dimensions and acquiring a solid crust, a hypothesis hardly less arbitrary yet more calculated for lasting popularity, because by referring the mind directly to the beginning of things it requires no support from observation nor from any ulterior hypothesis. But if, instead of forming vague conjectures as to what might have been the state of the planet at the era of its creation, we fix our thoughts on the connection at present existing between climate and the distribution of land and sea, and then consider what influence former fluctuations in the physical geography of the earth must have had on superficial temperature, we may perhaps approximate to a true theory. If doubts and obscurities still remain, they should be ascribed to our limited acquaintance with the laws of nature, not to revolutions in her economy. They should stimulate us to further research, not tempt us to indulge our fancies respecting the imaginary changes of internal temperature in an embryo world. Diffusion of Heat over the Globe In considering the laws which regulate the diffusion of heat over the globe, we must be careful, as Humboldt well remarks, not to regard the climate of Europe as the type of the temperature which all countries placed under the same latitude enjoy. The physical sciences observes this philosopher always bear the impress of the places where they began to be cultivated, and as, in geology, an attempt was at first made to refer all the volcanic phenomena to those of the volcanoes in Italy, so in meteorology, a small part of the Old World, the centre of the primitive civilisation of Europe, was for a long time considered a type to which the climate of all corresponding latitudes might be referred. But this region, constituting only one seventh of the whole globe, proved eventually to be the exception to the general rule. For the same reason, we may warn the geologist to be on his guard, and not hastily to assume that the temperature of the earth in the present era is a type of that which most usually obtains, since he contemplates far mightier alterations in the positions of land and sea at different epochs than those which now cause the climate of Europe to defer from that of other countries in the same parallels. It is now well ascertained that zones of equal warmth, both in the atmosphere and in the waters of the ocean, are neither parallel to the equator, nor to each other. It is also known that the mean annual temperature may be the same in two places which enjoy very different climates. For the seasons may be nearly uniform or violently contrasted, so that the lines of equal winter temperature do not coincide with those of equal annual heat or isothermal lines. The deviations of all these lines from the same parallel of latitude are determined by a multitude of circumstances, among the principle of which are the position, direction, and elevation of the continents and islands, the position and depths of the sea, and the direction of currents and of winds. On comparing the two continents of Europe and America, it is found that places in the same latitudes have sometimes a mean difference of temperature amounting to 7 degrees, or even in a few cases to 17 degrees Fahrenheit, and some places on the two continents which have the same mean temperature differ from 7 degrees to 17 degrees in latitude. Thus Cumberland House in North America having the same latitude 54 degrees north as the city of York in England stands on the isothermal line of 32 degrees which in Europe rises to the North Cape in latitude 71 degrees, but its summer heat exceeds that of Brussels or Paris. The principle cause of greater intensity of cold in corresponding latitudes of North America as contrasted with Europe is the connection of America with the polar circle by a large tract of land, some of which is from 3 to 5,000 feet in height and on the other hand the separation of Europe from the Arctic Circle by an ocean. The ocean has a tendency to preserve everywhere a mean temperature which it communicates to the contiguous land so that it tempers the climate moderating alike an excess of heat or cold. The elevated land on the other hand rising to the colder regions of the atmosphere becomes a great reservoir of ice and snow, arrests condenses and congeals vapor and communicates its cold to the adjoining country. For this reason Greenland forming part of a continent which stretches northward to the 82nd degree of latitude experiences under the 60th parallel a more rigorous climate than Lapland under the 72nd parallel. But if land be situated between the 40th parallel anti equator it produces unless it be of extreme height exactly the opposite effect for it then warms the tracts of land or sea that intervene between it and the polar circle. For the surface being in this case exposed to the vertical or nearly vertical rays of the sun absorbs a large quantity of heat which it diffuses by radiation into the atmosphere. For this reason the western parts of the old continent derive warmth from Africa which like an immense furnace distributes its heat to Arabia to Turkey in Asia and to Europe. On the contrary the northeastern extremity of Asia experiences in the same latitude extreme cold for it has land on the north between the 60th and 70th parallel while to the south it is separated from the equator by the Pacific ocean. In consequence of the more equal temperature of the waters of the ocean the climate of islands and of coasts differs essentially from that of the interior of continents. The more maritime climate being characterized by mild winters and more temperate summers for the sea breezes moderate the cold of winter as well as the heat of summer. When therefore we trace round the globe those in which the mean annual temperature is the same we often find great differences in climate for there are insular climates in which the seasons are nearly equalized and excessive climates as they have been termed where the temperature of winter and summer is strongly contrasted. The whole of Europe compared with the eastern parts of America and Asia has an insular climate. The northern part of China and the Atlantic region of the United States exhibit excessive climates. We find it New York says Humbord the summer of Rome and the winter of Copenhagen at Quebec the summer of Paris and the winter of Petersburg at Beijing in China where the mean temperature of the year is that of the coasts of Brittany the scorching heats of summer are greater than at Cairo and the winters as rigorous Uppsala. If lines be drawn round the globe through all those places which have the same winter