Principles of Geology Part 8

That a corresponding interchange takes place in the seas, is demonstrated, according to Humboldt, by the cold which is found to exist at great depths within the tropics; and, among other proofs, may be mentioned the ma.s.s 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.[189]

Currents of colder and therefore specifically heavier water pa.s.s 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 of higher lat.i.tudes in 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 s.p.a.ce 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 lat.i.tude, at one and 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 circ.u.mscribed 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 cla.s.s 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 lat.i.tudes; 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, 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 quant.i.ty of glaciers generated on or near the land, to cool down the sea. Captain Scoresby, 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."[190]

Now field ice is almost always covered with snow;[191] and thus not only land as extensive as our existing continents, but immense tracts of sea in the frigid and temperate zones, might present a 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 ma.s.ses 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 acc.u.mulated in it, during a different state of the surface. This heat would be dissipated in the s.p.a.ces surrounding our atmosphere, which, according to the calculations of M. Fourier, have a temperature much inferior to that of freezing water.

After the geographical revolution above a.s.sumed, the climate of equinoctial lands might be brought at last to resemble that of the present temperate zone, or perhaps be far more wintry. 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 wood and fruit of the cocoa-nut tree or the palm silicified by the waters of some ancient mineral spring, or incrusted with calcareous matter, they would muse on the 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 more than twenty 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 lat.i.tudes which we now call temperate, they would be astonished at the proof thus afforded, that forests had once grown where 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 as the 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 (p. 98), Captain Cook found the everlasting snows descending to the level of the sea, between lat. 54 and 55 S.; 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 gra.s.s. If such a climate can now exist at the level of the sea in a lat.i.tude 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 antic.i.p.ate 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 effected 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 lat.i.tudes to the south polar 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 at 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 (see figs. 3 and 4);[192] and, what is still more extraordinary, on comparing the extratropical 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 thirteen to one![193] To imagine all the lands, therefore, in high, and all the sea in low lat.i.tudes, as delineated in fig. 6, p. 111, would scarcely be a more anomalous state of the surface.

[Ill.u.s.tration: Map showing the present unequal Distribution of LAND and WATER on the Surface of the GLOBE.

Fig. 3. Here London is taken as a centre, and we behold the greatest quant.i.ty of land existing in one hemisphere.

Fig. 4. Here the centre is the antipodal point to London, and we see the greatest quant.i.ty of water existing in one hemisphere.

The black shading expresses land having land opposite or antipodal to it.]

[Ill.u.s.tration: Maps showing the position of LAND and SEA which might produce the Extremes of HEAT and COLD in the Climates of the GLOBE.

Fig. 5.

Extreme of Heat.

Fig. 6.

Extreme of Cold.

OBSERVATIONS.--These maps are intended to show that continents and islands having the same shape and relative dimensions as those now existing, might be placed so as to occupy either the equatorial or polar regions.

In fig. 5, scarcely any of the land extends from the equator towards the poles beyond the 30th parallel of lat.i.tude; and fig. 6, a very small proportion of it extends from the poles towards the Equator beyond the 40th parallel of lat.i.tude.

_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 of circ.u.mstances which would bring on the spring and summer. To imagine all the lands to be collected together in equatorial lat.i.tudes, and a few promontories only to project beyond the thirtieth parallel, as represented in the annexed maps (figs. 5 and 6), 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 general elevation of temperature. Nor can it be regarded as a visionary idea, that amidst the revolutions of the earth's surface, the quant.i.ty 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 fifteen thousand feet in the Andes under the line, and in the Himalaya mountains, which are without the tropic, to seventeen thousand 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.[194] 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 clothed 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 springs, which we know to be in general an index of the mean temperature of the climate, would be warmer in all lat.i.tudes. 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 archipelagoes of the temperate and polar oceans, where the tepid waters of equatorial currents would 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 pa.s.sage 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 snows 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 lat.i.tudes, at present, the vegetation of small islands 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 circ.u.mstances 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 phaenogamous plants is only as one to twenty-six; whereas, in the South-Sea Islands, it is as one to four, or even as one to three.[195]

We might expect, therefore, in the summer of the "great year," or 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 pterodactyle 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 and the narwal now abound; and droves of turtles might begin again to wander through regions now tenanted by the walrus and the seal.

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 labors of man 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 lat.i.tudes, 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 twenty 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 effected; 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 sixteen hundred feet above the plateau, have been thrown up by volcanic eruptions. It is by the repet.i.tion 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.

CHAPTER VIII.

ON FORMER CHANGES IN PHYSICAL GEOGRAPHY AND CLIMATE.

Geographical features of the northern hemisphere, at the period of the oldest fossiliferous strata--State of the surface when the mountain limestone and coal were deposited--Changes in physical geography, between the carboniferous period and the chalk--Abrupt transition from the secondary to the tertiary fossils--Accession of land, and elevation of mountain chains, after the consolidation of the secondary rocks--Explanation of Map, showing the area covered by sea, since the commencement of the tertiary period--Astronomical theories of the causes of variations in climate--Theory of the diminution of the supposed primitive heat of the globe.

