288 GEOLOGY 



It will be seen that the principle of partial liquefaction for which 

 Thompson laid the basis, the crystallization of descending water, 

 urged by Charpentier and Agassiz, and the granular growth on 

 which Hugi, Beaumont, Forel, and others founded their hypotheses, 

 are incorporated in the view already presented. Probably the 

 agencies on which some of the other views are based may also be 

 participants in producing glacial motion, sometimes as incidental 

 factors, and sometimes perhaps as important ones, for under 

 different conditions, different agencies may play roles of varying 

 importance. For example, in going over the brinks of precipices 

 of sufficient height, glaciers break into fragments which are re- 

 cemented below, and the " reconstructed" glacier moves on as 

 before. Here fracture and regelation are evident, though they are 

 hardly causes of motion. The movement of the gliding planes of 

 the ice crystals over each other, which has been looked upon as a 

 special kind of viscoid movement, probably plays a part in shearing. 

 But neither of these is probably a large factor in ordinary glacial 

 movement, and it seems highly improbable that any of them are 

 essential factors in the movements in the snowfields where glacial 

 motion begins. 



Map work. The effects of glaciation are well shown on many of the 

 topographic maps of the U. S. Geological Survey. Lists of such maps are 

 found on pages 230 and 289 of the junior author's Physiography, Advanced 

 Course. Plates XCV to CXXIX, found in Professional Paper 60, U. S. 

 Geological Survey, present a number of maps illustrating this topic. 



