322 CLIMATIC CONDITIONS OF THE GLACIAL EPOCH. 



any former time ; or, in otiier words, that we are not now nearer a period of 

 "general glaciation " than we have ever before been. 



It cannot fail to have been noticed in the synoptical view of present 

 glaciation which has been presented in the preceding pages, that glaciers 

 and permanent snow-fields are the exclusive appendages of mountain slopes 

 and high tal)le-lands. Nowhere on earth, not even in the highest northern 

 latitudes, cnn a glacier be seen which has originated on level land, and 

 developed itself independently of some adjacent higher region. That this 

 should be the case in low latitudes, it is very easy to understand ; it is be- 

 cause in such we cannot have a terajierature sufficiently low for snow to fall 

 or exist at all, unless we rise to a certain elevation above the sea-level. 

 That it is true also for high latitudes, where the mean temperature at the 

 sea-level is below the freezing-point, and where otiier conditions seem such 

 as to favor the formation of glaciers, is a fact not so easily explained. It is, 

 however, one of so much interest in connection with theories of the Glacial 

 epoch that it merits special examination, and we may perhaps best arrive at 

 a clear idea of the nature of the problem, by endeavoring to answer the 

 question: Can glaciers originate on level land under any circumstances? 

 That they do not, at present, will be admitted, and tlie point is, What pos- 

 sible modification of prevailing climatic or other conditions would permit 

 this to take place ? To obtain an answer to this important question, it will 

 be necessary first to ascertain what is the essential difference, as respects the 

 accumulatiou of snow and the resultant formation of ice from it, between 

 level areas and the summits and .slopes of mountains. The difference we 

 conceive to be simply this: that elevated regions may be, and often are, 

 regions of cold, and at the same time of large precipitation, while flat regions 

 are not. That this should be the case clearly results, as it appears to the 

 writer, from the general principles in regard to precipitation to which our 

 attention has been turned in the preceding chapter. That it is so, seems 

 evident enough when we survey the entire area covered by ice and snow 

 throughout the world. Over those immense tracts of low land which lie near 

 and beyond the Polar Circle, in both Asia and North America, the snows of 

 winter do not accumulate in such quantity that the next sunnner's sun does 

 not entirely melt them away. On the high plateaux of Central Asia, where the 

 mean temperature is very low, the snow-fall is nearly all intercepted by the 

 adjacent ranges, which rise still higher. A plateau which was itself the high- 

 est land of any region might become deeply covered by snow, if exposed to 



