358 GEOLOGY. 



the west, 142 feet per mile, for the first 1000 meters of ascent. For 

 the second 1000 meters of rise, the slopes were 93 and 63 feel per mile, 

 respectively; while for that pari of the snow-field more than 2000 

 meters high, and more than 50 miles from the east edge and more than 

 76 miles from the west edge, the slope ranged from 26 to 37 feet per 

 mile. From these data it is fair to conclude that if the ice-sheet were 

 much larger, like that of our continent during the glacial period, the 

 gradient would be still less toward its center. 



Stages in the history of an ice-sheet. — The history of an ice-sheet 

 which no longer exists involves at least two distinct stages. These 

 are (1) the period of growth, and (2) the period of decadence. If 

 the latter did not begin as soon as the former was completed, an inter- 

 vening stage, representing the period of maximum ice extension, must 

 be recognized. In the case of the ice-sheets of the glacial period, each 

 of these stages was probably more or less complex. The general period 

 of growth of each ice-sheet is believed to have been marked by tem- 

 porary, but by more or less extensive, intervals of decadence, while 

 during the general period of decadence, it is certain that the ice was 

 subject to temporary, but to more or less extensive, intervals of recru- 

 descence. 



Tn the study of the work accomplished by an ice-sheet, it is of 

 importance to distinguish between these main stages, and, in the last 

 analysis, to take account of the oscillations of the edge of the ice in 

 each. 



The Work of an Ice-sheet. 



Glacial erosion and glacial deposition have been briefly discussed 

 in Volume I (p. 281-305). It need only be added here that the surface 

 over which the ice-sheets moved is believed to have had a topography 

 which had been shaped, so far as details are concerned, by rain and 

 river erosion, and was covered by a layer of mantle-rock which origi- 

 nated in the decay of the formations beneath. The ice removed this 

 mantle of decomposed material, and cut deeply into the undecayed 

 rock beneath. The best rough measure of the ice erosion is the great 

 body of drift, much of which is composed of rock debris, which lay 

 beneath the decayed horizon at the surface. In effecting this erosion, 

 the ice modified the preexisting topography' to some extent, for 

 weaker terranes were eroded more than resistant ones, and the topog- 