temperature they are found to deviate from the terrestrial parallels much farther than the lines of equal mean annual heat. The lines of equal winter in Europe for example are often curved so as to reach parallels of latitude 9 or 10 degrees distant from each other whereas the isothermal lines or those passing through places having the same mean annual temperature differ only from 4 to 5 degrees in Europe. Influence of currents and drift ice on temperature. Among other influential causes both of remarkable diversity in the mean annual heat and of unequal division of heat in the different seasons are the direction of currents and the accumulation and drifting of ice in high latitudes. The temperature of the Lagoula's current is 10 degrees or 12 degrees Fahrenheit above that of the sea at the Cape of Good Hope for it derives the greater part of its waters from the Mozambique Channel and southeast coast of Africa and from regions in the Indian Ocean much nearer the line and much hotter than the Cape. An opposite effect is produced by the equatorial current which crosses the Atlantic from Africa to Brazil having a breadth varying from 160 to 450 nautical miles. Its waters are cooler by 3 degrees to 4 degrees Fahrenheit than those of the ocean under the line so that it moderates the heat of the tropics. But the effects of the Gulf stream on the climate of the North Atlantic Ocean are far more remarkable. The most powerful of known currents has its source in the Gulf for a sea of Mexico which like the Mediterranean and other close seas in temperate or low latitudes is warmer than the open ocean in the same parallels. The temperature of the Mexican sea in summer is, according to Rennell, 86 degrees Fahrenheit or at least 7 degrees above that of the Atlantic in the same latitude. In a great reservoir or a caldron of warm water a constant current pours forth through the Straits of Bahama at the rate of 3 or 4 miles an hour. It crosses the ocean in a northeasterly direction, skirting the Great Bank of New Foundland where it still retains a temperature of 8 degrees above that of the surrounding sea. It reaches the Azores in about 78 days after flowing nearly 3000 graphical miles and from thence it sometimes extends its course 1000 miles farther so as to reach the Bay of Biscay still retaining an excess of 5 degrees above the mean temperature of that sea. As it has been known to arrive there in the months of November and January it may tend greatly to moderate the cold of winter in countries on the west of Europe. There is a large tract in the center of the North Atlantic between the parallels of 33 degrees and 45 degrees north in latitude which Rennell calls the Recipient of the Gulf Water. A great part of it is covered by the weed called Sargassum Bacchiferum which the current floats in abundance from the Gulf of Mexico. This mass of water is nearly stagnant, is warmer by 7 or 10 degrees than the waters of the Atlantic and may be compared to the fresh water of a river overflowing the heavier salt water of the sea. Rennell estimates the area of the Recipient together with that covered by the main current as being 2000 miles in length from east to west and 350 in breadth from north to south which he remarks is a larger area than that of the Mediterranean. The heat of this great body of water is stepped up by the incessant and quick arrivals of fresh supplies of warm water from the south and there can be no doubt that the general climate of parts of Europe and America is materially affected by this cause. It is considered probable by Scorspe that the influence of the Gulf stream extends even to the sea near Spitzbergen where its waters may pass under those of melted ice for it has been found that in the neighborhood of Spitzbergen the water is warmer by 6 or 7 degrees at the depth of 100 and 200 fathoms than at the surface. This might arise from the known law that fresh water passes the point of greatest density when cooled down below 40 degrees and between that and the freezing point expands again. The water of melted ice might be lighter both as being fresh having lost its salt in the decomposing process of freezing and because its temperature is nearer the freezing point than the interior water of the Gulf stream. The great glaciers generated in the valleys of Spitzbergen in the 79th degree of north latitude are almost all cut off at the beach being melted by the feeble remnant of heat still retained by the Gulf stream. In Baffins Bay on the contrary on the west coast of old Greenland where the temperature of the sea is not mitigated by the same cause and where there is no warmer undercurrent the glaciers stretch out from the shore and furnish repeated crops of mountainous masses of ice which float off into the ocean. The number and dimensions of these Burgesses prodigies. Captain Sir John Ross saw several of them together in Baffins Bay a ground in water 1500 feet deep. Many of them are driven down to Hudson's Bay and accumulating their diffuse excessive cold over the neighbouring continent so that Captain Franklin reports that at the mouth of Hayes River which lies in the same latitude as the north of Prussia or the south of Scotland Isis found everywhere in digging wells in summer at the depth of four feet. Other Burgesses have been occasionally met with at midsummer in a state of rapid thaw as far south is latitude 40 degrees and longitude about 60 degrees west where they cool the water sensibly to the distance of 40 or 50 miles around. The thermometer is sinking sometimes 17 degrees or even 18 degrees Fahrenheit in their neighbourhood. It is a well known fact that every four or five years a large number of icebergs floating from Greenland double Cape Langanness and are stranded on the west coast of Iceland. The inhabitants are then aware that their crops of hay will fail in consequence of fogs which are generated almost incessantly and the depth of food is not confined to the land for the temperature of the water is so changed that the fish entirely desert the coast. Difference of climate of the northern and southern hemispheres. When we compare the climate of the northern and southern hemispheres we obtain still more instruction in regards to the influence of the distribution of land and sea on climate. The dry land in the southern hemisphere is to that of the northern in the ratio only of 1 to 3 excluding from our consideration that part which lies between the pole and the 78th degree of south latitude which has hitherto proved inaccessible. And whereas in the northern hemisphere between the pole and the 30th parallel of north