380 CLIMATIC CONDITIONS OF THE GLACIAL EPOCH. 



phenomena of the drift to be chiefly due to the agency of currents of water. 

 In one important respect, however, we should differ from these distinguished 

 pioneers of the science in this country, namely, in that the view adopted by 

 them Avas, almost without exception, to the effect that it was the water of 

 the ocean which did the work. Tlie currents were marine, as they believed, 

 and not fluviatile. The reference of work of this kind to the agency' of the 

 ocean was, in the early days of geological inquiry, a thing of almost universal 

 occurrence: at the present time the importance of rivers as agents of geolog- 

 ical change is much more generally recognized — that is, by those Avho do 

 not consider everj'thing the work of ice. In this case, however, there can 

 be no doubt as to the absence of the sea over a large part of the Drift region, 

 during at least a considerable portion of the Glacial period, for not only are 

 marine fossils entirely wanting there, but positive proofs exist of the jiresence 

 of fresh water, or of land surfaces, in the form of buried vegetation, and 

 remains of land and fresh-water animals. 



That the courses and positions of the former rivers cannot be made out 

 with much of any detail is not a matter to excite surprise. Even on the 

 west slope of the Sierra Nevada, a district so much smaller in extent than the 

 great Northern Drift region, with all the help afforded by extensive mining 

 operations, it has often been impossible to reconstruct the former drainage, 

 although its general character coidd be recognized without difficulty. And 

 if, in spite of the very marked topographical features of the Californian 

 region, there have been many cases in which isolated fragments of old river 

 channels could not be brought into relations with each other, although closely 

 adjacent and similarly situated, how much less likely that this could be done 

 over a country of so varied a character, with such irregular lines of water- 

 shed, and with so marked an absence of a dominating range as we find in 

 the Northern Drift region. 



No geologist denies the agency of water in the formation and deposition 

 of at least a considerable portion of the drift. The evidence is plain that 

 much of it has been deposited in lacustrine basins ; that a large part of 

 the finer materials has resulted from the decomposition or solution of the 

 rock in place, having been moved either not at all or but a short distance 

 from its original position ; that almost all the rounding and the arrangement 

 in stratified form has been the work of water. 



The question now arises. What are the obstacles to ascribing all the phe- 

 nomena of the drift to the agency of water? The principal one. no doubt, is 



