The Elements of Geology; Adapted to the Use of Schools and Colleges

by Justin R. Loomis

SECTION V.—CHANGES OF CLIMATE.

Our means of determining the climate of any former period consists in a comparison of the fossils of such period with the existing forms of life in warm and cold climates.

The earliest abundant vegetation consisted principally of ferns, rushes and mosses, and a larger growth was attained than is attained by any of the allied forms of the present time. We may infer that the circumstances under which these lower forms of vegetable life are now produced in the largest proportion, compared with other forms, and under which they grow to the largest size, are the circumstances approaching most nearly those under which the early vegetation was produced. These circumstances are found to be a position elevated but little above the level of the sea, a humid atmosphere, and the highest terrestrial temperature. Such facts favor the conclusion that during the coal period an ultra-tropical climate prevailed, and that the land existed in the form of low islands, thickly set in a general ocean.

The peculiar characters of some of the animal fossils, from the earliest fossiliferous to the tertiary series, indicate that a warmer climate prevailed during their formation than now exists. The remains of marine animals, such as the cephalopoda, are found in great numbers and in high latitudes, in a fossil state; but similar species, as the nautilus, now abound only between the tropics. The same is true of the crinoidea. Coralline limestone is also found in great abundance and in high northern latitudes; but the stone-producing coral now exists only in very warm seas. The remains of saurian reptiles are numerous in the oölite and Wealden; but all the larger recent species of the lizard tribe, such as the crocodile, are confined to the warmer regions of the earth.

A former warm climate in Siberia is indicated by the occurrence there of the remains of elephants. These animals were so abundant that their tusks are now collected as an article of commerce. The abundance and high state of preservation of these remains seem to preclude the explanation that they were conveyed there, from the present tropical regions, by any great geological convulsion. The species must therefore have inhabited the country, though the elephant is now found only between the tropics. The Siberian elephant was a different species from any now existing, and, unlike the recent species, had a covering of coarse hair. There is, however, no reason to conclude that it could endure a continued low temperature; and its sustenance would have been impossible, from the very stinted vegetation which that region now affords. We must therefore suppose that Siberia enjoyed, at the period when it supported these animals in such abundance, a tropical climate.

Most of the facts which go to prove a change of climate have been observed in the northern hemisphere; but the explorations in South America and New Holland furnish ground for believing that the geological phenomena of the two hemispheres are essentially alike, and that the indications of climate are the same for the same periods.

Such is, in general, the evidence in reference to climate; and it leads to the conclusion that a highly tropical climate prevailed in the temperate, and for some distance, at least, into the polar zones, in the early geological periods; while there is no reason for supposing that the tropical regions experienced a temperature too high for physical life to endure it. The climate of the earth was characterized then by a higher temperature than now, and by greater uniformity. This was the climate, with perhaps a gradual reduction of temperature, till the later portions of the tertiary period.

Before the close of the tertiary period, a change occurred, and probably a rapid one, to a more rigorous climate than now exists. The destruction of the elephant in Siberia was evidently sudden, and was followed by extreme cold; for the animals are in some cases entirely preserved in ice, and in so perfect a state that, when the ice which surrounds them becomes melted, the flesh is devoured by carnivorous animals. There are occasionally found, in the drift of the boulder period, shells similar to those of the Arctic regions, and in a condition to show that they have not been transported. The clay beds of the northern portion of the United States and of Canada were deposited during the last depression of that portion of the continent, and they contain the remains of marine animals identical in several instances with species now living, but confined to more northern regions. It must therefore be admitted that the interval between the middle tertiary and the modern era was one of great cold. It is generally referred to as the Glacial period.

Very considerable local changes of climate have also occurred within the historical period. Thus the mean temperature of the Alps has been so reduced that the ancient passes have in modern times become choked up with snow, and other passes have been sought,—a result, perhaps, of additional upheaval. It would seem that Siberia is now receiving a milder climate. The ice in which elephants have for centuries been imbedded has been slowly melting for at least thirty years.