JDr. James Croll — On Geological Time. 395 



According to others, elevation of the land in the regions glaciated 

 is assigned as the cause of that glaciation, and if the ice had been 

 mei-ely local, such an explanation might have sufficed. But we know 

 the whole Northern Hemisphere down to tolerably low latitudes has 

 been subjected in post-Tertiary times to the rigour of an Arctic climate ; 

 so that according to this theory we must assume an upheaval of the 

 entire hemisphere — an assumption too monstrous to be admitted and 

 as useless as absurd. 



Tendency in Geology to Cataclysmic Theories. — There has always 

 been in Geology a tendency to cataclysmic theories of causation ; a 

 proneness to attribute the grand changes experienced by the earth's 

 crust to extraordinary causes. Geologists have only slowly become 

 convinced that those changes were the eilects of the ordinary agencies 

 in daily operation around us. For example, hills were formerly 

 supposed to be due to sudden eruptions and upheavals ; valleys to 

 subsidences, and deep river gorges to violent dislocations of the earth. 

 All this is now changed, and geologists in general have become con- 

 vinced that the main features of the earth's surface owe their exist- 

 ence to the silent, gentle, and continuous working of such influences 

 as rain and rivers, heat and cold, frost and snow. 



It is not difficult to understand why a belief in cataclysms should 

 so long have prevailed, and geologists should have been so prone to 

 assume the existence of extraordinarj'' causes acting with great force. 

 Geological phenomena come directly under the eye in all their 

 magnitude, and consequently produce a powerful impression on the 

 mind. The quiet and gentle operations of nature's ordinary agencies 

 appear utterly inadequate to produce results so stupendous ; and one 

 naturally refers effects so striking to extraordinary causes. Beholding 

 in a moment the effect, we forget that the cause has been in operation 

 for countless ages. 



We look for example at a gorge, perhaps a thousand feet in depth, 

 with a small streamlet running along its bottom. Our first im- 

 pression is that this enormous chasm has been formed by some 

 earthquake or other convulsion of nature rending the rocks asunder. 

 And it is only when we examine the chasm more minutely, and find 

 that it has been actually excavated out of the 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, 