In the sixth chapter, I stated the arguments derived from organic remains for concluding that in the period when the carboniferous strata were deposited, the temperature of the ocean and the air was more uniform in the different seasons of the year, and in different lat.i.tudes, than at present, and that there was a remarkable absence of cold as well as great moisture in the atmosphere. It was also shown that the climate had been modified more than once since that epoch, and that it had been reduced, by successive changes, more and more nearly to that now prevailing in the same lat.i.tudes. Farther, I endeavored, in the last chapter, to prove that vicissitudes in climate of no less importance may be expected to recur in future, if it be admitted that causes now active in nature have power, in the lapse of ages, to produce considerable variations in the relative position of land and sea. It remains to inquire whether the alterations, which the geologist can prove to have _actually taken place_ at former periods, in the geographical features of the northern hemisphere, coincide in their nature, and in the time of their occurrence, with such revolutions in climate as might naturally have resulted, according to the meteorological principles already explained.

_Period of the primary fossiliferous rocks._--The oldest system of strata which afford by their organic remains any evidence as to climate, or the former position of land and sea, are those formerly known as the _transition rocks_, or what have since been termed Lower Silurian or "primary fossiliferous" formations. These have been found in England, France, Germany, Sweden, Russia, and other parts of central and northern Europe, as also in the great Lake district of Canada and the United States. The multilocular or chambered univalves, including the Nautilus, and the corals, obtained from the limestones of these ancient groups, have been compared to forms now most largely developed in tropical seas.

The corals, however, have been shown by M. Milne Edwards to differ generally from all living zoophytes; so that conclusions as to a warmer climate drawn from such remote a.n.a.logies must be received with caution.

Hitherto, few, if any, contemporaneous vegetable remains have been noticed; but such as are mentioned agree more nearly with the plants of the carboniferous era than any other, and would therefore imply a warm and humid atmosphere entirely free from intense cold throughout the year.

This absence or great scarcity of plants as well as of freshwater sh.e.l.ls and other indications of neighboring land, coupled with the wide extent of marine strata of this age in Europe and North America, are facts which imply such a state of physical geography (so far at least as regards the northern hemisphere) as would, according to the principles before explained, give rise to such a moist and equable climate. (See p.

109, and fig. 5, p. 111.)

_Carboniferous group._--This group comes next in the order of succession; and one of its princ.i.p.al members, the mountain limestone, was evidently a marine formation, as is shown by the sh.e.l.ls and corals which it contains. That the ocean of that period was of considerable extent in our lat.i.tudes, we may infer from the continuity of these calcareous strata over large areas in Europe, Canada, and the United States. The same group has also been traced in North America, towards the borders of the arctic sea.[196]

There are also several regions in Scotland, and in the central and northern parts of England, as well as in the United States, where marine carboniferous limestones alternate with strata containing coal, in such a manner as to imply the drifting down of plants by rivers into the sea, and the alternate occupation of the same s.p.a.ce by fresh and salt water.

Since the time of the earlier writers, no strata have been more extensively investigated, both in Europe and North America, than those of the ancient carboniferous group, and the progress of science has led to a general belief that a large portion of the purest coal has been formed, not, as was once imagined, by vegetable matter floated from a distance, but by plants which grew on the spot, and somewhat in the manner of peat on the s.p.a.ces now covered by the beds of coal. The former existence of land in some of these s.p.a.ces has been proved, as already stated, by the occurrence of numerous upright fossil trees, with their roots terminating downwards in seams of coal; and still more generally by the roots of trees (stigmariae) remaining in their natural position in the clays which underlie almost every layer of coal.

As some nearly continuous beds of such coal have of late years been traced in North America, over areas 100 or 200 miles and upwards in diameter, it may be asked whether the large tracts of ancient land implied by this fact are not inconsistent with the hypothesis of the general prevalence of islands at the period under consideration? In reply, I may observe that the coal-fields must originally have been low alluvial grounds, resembling in situation the cypress-swamps of the Mississippi, or the sunderbunds of the Ganges, being liable like them to be inundated at certain periods by a river or by the sea, if the land should be depressed a few feet. All the phenomena, organic and inorganic, imply conditions nowhere to be met with except in the deltas of large rivers. We have to account for an abundant supply of fluviatile sediment, carried for ages towards one and the same region, and capable of forming strata of mud and sand thousands of feet, or even fathoms, in thickness, many of them consisting of laminated shale, inclosing the leaves of ferns and other terrestrial plants. We have also to explain the frequent intercalations of root-beds, and the interposition here and there of brackish and marine deposits, demonstrating the occasional presence of the neighboring sea. But these forest-covered deltas could only have been formed at the termination of large hydrographical basins, each drained by a great river and its tributaries; and the acc.u.mulation of sediment bears testimony to contemporaneous denudation on a large scale, and, therefore, to a wide area of land, probably containing within it one or more mountain chains.

