324 CLIMATIC CONDITIONS OF THE GLACIAL EPOCH. 



line on mountain slopes is clearly dependent on climatic conditions, and not 

 on the pressure to which the mass may happen to be subjected. The phe- 

 nomenon of the conversion of snow into ice by imbibition with water and 

 subsequent freezing is one which occasionally takes place on a large scale 

 and over an extensive area in our own land and climate. A deep snow-fall 

 may in a few liours be entirely changed to ice, and those who live in New 

 England have Iiad, within a few years, occasion to observe this process occur- 

 rino; on a larg-e scale.* 



That mere pressure alone will not, as maintained by Tyndall and others, 

 change snow into ice in nature — whatever it may do in the laboratory — 

 seems proved by the conditions presented by snow masses in regions where 

 rain never, or hardly ever, falls. For instance, accumulations of snow miles 

 in length and hundreds of feet in depth lie from year to year on the slopes 

 of Mount Shasta without exhibitino; the slio;htest sign of conversion into ice. 

 The climate is too diy to allow any moisture to remain in contact with the 

 snow long enough to allow of imbibition. Neither does it ever rain in sum- 

 mer; and in winter all the precipitation, which is extremely abundant, is 

 in the form of snow ; so that the sno\y remains dry, and never shows any 

 tendency to become converted into ice. It is only on the north side of the 

 mountain, directly imder the summit, where the prevailing winds would carry 

 what little moisture is formed, and allow it in part at least to become con- 

 densed, that small masses of ice exist ; these may, however, be the relics of 

 the once largely developed system of glaciers occurring on the summit and 

 western slope of the Sierra Nevada, formed when climatic conditions were 

 different from what they are at present : it is hy no means certain that, if 

 removed now, they would be replaced by formation out of the snow-fall of 

 the existing period. 



Glacier ice is not simple ice, but a mixture of ice and water, and it is to 

 the presence of the latter that the whole mass owes its flexibility. The larger 

 the amount of water, other things being equal, the more easily the glacial 

 mass moves. When the water increases so as to get the upper hand, the ice 

 gives way witli a rush, and becomes an avalanche, as is well known to be the 

 case with certain glaciers in Tvrol and the Caucasus. The extreme varia- 

 bility of the rate of motion of different glaciers coming down from the inland 

 ice of Greenland is due to the different amounts of water which they have 



* During the winter of 1875 - 76 there was :i miirkeil instance of this kind ; the surface of New England from 

 Connecticut to bevonil llassuchnsetts became covered with a continuous sheet of ice. 



