Principles of Geology Part 7

The warmth and humidity of the air, and the uniformity of climate, both in the different seasons of the year, and in different lat.i.tudes, 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 lat.i.tudes 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 pliocene, 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 lat.i.tudes during the glacial epoch, or the epoch immediately antecedent to that in which all the species now contemporary with man were in being.

CHAPTER VII.

FARTHER EXAMINATION OF THE QUESTION AS TO THE a.s.sUMED 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._[165]--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 cosmogonist 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 cla.s.s 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 a.s.sumed, 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,--an 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 farther 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 a type of the temperature which all countries placed under the same lat.i.tude 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 civilization of Europe, was for a long time considered a type to which the climate of all corresponding lat.i.tudes might be referred. But this region, const.i.tuting 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 a.s.sume 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 position of land and sea, at different epochs, than those which now cause the climate of Europe to differ 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.[166] 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 lat.i.tude are determined by a mult.i.tude of circ.u.mstances, among the princ.i.p.al 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 lat.i.tudes have sometimes a mean difference of temperature amounting to 11, or even in a few cases to 17 Fahr.; and some places on the two continents, which have the same mean temperature, differ from 7 to 17 in lat.i.tude. Thus, c.u.mberland House, in North America, having the same lat.i.tude (54 N.) as the city of York in England, stands on the isothermal line of 32, which in Europe rises to the North Cape, in lat. 71, but its summer heat exceeds that of Brussels or Paris.[167] The princ.i.p.al cause of greater intensity of cold in corresponding lat.i.tudes 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 three to five thousand 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 82d degree of lat.i.tude, experiences under the 60th parallel a more rigorous climate than Lapland under the 72d parallel.

But if land be situated between the 40th parallel and the 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 quant.i.ty 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."[168] On the contrary, the northeastern extremity of Asia experiences in the same lat.i.tude 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 belts 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 at New York, says Humboldt, the summer of Rome and the winter of Copenhagen; at Quebec, the summer of Paris and the winter of Petersburg.

At Pekin, 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 as at Upsala.[169]

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 lat.i.tude 9 or 10 distant from each other, whereas the isothermal lines, or those pa.s.sing through places having the same mean annual temperature, differ only from 4 to 5 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 acc.u.mulation and drifting of ice in high lat.i.tudes. The temperature of the Lagullas current is 10 or 12 Fahr.

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.[170] 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 or 4 Fahr. than those of the ocean under the line, so that it moderates the heat of the tropics.[171]

But the effects of the Gulf stream on the climate of the North Atlantic Ocean are far more remarkable. This most powerful of known currents has its source in the Gulf or Sea of Mexico, which, like the Mediterranean and other close seas in temperate or low lat.i.tudes, is warmer than the open ocean in the same parallels. The temperature of the Mexican sea in summer is, according to Rennell, 86 Fahr., or at least 7 above that of the Atlantic in the same lat.i.tude.[172] From this great reservoir or 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 Newfoundland, where it still retains a temperature of 8 above that of the surrounding sea. It reaches the Azores in about 78 days, after flowing nearly 3000 geographical miles, and from thence it sometimes extends its course a thousand miles farther, so as to reach the Bay of Biscay, still retaining an excess of 5 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 centre of the North Atlantic, between the parallels of 33 and 45 N. lat., which Rennell calls the "recipient of the gulf water." A great part of it is covered by the weed called sarga.s.so (_Sarga.s.sum bacciferum_), which the current floats in abundance from the Gulf of Mexico. This ma.s.s of water is nearly stagnant, is warmer by 7 or 10 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 E. to W., and 350 in breadth from N.

to S., which, he remarks, is a larger area than that of the Mediterranean. The heat of this great body of water is kept 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 Scoresby that the influence of the Gulf stream extends even to the sea near Spitzbergen, where its waters may pa.s.s under those of melted ice; for it has been found that in the neighborhood of Spitzbergen, the water is warmer by 6 or 7 at the depth of one hundred and two hundred fathoms than at the surface. This might arise from the known law that fresh water pa.s.ses the point of greatest density when cooled down below 40, 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 inferior water of the Gulf stream.

