282 THE POPULAR SCIENCE MONTHLY. 



which blow over them ; for these determine, to a considerable degree, 

 the temperature and the annual rainfall ; hence the volume and exca- 

 vating 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. Kecognizing the 

 potency of topographical causes, Lyell sought for, and thought he 

 had found, a sufficient explanation of the contrast between the cli- 

 mates 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 conse- 

 quence 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 investiga- 

 tion. 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 substan- 

 tial 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," w T here he says (p. 541), "The occurrence of an Ice period 

 was probably dependent mainly, as suggested by Lyell, on the exten- 

 sion and elevation of the land over the higher latitudes." Prof. Dana 

 has further elaborated and applied the Lyellian hypothesis by sug- 

 gesting that in the Glacial period barriers of land connected the 

 continents of the two hemispheres, and excluded the tropical cur- 

 rents 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 



