Page:Popular Science Monthly Volume 1.djvu/248

236 The exhaustion of all other means of solution, joined to the mass of positive evidence accumulated by recent science, throws us more and more conclusively upon the idea to which Sir Charles Lyell has firmly held from the first, and which may be taken as the culminating point of his latest achievements in geology, that the predominant cause of the great changes in climate is to be found in the distribution and elevation of the land. The Glacial period may be traced to an excessive and abnormal accumulation of land around the Pole. There is absolutely no limit to the alternations which the surface of our globe may have, or indeed has gone through. There is hardly a spot of what is now land which has not been covered by the sea, probably not a space now covered by the ocean which has not been at some time, if not many times, dry land. In one epoch the land may have been chiefly equatorial, at another polar or circumpolar. At present we may readily divide the globe into two equal parts, the land hemisphere and the water hemisphere; the former of which exhibits almost as much land as water, or as 1 to 1.106; while in the latter the proportion of land to water, as made out by Mr. Trelawny Saunders, is only as 1 to 7.988. The general proportion of land to sea may be taken throughout the globe as 1 to 2½. Were the land, by the action of subterranean forces, its total amount being unchanged, now gathered together in masses along the equator and around the Poles alternately, such geographical changes would amply suffice, as Sir C. Lyell makes it his task to show, to explain the utmost vicissitudes which the climate of the earth has undergone. This course of reasoning by no means precludes such aid as may be brought in by independent verce causæ, by the concurrence of the cold period induced by excessive piling of land around the Pole with wintering in aphelion, or at a period when the earth's axis was abnormally inclined. These causes, especially in combination, would greatly intensify what, after all, must remain the ruling and inherent principle of climatic revolutions. We have only to look at the present aspect of Greenland to satisfy ourselves what might become the state of the British Isles by a mere substitution of other local conditions under the same parallel of latitude. Were the Gulf Stream done away with, the equatorial continents which now form vast reservoirs of heat transferred to the Northern regions, and their snow-clad frozen surface swept by Polar currents, how far south would the ice-sheet cover the unsubmerged tracts of land, and the glaciers come down to the level of the sea? The chain of facts and reasonings by which Sir Charles Lyell binds together the phenomena which science and discovery contribute to this intricate problem forms one of the most characteristic features of his book. Every new link, and every additional degree of tenacity given to his argument, enhances the value of this standard work as a steadfast, clear-sighted, and consistent witness to the great law of uniformity and continuity in Nature.

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