The great glaciers generated in the valleys of Spitzbergen, in the 79 of north lat.i.tude, are almost all cut off at the beach, being melted by the feeble remnant of heat still retained by the Gulf stream. In Baffin's 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 under-current, the glaciers stretch out from the sh.o.r.e, and furnish repeated crops of mountainous ma.s.ses of ice which float off into the ocean.[173] The number and dimensions of these bergs is prodigious. Captain Sir John Ross saw several of them together in Baffin's Bay aground in water fifteen hundred feet deep! Many of them are driven down into Hudson's Bay, and acc.u.mulating there, diffuse excessive cold over the neighboring continent; so that Captain Franklin reports, that at the mouth of Hayes' River, which lies in the same lat.i.tude as the north of Prussia or the south of Scotland, ice is found everywhere in digging wells, in summer, at the depth of four feet! Other bergs have been occasionally met with, at midsummer, in a state of rapid thaw, as far south as lat. 40 and longitude about 60 west, where they cool the water sensibly to the distance of forty or fifty miles around, the thermometer sinking sometimes 17, or even 18, Fahrenheit, in their neighborhood.[174] It is a well-known fact that every four or five years a large number of icebergs, floating from Greenland, double Cape Langaness, 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 dearth 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 regard 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 one to three, excluding from our consideration that part which lies between the pole and the 78 of south lat.i.tude, which has. .h.i.therto proved inaccessible.

And whereas in the northern hemisphere, between the pole and the thirtieth parallel of north lat.i.tude, the land and sea occupy nearly equal areas, the ocean in the southern hemisphere covers no less than fifteen parts in sixteen of the entire s.p.a.ce included between the antarctic circle and the thirtieth parallel of south lat.i.tude.

This great extent of sea gives a particular character to climates south of the equator, the winters being mild and the summers cool. Thus, in Van Dieman's Land, corresponding nearly in lat.i.tude to Rome, the winters are more mild than at Naples, and the summers not warmer than those at Paris, which is 7 farther from the equator.[175] The effects on animal and vegetable life are remarkable. Capt. 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. He states also that humming birds 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's bark, south of lat. 55, near Cape Horn.[176]

So the orchideous plants which are parasitical on trees, and are generally characteristic of the tropics, advance to the 38th and 42d degree of S. lat., and even beyond the 45th degree in New Zealand, where they were found by Forster. In South America also arborescent gra.s.ses abound in the dense forests of Chiloe, in lat. 42 S., where "they entwine the trees into one entangled ma.s.s to the height of thirty or forty feet above the ground. Palm-trees in the same quarter of the globe grow in lat. 37, an arborescent gra.s.s very like a bamboo in 40, and another closely allied kind, of great length, but not erect, even as far south as 45."[177]

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 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 thirty-fifth and fortieth 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 than the ice around the north pole. All the recent voyages of discovery have tended to confirm this opinion, although Capt.

Weddel penetrated, in 1823, three degrees farther south than Capt. Cook, reaching lat. 74 15' South, long. 34 17' West, and Sir James Ross, in 1842, arrived at lat. 78 10' S., as high a lat.i.tude, within three degrees, as the farthest point attained by Captain Parry in the arctic circle, or lat. 81 12' North.

The description given by ancient as well as modern navigators of the sea and land in high southern lat.i.tudes, clearly attests the greater severity of the climate as compared to arctic regions. In Sandwich Land, in lat. 59 S., or in nearly the same parallel as the north of Scotland, Capt. 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 astonis.h.i.+ng, in the island of S. Georgia, which is in the 54 south lat.i.tude, or the same parallel as Yorks.h.i.+re, the line of perpetual snow descends to the level of the ocean.[178] 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 four degrees farther to the north, do not attain the limit of perpetual snow on our side of the equator, we learn that lat.i.tude is one only of many powerful causes, which determine the climate of particular regions of the globe. Capt. Sir James Ross, in his exploring expedition in 1841-3, found that the temperature south of the 60th degree of lat.i.tude seldom rose above 32 Fahr. During the two summer months of the year 1841 (January and February) the range of the thermometer was between 11 and 32 Fahr.; 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 rarefy the air. The heated aerial currents would then ascend and flow more 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 68 and 71 of south lat.i.tude, 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 circ.u.mstance seems explicable only on 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 seventieth degree of south lat.i.tude and the pole. The justness of these and other speculations of that great navigator have since been singularly confirmed by the investigation made by Sir James Ross in 1841. He found Victoria Land, extending from 71 to 79 S. lat.i.tude, skirted by a great barrier of ice, the height of the land ranging from 4000 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 alt.i.tude, 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 1831-2 (between lat. 64 and 68 S.), presented a most wintry aspect, covered even in summer with ice and snow, and nearly dest.i.tute of animal life. In corresponding lat.i.tudes 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.[179]

The distance to which icebergs float from the polar regions on the opposite sides of the line is, as might have been antic.i.p.ated, very different. Their extreme limit in the northern hemisphere is lat. 40, as before mentioned, and they are occasionally seen in lat. 42 N., near the termination of the great bank of Newfoundland, and at the Azores, lat. 42 N., to which they are sometimes drifted from Baffin's Bay. But in the other hemisphere they have been seen, within the last few years, at different points off the Cape of Good Hope, between lat. 36 and 39.[180] One of these (see fig. 2) was two miles in circ.u.mference, and 150 feet high, appearing like chalk when the sun was obscured, and having the l.u.s.tre of refined sugar when the sun was s.h.i.+ning 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 sea-water, that for every cubic foot seen above, there must at least be eight cubic feet below water.[181] 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, so that the serene sky of that delightful region might soon be deformed by clouds and mists.