latitude the land and sea occupy nearly equal areas the ocean in the southern hemisphere covers no less than 15 parts in 16 of the entire space included between the Antarctic circle and the 30th parallel of south latitude. This great extent of sea gives a particular character to climate south of the equator, the winters being mild and the summers cool. Thus in Van Diemen's land corresponding nearly in latitude to Rome the winters are more mild than at Naples and the summer is not warmer than those at Paris which is 7 degrees farther from the equator. The effects on animal and vegetable life are remarkable. Captain King observed large shrubs of Fuchsia and Veronica which in England are treated as tender plants thriving and in full flower in Tierra del Fuego with the temperature at 36 degrees. He states also that hummingbirds were seen sipping the sweets of the flowers after two or three days of constant rain, snow and sleet during which time the thermometer had been at the freezing point. Mr Darwin also saw parrots feeding on the seeds of a tree called the winter spark south of latitude 55 degrees near Cape Horn. So the orchidious plants which are parasitical on trees and are generally characteristic of the tropics advance to the 38th and 42nd degree of southern latitude and even beyond the 45th degree in New Zealand where they were found by forester. In South America also arborescent grasses abound in the dense forests of Chiloé in latitude 42 degrees south where they entwined the trees into one entangled mass to the height of 30 or 40 feet above the ground. Palm trees in the same quarter of the globe grow in latitude 37 degrees and arborescent grass very like a bamboo in 40 degrees and other closely allied kind of great length but not erect even as far south as 45 degrees. It has long been supposed that the general temperature of the southern hemisphere was considerably lower than that of the northern and that the difference amounted to at least 10 degrees fahrenheit. Baron Humboldt after collecting and comparing a great number of observations came to the conclusion that even a much larger difference existed but that none was to be observed within the tropics and only a small difference as far as the 35th and 40th parallel. Captain Cook was of opinion that the ice of the Antarctic predominated greatly over that of the Arctic region that encircling the southern pole coming nearer to the equator by 10 degrees than the ice around the North Pole. All the recent voyages of discovery have tended to confirm this opinion although Captain Weddell penetrated in 1823, 3 degrees farther south than Captain Cook reaching latitude 74 degrees 15 minutes south, longitude 34 degrees 17 minutes west and Sir James Ross in 1842 arrived at latitude 78 degrees 10 minutes south as higher latitude within 3 degrees as the farthest point attained by Captain Perry in the Arctic Circle or latitude 81 degrees 12 minutes north. The description given by ancient as well as modern navigators of the sea and land in high southern latitudes clearly attests the greater severity of the climate as compared to Arctic regions. In Sandwich land in latitude 59 degrees south where in nearly the same parallel as the north of Scotland Captain Cook found the whole country from the summits of the mountains down to the very brink of the sea cliffs covered many fathoms thick with everlasting snow and this on the 1st of February the hottest time of the year and what is still more astonishing in the island of south Georgia which is in the 54th degree south latitude in parallel as Yorkshire the line of perpetual snow descends to the level of the ocean. When we consider this fact and then recollect that the highest mountains in Scotland which ascend to an elevation of nearly 5000 feet and are 4 degrees farther to the north do not attain the limit of perpetual snow on our side of the equator we learn that latitude is one only of many powerful causes which determine the climate of particular regions of the globe Captain Sir James Ross in his exploring expedition in 1841 to 3 found that the temperatures south of the 60th degree of latitude sell them rows above 32 degrees Fahrenheit. During the two summer months of the year 1841, January and February the range of the thermometer was between 11 degrees to 32 degrees Fahrenheit and scarcely once rose above the freezing point. The permanence of snow in the southern hemisphere is in this instance partly due to the floating ice which chills the atmosphere and condenses the vapor so that in summer the sun cannot pierce through the foggy air but besides the abundance of ice which covers the sea to the south of Georgia and Sandwich land we may also as Humboldt suggests ascribe the cold of those countries in part to the absence of land between them and the tropics if Africa and New Holland extended farther to the south a diminution of ice would take place in consequence of the radiation of heat from these continents during summer which would warm the contiguous sea and rarify the air the heated aerial currents would then ascend and flow rapidly towards the south pole and moderate the winter in confirmation of these views it is stated that the ice which extends as far as the 68th degree and 71st degree of south latitude advances more towards the equator whenever it meets an open sea that is where the extremities of the present continents are not opposite to it and this circumstance seems explicable only the principle above alluded to of the radiation of heat from the lands so situated the cold of the Antarctic regions was conjectured by Cook to be due to the existence of a large tract of land between the 70th degree of south latitude and the pole the justness of these and other speculations of that great navigator have since been singularly confirmed by the investigation made by James Ross in 1841 he found Victoria land extending from 71 degrees to 79 degrees south latitude skirted by a great barrier of ice the height of the land ranging from 4,000 to 14,000 feet the whole entirely covered with snow except a narrow ring of black earth surrounding the huge crater of the active volcano of Mount Erebus rising 12,400 feet above the level of the sea the position of a mountainous territory of such altitude so near the pole and so obvious a source of intense cold fully explains why Graham's and Enderby's land discovered by Captain Biscoe in 1831 to 2 between latitude 64 degrees and 68 degrees south presented a most wintry aspect covered even in summer with ice and snow and nearly destitute of animal life in corresponding latitudes of the northern hemisphere