In the case of the great Ohio or Appalachian coal-field, the largest in the world, it seems clear that the uplands drained by one or more great rivers were chiefly to the eastward, or they occupied a s.p.a.ce now filled by part of the Atlantic Ocean, for the mechanical deposits of mud and sand increase greatly in thickness and coa.r.s.eness of material as we approach the eastern borders of the coal-field, or the southeast flanks of the Alleghany mountains, near Philadelphia. In that region numerous beds of pebbles, often of the size of a hen's egg, are seen to alternate with beds of pure coal.

But the American coal-fields are all comprised within the 30th and 50th degrees of north lat.i.tude; and there is no reason to presume that the lands at the borders of which they originated ever penetrated so far or in such ma.s.ses into the colder and arctic regions, so as to generate a cold climate. In the southern hemisphere, where the predominance of sea over land is now the distinguishing geographical feature, we nevertheless find a large part of the continent of Australia, as well as New Zealand, placed between the 30th and 50th degrees of S. lat.i.tude.

The two islands of New Zealand taken together, are between 800 and 900 miles in length, with a breadth in some parts of ninety miles, and they stretch as far south as the 46th degree of lat.i.tude. They afford, therefore, a wide area for the growth of a terrestrial vegetation, and the botany of this region is characterized by abundance of ferns, one hundred and forty species of which are already known, some of them attaining the size of trees. In this respect the southern sh.o.r.es of New Zealand in the 46th degree of lat.i.tude almost vie with tropical islands.

Another point of resemblance between the Flora of New Zealand and that of the ancient carboniferous period is the prevalence of the fir tribe or of coniferous wood.

An argument of some weight in corroboration of the theory above explained respecting the geographical condition of the temperate and arctic lat.i.tudes of the northern hemisphere in the carboniferous period may also be derived from ah examination of those groups of strata which immediately preceded the coal. The fossils of the Devonian and Silurian strata in Europe and North America have led to the conclusion, that they were formed for the most part in deep seas, far from land. In those older strata land plants are almost as rare as they are abundant or universal in the coal measures. Those ancient deposits, therefore, may be supposed to have belonged to an epoch when dry land had only just begun to be upraised from the deep; a theory which would imply the existence during the carboniferous epoch of islands, instead of an extensive continent, in the area where the coal was formed.

Such a state of things prevailing in the north, from the pole to the 30th parallel of lat.i.tude, if not neutralized by circ.u.mstances of a contrary tendency in corresponding regions south of the line, would give rise to a general warmth and uniformity of climate throughout the globe.

_Changes in physical geography between the formation of the carboniferous strata and the chalk._--We have evidence in England that the strata of the ancient carboniferous group, already adverted to, were, in many instances, fractured and contorted, and often thrown into a vertical position, before the deposition of some even of the oldest known secondary rocks, such as the new red sandstone.

Fragments of the older formations are sometimes included in the conglomerates of the more modern; and some of these fragments still retain their fossil sh.e.l.ls and corals, so as to enable us to determine the parent rocks from whence they were derived. There are other proofs of the disturbance at successive epochs of different secondary rocks before the deposition of others; and satisfactory evidence that, during these reiterated convulsions, the geographical features of the northern hemisphere were frequently modified, and that from time to time new lands emerged from the deep. The vegetation, during some parts of the period in question (from the lias to the chalk inclusive), when genera allied to Cycas and Zamia were abundant, appears to have approached to that of the larger islands of the equatorial zone; such, for example, as we now find in the West Indian archipelago.[197] These islands appear to have been drained by rivers of considerable size, which were inhabited by crocodiles and gigantic oviparous reptiles, both herbivorous and carnivorous, belonging for the most part to extinct genera. Of the contemporary inhabitants of the land we have as yet acquired but scanty information, but we know that there were flying reptiles, insects, and small mammifers, allied to the marsupial tribes.

A freshwater deposit, called the Wealden, occurs in the upper part of the secondary series of the south of England, which, by its extent and fossils, attests the existence in that region of a large river draining a continent or island of considerable dimensions. We know that this land was clothed with wood, and inhabited by huge terrestrial reptiles and birds. Its position so far to the north as the counties of Surrey and Suss.e.x, at a time when the mean temperature of the climate is supposed to have been much hotter than at present, may at first sight appear inconsistent with the theory before explained, that the heat was caused by the gathering together of all the great ma.s.ses of land in low lat.i.tudes, while the northern regions were almost entirely sea. But it must not be taken for granted that the geographical conditions already described (p. 109, and fig. 5, p. 111) as capable of producing the extreme of heat were ever combined at any geological period of which we have yet obtained information. It is more probable, from what has been stated in the preceding chapters, that a slight approximation to such an extreme state of things would be sufficient; in other words, if most of the dry land were tropical, and scarcely any of it arctic or antarctic, a prodigious elevation of temperature must ensue, even though a part of some continents should penetrate far into the temperate zones.