Page:Popular Science Monthly Volume 6.djvu/696

 influences by which the climate of a given region is vastly controlled; that is, it is to the warmth of the waters of the Gulf Stream that the present mildness of the climate of the, islands and coast-countries of Northern Europe is owing.

Adhemar has not been without followers, indeed, in the attempt to prove that the low temperature of the Ice period was due to astronomical rather than terrestrial causes—namely, to the change, in periods of 21,000 years, of the obliquity of the earth's axis to its orbit, and the still slower change in the eccentricity of that orbit itself. From the coöperation of these two causes, it is said the winters of either hemisphere would become longer and the summers shorter, and vice versa; and so on, in alternate periods. For example, the earth's axis reached the position most favorable to the climate of the Northern Hemisphere in 1248, since which time we have been advancing toward a new Ice period; whereas, antarctic regions then passed the point of greatest intensity of cold. Even the relative distribution of land and sea, it is affirmed, would be changed by these alternating accumulations and diminutions at either pole. To all this, we can here merely reply that if these astronomical facts have any influence whatever upon the earth's climate, we are wholly ignorant as to its amount; and since Herschel's time astronomers have been disinclined to ascribe to them any considerable share in the production of climatic variations.

But there is abundant evidence that during the Drift period there took place extensive and considerable elevations and subsidences of the earth's surface. Such elevations are still going on, as witness the rise of the coast-terraces of Scotland, Sweden, Norway, Sardinia, Sicily, and other lands, to a height far above the present sea-level. Confusion here is easy, however, and it must not be forgotten that the retreat of the shore-line would be apparently and practically an emergence of the land, although the latter remained fixed all the time; and curiously, apparent encroachments of the sea upon the shore may be an actual subsidence of the shore itself.

A study of the drift deposits, and the organic remains found therein, compels the belief that at one part of that period the lowlands of Europe—i. e., Holland, the plains of Northern Germany, and parts of Russia, Great Britain, Denmark, Sweden, and Norway—were covered by the North and Baltic Seas, which, thus united and enlarged, extended southward through Russia and Siberia, and possibly connected the Black and Caspian Seas. The Desert of Sahara was also under water, as Desor and Escher show. The shells of marine species yet mostly extant, now found in the extensive lowlands of North America, show these also to have been submerged.

In an earlier epoch of the Diluvial period, however, that is, previous