we not only meet with herds of wild herbivorous animals but with land which man himself inhabits and where he has even built ports and inland villages the distance to which icebergs float from the polar regions on the opposite side of the line is as might have been anticipated different their extreme limit in the northern hemisphere is latitude 40 degrees as before mentioned and they are occasionally seen in latitude 42 degrees north near the termination of the great bank of Newfoundland and at the Azores latitude 42 degrees north to which they are sometimes drifted from Baffins Bay but in the other hemisphere they have been seen within the last few years with two points of the Cape of Good Hope between latitudes 36 degrees and 39 degrees one of these was two miles in circumference and 150 feet high appearing like chalk when the sun was obscured and having the luster of refined sugar when the sun was shining on it others rose from 250 to 300 feet above the level of the sea and were therefore of great volume below since it is ascertained by experiments on the buoyancy of ice floating in seawater that for every cubic foot seen above there must at least be eight cubic feet below water if ice islands from the north polar regions floated as far they might reach Cape St Vincent and there being drawn by the current that always sets in from the Atlantic through the Straits of Gibraltar be drifted into the Mediterranean but the serene sky of that delightful region might soon be deformed by clouds and mist before the amount of difference between the temperature of the two hemispheres was ascertained it was referred by many astronomers to the precession of the equinoxes or the acceleration of the earth's motion in its perihelium in consequence of which the spring and summer of the southern hemisphere are now shorter by nearly eight days than those in the seasons north of the equator but Sir J Herschel reminds us that the excess of eight days in the duration of the sun's presence in the northern hemisphere is not productive of an excess of annual light and heat since according to the laws of elliptic motion it is demonstrable that whatever be the ellipticity of the earth's orbit the two hemispheres must receive equal absolute quantities of light per annum the proximity of the sun in period G exactly compensating the effect of its swift emotion Humboldt however observes that there must be a greater loss of heat by radiation in the southern hemisphere during a winter longer by eight days than that on the other side of the equator perhaps no very sensible effect may be produced by this source of disturbance yet the geologist should bear in mind that to a certain extent it operates alternately on each of the two hemispheres for a period of upwards of 10,000 years dividing unequally the times during which the annual supply of solar light and heat is received this cause may sometimes tend to counterbalance inequalities of temperature resulting from other far more influential circumstances but on the other hand it must sometimes tend to increase the extreme of deviation arising from particular combinations of causes but whatever may be at present the inferiority of heat in the temperate and frigid zones south of the line it is quite evident that the cold would be far more intense if there happened instead of open sea to be tracts of elevated land between the 55th and 70th parallel and on the other hand the cold would be moderated if there were more land between the line and the 45th degree of south latitude end of chapter 7 part 1 chapter 7 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 Abayi in August 2019 principles of geology by Charles Lyall chapter 7 part 2 changes in the position of land and sea may give rise to vicissitudes and climate having offered these brief remarks on the diffusion of heat over the globe in the present state of the surface I shall now proceed to speculate on the vicissitudes of climate which must attend those endless variations in the geographical features of our planet which are contemplated in geology that our speculations may be confined within the strict limits of analogy I shall assume first that the proportion of dry land to sea continues always the same secondly that the volume of the land rising above the level of the sea is a constant quantity and not only that it's mean but that it's extreme height is liable only to trifling variations thirdly that both the mean and extreme depth of the sea are invariable and fourthly it may be consistent with due caution to assume that the grouping together of the land in continents is a necessary part of the economy of nature for it is possible that the laws which govern the subterranean forces and which act simultaneously along certain lines cannot but produce at every epoch continuous mountain chains so that the subdivision of the whole land into innumerable islands may be precluded if it be objected that the maximum of elevation of land and depth of sea are probably not constant nor the gathering together of all the land in certain parts nor even perhaps the relative extent of land and water I reply that the arguments about to be reduced will be strengthened if in these peculiarities of the surface there be considerable deviations from the present type if for example all other circumstances being the same the land is at one time more divided into islands than at another a greater uniformity of climate might be produced the mean temperature remaining unaltered or if at another era there were mountains higher than the Himalayas these when placed in high latitudes would cause a greater excess of cold or if we suppose that at certain periods no chain of hills in the world rose beyond the height of 10,000 feet a greater heat might then have prevailed then is compatible with the presence of mountains thrice that elevation however constant may be the relative proportion of sea and land we know that there is annually some small variation in their respective geographical positions and that in every century the land is in some parts raised and in others depressed in level and so likewise is the bed of the sea by these and other ceaseless changes configuration of the earth's surface has been remodeled again and again since it was the habitation of organic beings and the bed of the ocean has been lifted up to the height of some of the loftiest mountains the imagination is apt to take alarm when called upon to admit the formation of such irregularities in the crust of the earth after it had once become the habitation of living creatures but if time be allowed the migration