124 SCIENCE IN SHORT CHAPTERS. 



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 regeneration 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 

 progressively diminish until it reaches a line where it would 

 just graze the bottom with a touch of feathery lightness. 

 Somewhere 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 tena- 

 city of the deposit, or its friction against the sea-bottom, or 

 both combined, becomes sufficient to overpower the now- 

 diminished pressure and forward thrusting, or erosive power 

 of the glacier. 



Father 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 warmer latitudes this thawing must be in- 

 creased by the currents that would necessarily be formed by 

 the interchange of water of varying specific gravities. De- 

 position would thus take place in this deeper water, contin- 

 ually shallowing it or bringing up the sea-bottom nearer to 

 the ice-bottom. 



This raising of the sea-bottom must occur not only here, 

 but farther back, i.e., from the limit at which deposition 

 commenced. This neutral ground, whereat the depth is 

 just sufficient to allow the ice to rest lightlj 7 on its own de- 

 posit and slide over it without either sweeping it forward 

 or depositing any more upon it, becomes an interesting 

 critical region, subject to continuous forward extension dur- 

 ing the lifetime of the glacier, as the deposition beyond it 

 must continually raise the sea-bottom until it reaches the 

 critical depth at which the deposition must cease. This 

 would constitute what I may designate the normal depth of 

 the glaciated sea, or the depth towards which it would be 

 continually tending, during a great glacial epoch, by the 

 formation of a submarine bank or plain of glacier deposit, 



