THE WORK OF SNOW AND ICE. 323 



traction of the glacier, analogous to the creeping of a mass of lead on a 

 roof; 



(7) James Thompson demonstrated that pressure lowers the melting- 

 point, and while this effect is so small as probably to be ineffectual, it is 

 correlated with the very important fact that compression may cause 

 melting, which is not the case in most other rocks. He recognized that 

 under pressure partial liquefaction took place, that the water so liber- 

 ated might be refrozen as it escaped from pressure, and appears to have 

 regarded this as a vital factor ; 



(8) CroU held that the movement was due to a consecutive series 

 of molecular changes somewhat like the chain of chemical combinations 

 in electrolysis; 



(9) Hugi, Eli de Beaumont, Bertin, Forel, and others thought that 

 the growth of the granules was the leading factor in the ice movement ; 



(10) McConnel and Miigge have made the gUding planes of the ice 

 crystals serve an important function in glacial movement. 



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

 Thompson laid the basis, the crystaUization of descending w^ater, 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 w^hich 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. The move- 

 ment of the gUding planes of the ice crystals over each other, which has 

 been looked upon as a special kind of viscoid movement, probably plays 

 a large part in the shearing movements in certain cases. 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 primary movements in the snow-fields where glacial action 

 begins. 