need not subvert the ordinary repose of nature and the result is in a general view insignificant if we consider how slightly the highest mountain chains cause our globe to defer from a perfect sphere. The word rises to more than 21,000 feet above the sea would be represented on a globe of about six feet in diameter by a grain of sand less than one twentieth of inch in thickness. The superficial inequalities of the earth then may be deemed minute in quantity and their distribution at any particular epoch must be regarded in geology as temporary peculiarities like the height and outline of the cone of Vesuvius in the interval between two eruptions but although in reference to the magnitude of the globe the unevenness of the surface so unimportant it is on the position and direction of these small inequalities that the state of the atmosphere and both the local and general climate are mainly dependent. Before considering the effect which a material change in the distribution of land and sea must occasion it may be well to remark how greatly organic life may be affected by those minor variations which need not in the least degree alter the general temperature. Thus for example if we suppose by a series of convulsions a certain part of Greenland to become sea and in compensation attract of land to rise and connect Spitzbergen with Lapland an accession not greater in amount than one which the geologist can prove to have occurred in certain districts bordering the Mediterranean within a comparatively modern period. This altered form of the land might cause an interchange between the climate of certain parts of North America and of Europe which lie in corresponding latitudes. Many European species of plants and animals would probably perish in consequence because the mean temperature would be greatly lowered and others would fail in America because it would there be raised. On the other hand in places where the mean annual heat remained unaltered some species which flourish in Europe where the seasons are more uniform would be unable to resist the greater heat of the North American summer or the intense cold of the winter while others now fitted by their habitats for the great contrast of the American seasons would not be fitted for the insular climate of Europe. The vine for example according to Bolton Bolt can be cultivated with advantage 10 degrees farther north in Europe than in North America. Many plants endure severe frost but cannot ripen their seeds without a certain intensity of summer heat and a certain quantity of light others cannot endure a similar intensity either of heat or cold. It is now established that many of the existing species of animals have survived great changes in the physical geography of the globe. If such species be termed modern in comparison to races which preceded them their remains nevertheless enter into submarine deposits many hundred miles in length and which have since been raised from the deep to no inconsiderable altitude. When therefore it is shown that changes in the temperature of the atmosphere may be the consequence of such physical fluctuations of the surface we ought no longer to wonder that we find the distribution of existing species to be local in regards to longitude as well as latitude. If all species were now by an exertion of creative power to be diffused uniformly throughout those zones where there is an equal degree of heat and in all respects a similarity of climate they would begin from this moment to depart more and more from their original distribution. Aquatic and terrestrial species would be displaced as hook long ago observed so often is land and water exchanged places and there would also by the formation of new mountains and other changes be transpositions of climate contributing in the manner before alluded to to the local extermination of species. If we now proceed to consider the circumstances required for a general change of temperature it will appear from the facts and principles already laid down that whenever a greater extent of high land is collected in the polar regions the cold will augment and the same result will be produced when there is more sea between or near the tropics while on the contrary so often as the above conditions are reversed the heat will be greater. If this be admitted it will follow that unless the superficial inequalities of the earth be fixed and permanent there must be never ending fluctuations in the mean temperature of every zone and that the climate of one era can no more be a type of every other than is one of our four seasons of all the rest. It has been well said that the earth is covered by an ocean in the midst of which are two great islands and many smaller ones for the whole of the continents and islands occupy an area scarcely exceeding one fourth of the whole superficies of the spheroid. Now according to this analogy we may fairly speculate on the probability that there would not be usually at any given epoch of the past more than about one fourth dry land in a particular region as for example near the poles or between them and 75th parallels of north and south latitude. If therefore at present there should happen to be in both these quarters of the globe much more than this average proportion of land some of it in the Arctic region being above 5,000 feet in height and if in Antarctic latitudes a mountainous country has been found varying from 4,000 to 14,000 feet in height this alone affords ground for concluding that in the present state of things the mean heat of the climate is below that which the earth's surface in its more ordinary state would enjoy. This presumption is heightened when we reflect on the results of the recent soundings made by Sir James Ross in the Southern Ocean and continued for four successive years ending 1844 which seemed to prove that the mean depth of the Atlantic and Pacific is as great as Laplace and other eminent astronomers had imagined for then we might look not only for more than two-thirds sea in the frigid zones but for water of great depth which could not readily be reduced to the freezing point. The same opinion is confirmed when we compare the quantity of land lying between the poles and the 30th parallels of North and South latitude with the quantity placed between those parallels and the equator for it is clear that we have it present not only more than the usual degree of cold in the polar regions but also less than the average quantity of heat within the tropics. Position of land and sea which might produce the extreme of cold of which the earth's surface is susceptible. To simplify