12 DISCUSSIONS IN CLIMATOLOGY. 



solid rock, that we begin to see that the work has been 

 done by running water. At first, however, we do not 

 imagine that such a chasm can have been made by the 

 streamlet in its present puny form. We conclude that 

 in former ages a great river ran down the channel. 

 We fail to give the element of time due influence in 

 our speculations. We overlook the fact that the 

 streamlet has been deepening its bed for perhaps 

 millions of years. Why, London itself might have 

 been built by one man had he been at work during 

 all the time that the streamlet was cutting out its 

 gorge. When such considerations cross the mind, 

 every difficulty vanishes, and we feel satisfied that all 

 the work has been performed by the streamlet. 



The very same may be said in regard to the origin 

 of hills, valleys, and other features of the earth's 

 surface. Yet how difficult it is still to convince some 

 geologists that our mountains have been formed, as a 

 rule, not by eruptions and upheavals, but by the slow 

 process of sub-aerial denudation. 



Cataclysmic explanations of phenomena have to a 

 large extent disappeared from the field of physical 

 geology. But there is one department in which they 

 still monopolize the field, viz., in that which treats of 

 great climatic changes in former ages. Just as in 

 physical geology great and imposing effects have been 

 attributed to extraordinary causes, so in questions of 

 geological climate vast vicissitudes have been referred 

 to equally vast and unusual agencies. 



We know that at a period comparatively recent 

 almost the entire Northern hemisphere down to toler- 

 ably low latitudes was buried under snow and ice, the 

 climate being perhaps as rigorous as that of Greenland 

 at the present day. And we know further that at 

 other periods, Greenland and the Arctic regions were 



