which blow over them; for these determine, to a considerable degree, the temperature and the annual rainfall; hence the volume and excavating power of rivers, etc. The higher portions of continents, as mountain-chains and plateaux, are colder than the lowlands, and hence become condensers of moisture—places where snow accumulates and glaciers are formed.
A striking illustration of the influence of topography on climate is shown by the high mountains of the tropics, where perpetual snow and glaciers are coexistent with extreme tropical conditions, not only on the same parallel, but within a narrow area. It is evident, then, that topographical changes such as could be easily conceived would readily and perfectly accomplish all the alternations of climate of which we have any evidence in geological history. Recognizing the potency of topographical causes, Lyell sought for, and thought he had found, a sufficient explanation of the contrast between the climates of the Ice period and the present, in changes in the physical geography of the northern hemisphere; assuming and believing that the Glacial period was marked and caused by great elevation and breadth of land-surface about the pole, and, as a corollary and consequence of this proposition, a depression of land and a broadening of oceanic surfaces in the temperate and tropical zones.
This theory affords so simple an explanation of the problem of the Ice period, that it at first strongly commends itself to those who are most cautious and logical in their modes of thought and investigation. Modern science is eminently conservative, and one of the first lessons learned by the investigator of this age is, to exhaust all known causes of phenomena before appealing to the unknown. Still, however plausible this view may be, it must be sustained by solid and substantial proof before it deserves to be regarded as anything but a theory, and before it can be accepted as a rule of faith and practice among geologists. Unfortunately, such proof is not only yet wanting, but there are many facts which, in the light of our present knowledge, seem to indicate that it will never be obtained. The theory of Lyell has, however, been adopted by Prof. Dana, in the last edition of his "Manual," where he says (p. 541), "The occurrence of an Ice period was probably dependent mainly, as suggested by Lyell, on the extension and elevation of the land over the higher latitudes." Prof. Dana has further elaborated and applied the Lyellian hypothesis by suggesting that in the Glacial period barriers of land connected the continents of the two hemispheres, and excluded the tropical currents from the polar seas, in this way cutting off the most powerful equalizing influences, and inducing an exaggeration of the heat of the tropics and the cold of the polar regions. He also claims that high and broad land-surfaces in the circumpolar areas formed great condensers and refrigerators, upon which the moisture, freely and rapidly evaporated from the seething caldron of the circumscribed