our view of the various changes in climate which different combinations of geographical circumstances may produce we shall first consider the conditions necessary for bringing about the extreme of cold or what would have been termed in the language of the old writers the winter of the great year or geological cycle and afterwards the conditions that produced the maximum of heat or the summer of the same year. To begin with the northern hemisphere let us suppose those hills of the Italian peninsula and of Sicily which are of comparatively modern origin and contain many fossil shells identical with living species to subside again into the sea from which they have been raised and that an extent of land of equal area and height varying from one to three thousand feet should rise up in the Arctic ocean between Siberia and the North Pole. In speaking of such changes I shall not allude to the manner in which I conceive it possible that they may be brought about nor of the time required for their accomplishment reserving for a future occasion not only the proofs that revolutions of equal magnitude have taken place but that analogous operations are still in progress. The alteration now supposed in the physical geography of the northern regions would cause additional snow and ice to accumulate where now there is usually an open sea and the temperature of the greater part of Europe would be somewhat lowered so as to resemble more nearly that of corresponding latitudes of North America or in other words it might be necessary to travel about ten degrees farther south in order to meet with the same climate which we now enjoy. No compensation would be derived from the disappearance of land in the Mediterranean countries but the contrary since the mean heat of the soil in those latitudes probably exceeds that which would belong to the sea by which we imagine it to be replaced. But let the configuration of the surface be still farther varied and let some large district within or near the tropics such as Brazil with its plains and hills of moderate height be converted into sea while lands of equal elevation and extent rise up in the Arctic circle. From these change their wood in the first place result a sensible diminution of temperature near the tropic for the Brazilian soil would no longer be heated by the sun so that the atmosphere would be less warm as also the neighboring Atlantic. On the other hand the whole of Europe North and Asia and North America would be chilled by the enormous quantity of ice and snow thus generated on the new Arctic continent. If as we have already seen there are now some points in the southern hemisphere where snow is perpetual down to the level of the sea in latitudes as low as Central England such might assuredly be the case throughout a great part of Europe under the change of circumstances above supposed. And if at present the extreme range of drifted icebergs is the Azores they might easily reach the equator after the assumed alteration. But to pursue the subject still farther let the Himalaya mountains with the whole of Hindustan sink down and their place be occupied by the Indian ocean while an equal extent of territory and mountains of the same vast height rise up between North Greenland and the Orkney Islands. It seems difficult to exaggerate the amount to which the climate of the northern hemisphere would then be cooled. But the refrigeration brought about at the same time in the southern hemisphere would be nearly equal and the difference of temperature between the Arctic and equatorial latitudes would not be much greater than at present for important disturbance can occur in the climate of a particular region without its immediately affecting all other latitudes however remote. The heat and cold which surround the globe are in a state of constant and universal flux and reflux. The heated and rarefied air is always rising and flowing from the equator towards the poles in the higher regions of the atmosphere while in the lower the colder air is flowing back to restore the equilibrium. The discirculation is constantly going on in the aerial currents is not disputed. It is often proved by the opposite course of the clouds at different heights and the fact has been farther illustrated in a striking manner by two recent events. The trade wind continually blows with great force from the island of Barbados to that of St. Vincent, not withstanding which during the eruption of the volcano in the island of St. Vincent in 1812 ashes fell in profusion from a great height in the atmosphere upon Barbados. In like manner during the great eruption of Zumbawa in 1815 ashes were carried to the islands of Amboina and Banda which last is about 800 miles east from the site of the volcano. Yet the southeast monsoon was then at its height. Disapparent transposition of matter against the wind confirmed the opinion of the existence of a countercurrent in the higher regions which had previously rested on theoretical conclusions only. That a corresponding interchange takes place in the seas is demonstrated, according to Humboldt, but a cold which is found to exist at great depths within the tropics and other proofs maybe mentioned the mass of warmer water which the gulf stream is constantly bearing northwards while a cooler current flows from the north along the coast of Greenland and Labrador and helps to restore the equilibrium. Currents of colder and therefore specifically heavier water pass from the poles towards the equator which cool the inferior parts of the ocean so that the heat of the torrid zone and the cold of the polar circle balance each other. The refrigeration therefore of the polar regions resulting from the supposed alteration in the distribution of land and sea would be immediately communicated to the tropics and from them its influence would extend to the Antarctic circle where the atmosphere and the ocean would be cooled so that ice and snow would augment. Although the mean temperature and the southern hemisphere is, as before stated for the most part, lower than that of the same parallels in the northern yet for a considerable space on each side of the line the mean annual heat of the waters is found to be the same in corresponding parallels. If therefore by the new position of the land the formation of icebergs had become of common occurrence in the northern temperate zone and if these were frequently drifted as far as the equator the same degree of cold which they generated would immediately be communicated as far as the tropic of Capricorn and from thence to the lands or ocean to the south. The freedom