68 THE POPULAR SCIENCE MONTHLY. 



tiary periods, temperate conditions seem to have prevailed even to tlie 

 pole. Of the many theories as to these changes which have been 

 proposed, two seem at present to divide the suffrages of geologists, 

 either alone, or combined with each other. These are — 1. The theory 

 of the precession of the equinoxes in connection with the varying ec- 

 centricity of the earth's orbit, advocated more especially by Croll ; 

 and, 2. The different distribution of land and water as affecting the 

 reception and radiation of heat and the ocean-currents — a theory ably 

 propounded by Lyell, and subsequently extensively adopted, either 

 alone or with the previous one. One of these views may be called the 

 astronomical ; the other, the geographical. I confess that I am in- 

 clined to accept the second or Lyellian theory, for such reasons as the 

 following : 1. Great elevations and depressions of land have occurred 

 in and since the pleistocene, while the alleged astronomical changes 

 are not certain, more especially in regard to their probable effect on 

 the earth. 2. When the rival theories are tested by the present phe- 

 nomena of the southern polar region and the North Atlantic, there 

 seem to be geographical causes adequate to account for all except ex- 

 treme and unproved glacial conditions. 3. The astronomical cause 

 would suppose regularly recurring glacial periods of which there is no 

 evidence, and it would give to the latest glacial age an antiquity which 

 seems at variance with all other facts. 4. In those more northern re- 

 gions where glacial phenomena are most pronounced, the theory of 

 floating sheets of ice, with local glaciers descending to the sea, seems 

 to meet all the conditions of the case ; and these would be obtained, in 

 the North Atlantic at least, by very moderate changes of level, caus- 

 ing, for example, the equatorial current to flow into the Pacific, instead 

 of running northward as a gulf stream. 5. The geographical theory 

 allows the supposition not merely of vicissitudes of climate quickly 

 following each other in unison with the movements of the surface, but 

 allows also of that near local approximation of regions wholly covered 

 with ice and snow, and others comparatively temperate, which we see 

 at present in the north. 



If, however, we are to adopt the geographical theory, we must 

 avoid extreme views ; and this leads to the inquiry as to the evidence 

 to be found for any such universal and extreme glaciation as is de- 

 manded by some geologists. 



The only large continental area in the northern hemisphere sup- 

 posed to be entirely ice- and snow-clad is Greenland ; and this, so far 

 as it goes, is certainly a local case, for the ice and snow of Greenland 

 extend to the south as far as 60° north latitude, while both in Norway 

 and in the interior of North America the climate in that latitude per- 

 mits the growth of cereals. Further, Grinnell Land, which is separated 

 from North Greenland only by a narrow sound, has a comparatively 

 mild climate, and, as Nares has shown, is covered with verdure in 

 summer. Still further, Nordenskiold, one of the most experienced 



