THE "GREAT ICE AGE." 167 



The thinning agency at work upon such glaciers during their 

 journey over the terra firma being the outflow of terrestrial 

 heat and that due to their friction upon their beds, this thin- 

 ning must all take place from below, and thus, as the glacier 

 proceeds downward, these rock fragments must be continually 

 approaching the bottom instead of continually approaching the 

 top, as in the case of modern Alpine glaciers flowing below the 

 snow-line, and thawing from surface downward. 



It follows, therefore, that such glaciers could not deposit any 

 moraines such as are in course of deposition by existing Alpine 

 and Scandinavian glaciers. 



What, then, must become of the chips and filings of these 

 outfloating glaciers ? They must be carried along with the 

 ice so long as that ice rests upon the land ; for this debris must 

 consist partly of fragments imbedded in the ice, and partly of 

 ground and reground excessively subdivided particles, that 

 must either cake into what I may call ice-mud, and become a 

 part of the glacier, or flow as liquid mud or turbid water be- 

 neath it, as with ordinary glaciers. The quantity of water be- 

 ing relatively small under the supposed conditions, the greater 

 part would be carried forward to the sea by the ice rather than 

 by the water. 



An important consequence of this must be that the erosive 

 power of these ancient glaciers was, cceteris paribus, greater 

 than that of modern Alpine glaciers, especially if we accept 

 those theories which ascribe an actual internal growth or regen- 

 eration of glaciers by the relegation below of some of the water 

 resulting from the surface-thaw. 



As the glacier with its lower accumulation advances into 

 deeper and deeper water, its pressure upon its bed must pro- 

 gressively diminish until it reaches a line where it would just 

 graze the bottom with a touch of feathery lightness. Some- 

 where before reaching this it would begin to deposit its burden 

 on the sea-bottom, the commencement of this deposition being 

 determined by the depth whereat the tenacity of the deposit, 

 or its friction against the sea-bottom, or both combined, be- 

 . comes sufficient to overpower the now-diminished pressure and 

 forward thrusting, or erosive power of the glacier. 



Further forward, in deeper water, where the ice becomes 

 fairly floated above the original sea-bottom, a rapid under- 

 thawing must occur by the action of the sea-water, and if any 

 communication exists between this ice-covered sea and the 

 waters of wanner latitudes, this thawing must be increased by 