then of the circulation of heat and cold from pole to pole being duly considered it will be evident that the mean temperature which may prevail at the same point at two distinct periods may differ far more widely than that of any two points in the same parallels of latitude at one at the same period. For the range of temperature or in other words the curvature of the isothermal lines in a given zone and at a given period must always be circumscribed within narrow limits the climate of each place in that zone being controlled by the combined influence of the geographical peculiarities of all other parts of the earth. Whereas if we compare the state of things at two distinct and somewhat distant epochs a particular zone may at one time be under the influence of one class of disturbing causes and at another time may be affected by an opposite combination. The lands for example to the north of Greenland cause the present climate of North America to be colder than that of Europe in the same latitudes but the excess of cold is not so great as it would have been if the western hemisphere had been entirely isolated or separated from the eastern like a distinct planet. For not only does the refrigeration produced by Greenland chill to a certain extent the atmosphere of northern and western Europe but the mild climate of Europe reacts also upon North America and moderates the chilling influence of the adjoining polar lands. To return to the state of the earth after the changes above supposed we must not omit to dwell on the important effects to which a wide expanse of perpetual snow would give rise. It is probable that nearly the whole sea from the poles to the parallels of 45 degrees would be frozen over for it is well known that the immediate proximity of land is not essential to the formation and increase of field ice provided there be in some part of the same zone a sufficient quantity of glaciers generated on or near the land to cool down the sea. Captain Scorsby in his account of the Arctic Regions observes that when the sun's rays fall upon the snow clad surface of the ice or land they are in a great measure reflected without producing any material elevation of temperature but when they impinge on the black exterior of a ship the pitch on one side occasionally becomes fluid while ice is rapidly generated at the other. Now field ice is almost always covered with snow and thus not only land as extensive as our existing continents but immense tracts of sea in the frigid and temperate zones solid surface covered with snow and reflecting the sun's rays for the greater part of the year. Within the tropics moreover where the ocean now predominates the sky would no longer be serene and clear as in the present era but masses of floating ice would cause quick condensations of vapor so that fogs and clouds would deprive the vertical rays of the sun of half their power. The whole planet therefore would receive annually a smaller portion of the solar influence and the external crust would part by radiation with some of the heat which had been accumulated in it during a different state of the surface. This heat would be dissipated in the spaces surrounding our atmosphere which according to the calculations of Monsieur Fourier have a temperature much inferior to that of freezing water. In the political revolution above assumed the climate of equinoctial lands might be brought at last to resemble that of the present temperate zone or perhaps be far more wind-tree. They who should then inhabit such small isles and coral reefs as are now seen in the Indian Ocean and South Pacific would wonder that zoophytes of large dimensions had once been so prolific in their seas or if, perchance, they found the same fruit of the coconut tree or the palm silicified by the waters of some ancient mineral spring or encrusted with calcareous matter they would muse under revolutions which had annihilated such genera and replaced them by the oak, the chestnut and the pine. With equal admiration would they compare the skeletons of their small lizards with the bones of fossil alligators and crocodiles 20 feet in length which at a former epoch had multiplied between the tropics and when they saw a pine included in an iceberg drifted from latitudes which we now call temperate they would be astonished at the proof thus afforded that forests had once grown when nothing could be seen in their own times but a wilderness of snow. If the reader hesitate to suppose so extensive an alteration of temperature and probable consequence of geographical changes confined to one hemisphere he should remember how great are the local anomalies in climate now resulting from the peculiar distribution of land and sea in certain regions. Thus in the island of South Georgia before mentioned Captain Cook found the everlasting snows descending to the level of the seas between latitude 54 and 55 degrees no trees or shrubs were to be seen and in summer a few rocks only after a partial melting of the ice and snow were scantily covered with moss and tufts of grass. If such a climate can now exist at the level of the sea in a latitude corresponding to that of Yorkshire in spite of all those equalizing causes before enumerated by which the mixture of the temperatures of distant regions is facilitated throughout the globe what rigors might we not anticipate in a winter generated by the transfer of the mountains of India to our Arctic circle. But we have still to contemplate the additional refrigeration which might be affected by changes in the relative position of land and sea in the southern hemisphere. If the remaining continents were transferred from the equatorial and contiguous latitudes to the south of all the regions the intensity of cold produced might perhaps render the globe uninhabitable. We are too ignorant of the laws governing the direction of subterranean forces to determine whether such a crisis be within the limits of possibility. At the same time it may be observed that no distribution of land can well be imagined more irregular or as it were capricious than that which now prevails. For it present the globe may be divided into two equal parts in such a manner that one hemisphere shall be almost entirely covered with water while the other shall contain less water than land and what is still more extraordinary on comparing the extra tropical lands in the northern and southern hemispheres the lands in the northern are found to be to those in the southern in the proportion of 13 to 1. To imagine all the lands therefore in high and all the sea in low latitudes as delineated in figure 6 would