[Ill.u.s.tration: Fig. 2.

Iceberg seen off the Cape of Good Hope, April, 1829. Lat. 89 18' S.

Long. 48 46' E.]

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 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...o...b..t, the two hemispheres must receive _equal absolute quant.i.ties_ of light and heat per annum, the proximity of the sun in perigee exactly compensating the effect of its swifter motion.[182] 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.[183]

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 circ.u.mstances; 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 forty-fifth degree of south lat.i.tude.

_Changes in the position of land and sea may give rise to vicissitudes in 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 a.n.a.logy, I shall a.s.sume, 1st, That the proportion of dry land to sea continues always the same. 2dly, That the volume of the land rising above the level of the sea is a constant quant.i.ty; and not only that its mean, but that its extreme height, is liable only to trifling variations. 3dly, That both the mean and extreme depth of the sea are invariable; and 4thly, It may be consistent with due caution to a.s.sume 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 adduced will be strengthened if, in these peculiarities of the surface, there be considerable deviations from the present type. If, for example, all other circ.u.mstances 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 Himalaya, these, when placed in high lat.i.tudes, 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 than is compatible with the existence 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, the configuration of the earth's surface has been remodelled 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 operation 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 differ from a perfect sphere.

Chimborazo, though it 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 an inch in thickness.

The superficial inequalities of the earth, then, may be deemed minute in quant.i.ty, 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 is 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, a tract 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 lat.i.tudes. 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 intenser cold of the winter; while others, now fitted by their habits for the great contrast of the American seasons, would not be fitted for the _insular_ climate of Europe. The vine, for example, according to Humboldt, can be cultivated with advantage 10 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 quant.i.ty 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 alt.i.tude. When, therefore, it is shown that changes in the temperature of the atmosphere may be the consequence of such physical revolutions of the surface, we ought no longer to wonder that we find the distribution of existing species to be _local_, in regard to _longitude_ as well as lat.i.tude. 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 Hooke long ago observed, so often as 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.[184]

If we now proceed to consider the circ.u.mstances 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. (See figs. 5 and 6, p. 111.) 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 a.n.a.logy, 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 the 75th parallels of N. and S. lat.i.tude.

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, five thousand feet in height, and if in antarctic lat.i.tudes a mountainous country has been found varying from 4000 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 seem to prove that the mean depth of the Atlantic and Pacific is as great as Laplace and other eminent astronomers had imagined;[185] 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 quant.i.ty of land lying between the poles and the 30th parallels of north and south lat.i.tude, with the quant.i.ty placed between those parallels and the equator; for, it is clear, that we have at present not only more than the usual degree of cold in the polar regions, but also less than the average quant.i.ty 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 circ.u.mstances 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 requisite to produce 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 sh.e.l.ls 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 a.n.a.logous operations are still in gradual progress. The alteration now supposed in the physical geography of the northern regions, would cause additional snow and ice to acc.u.mulate 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 lat.i.tudes of North America: or, in other words, it might be necessary to travel about 10 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 lat.i.tudes 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 this change there would, 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, Northern Asia, and North America, would be chilled by the enormous quant.i.ty 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 lat.i.tudes as low as central England, such might a.s.suredly be the case throughout a great part of Europe, under the change of circ.u.mstances above supposed: and if at present the extreme range of drifted icebergs is the Azores, they might easily reach the equator after the a.s.sumed alteration. But to pursue the subject still farther, let the Himalaya mountains, with the whole of Hindostan, 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.[186]

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 lat.i.tudes would not be much greater than at present; for no important disturbance can occur in the climate of a particular region without its immediately affecting all other lat.i.tudes, 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. That this circulation 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 ill.u.s.trated in a striking manner by two recent events. The trade wind continually blows with great force from the island of Barbadoes to that of St. Vincent; notwithstanding 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 Barbadoes.[187] In like manner, during the great eruption of Sumbawa, in 1815, ashes were carried to the islands of Amboyna and Banda, which last is about 800 miles east from the site of the volcano. Yet the southeast monsoon was then at its height.[188] This apparent transposition of matter against the wind, confirmed the opinion of the existence of a counter-current in the higher regions, which had previously rested on theoretical conclusions only.