scarcely be a more anomalous state of the surface. Position of land and sea which might give rise to the extreme of heat. Let us now turn from the contemplation of the winter of the great year and consider the opposite train circumstances which would bring on the spring and summer. To imagine all the lands to be collected together in equatorial latitudes and a few promontories only to project beyond the 30th parallel as represented in the annexed maps would be undoubtedly to suppose an extreme result of geological change. But if we consider a mere approximation to such a state of things it would be sufficient to cause a gradual elevation of temperature. Nor can it be regarded as a visionary idea that amidst the revolutions of the Earth's surface the quantity of land should at certain periods have been simultaneously lessened in the vicinity of both the poles and increased within the tropics. We must recollect that even now it is necessary to ascend to the height of 15,000 feet in the Andes under the line and in the Himalaya mountains which are without the tropic to 17,000 feet before we reach the limit of perpetual snow. On the northern slope indeed of the Himalaya range where the heat radiated from a great continent moderates the cold there are meadows and cultivated land at an elevation equal to the height of Mont Blanc. If then there were no arctic lands to chill the atmosphere and freeze the sea and if the loftiest chains were near the line it seems reasonable to imagine that the highest mountains might be closed with a rich vegetation to their summits and that nearly all signs of frost would disappear from the Earth. When the absorption of the solar rays was in no region impeded even in winter by a coat of snow the mean heat of the Earth's crust would augment to considerable depths and readings which we know to be in general an index of the mean temperature of the climate would be warmer in all latitudes. The waters of lakes, therefore, and rivers would be much hotter in winter and would be never chilled in summer by melted snow and ice. A remarkable uniformity of climate would prevail amid the archipelagos of the temperate and polar oceans where the tepid waters of equatorial currents freely circulate. The general humidity of the atmosphere would far exceed that of the present period for increased heat would promote evaporation in all parts of the globe. The winds would be first heated in their passage over the tropical plains and would then gather moisture from the surface of the deep till charged with vapor they arrived at extreme northern and southern regions and there encountering a cooler atmosphere discharged their burden in warm rain. If during the long night of a polar winter the snow should whiten the summits of some arctic islands they would be dissolved as rapidly by the returning sun as are the snows of Etna by the blasts of the Sirocco. We learn from those who have studied the geographical distribution of plants that in very low latitudes at present the vegetation of small remote from continents has a peculiar character. The ferns and allied families in particular bearing a great proportion to the total number of other plants. Other circumstances being the same the more remote the isles are from the continents the greater does this proportion become. Thus in the continent of India and the tropical parts of New Holland the proportion of ferns to the phenogamous plants only as one to twenty six whereas in the south sea islands it is as one to four or even as one to three. We might expect therefore in the summer of the great year or a cycle of climate that there would be a predominance of tree ferns and plants allied to genera now called tropical in the islands of the wide ocean while many forms now confined to arctic and temperate regions or only found near the equator on the summit of the loftiest mountains would almost disappear from the earth. Then might those genera of animals return of which the memorials are preserved in the ancient rocks of our continents. The pterodactyl might flit again through the air the huge iguanodon reappear in the woods and the ichthyosaurs swarm once more in the sea. Coral reefs might be prolonged again beyond the arctic circle where the whale at the nar-wall now abound and droves of turtles might begin again to wander through regions now tenanted by the walrus and the sea. But not to indulge too far in these speculations I may observe in conclusion that however great during the lapse of ages may be the vicissitudes of temperature in every zone it accords with this theory that the general climate should not experience any sensible change in the course of a few thousand years because that period is insufficient to affect the leading features of the physical geography of the globe. Notwithstanding the apparent uncertainty of the seasons it is found that the mean temperature of particular localities is very constant when observations made for a sufficient series of years are compared. Yet there must be exceptions to this rule and even the labours of men have by the drainage of lakes and marshes and the felling of extensive forests caused such changes in the atmosphere as greatly to raise our conception of the more important influence of those forces to which in certain latitudes even the existence of land or water, hill or valley, lake or sea must be ascribed. If we possessed accurate information of the amount of local fluctuation in climate in the course of 20 centuries it would often undoubtedly be considerable. Certain tracts for example on the coast of Holland and of England consisted of cultivated land in the time of the Romans which the sea by gradual encroachments has at length occupied. Here at least a slight alteration has been affected for neither the distribution of heat in the different seasons nor the mean annual temperature of the atmosphere investing the sea is precisely the same as that which rests upon the land. In those countries also where earthquakes and volcanoes are in full activity a much shorter period may produce a sensible variation. The climate of the great table land of Malpais in Mexico must differ materially from that which prevailed before the middle of the last century. For since that time six mountains the highest of them rising 1600 feet above the plateau have been thrown up by volcanic eruptions. It is by the repetition of an indefinite number of such local revolutions and by slow movements extending simultaneously over wider areas as will be afterwards shown that a general change of climate may finally be brought about. End of chapter 7 part 2