Davis.] 34 [May 3, 



ice, and its upper and lower members are often separated by lay- 

 ers of gravel and other deposits, which have given rise to the 

 theory of two or more periods of glacial extension in quaternary 

 time. Further, on an apparently intermediate area, which I am not 

 yet able to define, the ground moraine has frequently resisted the 

 efforts of the ice to rub it along to the margin, and has accumu- 

 lated in masses of considerable size, which took a form of least 

 resistance as they grew beneath the glacier : these are known as 

 drums, drumlins, whalebacks or lenticular hills (see B 10). 



J. Geikie says " wherever the flow of the ice-sheet slackened there would 

 necessarily be less erosive action, and therefore a greater chance of pregla- 

 ial and interglacial land-surfaces being preserved. For the same reason 

 we ought to find, as we approach the limits reached by the old confluent 

 glaciers, that preglacial and interglacial deposits have suffered less erosion. 1 ' 

 " In exposed positions, such as hilltops and hill slopes, the till never con- 

 tains intercalated beds." (Great Ice Age, 1877, 137, 133.) 



Later he compares the old ice-streams to rivers, inasmuch as they erode 

 in their upper valleys, and deposit on the more open flood-plains. In the 

 central regions, where the erosive action was great, little or no boulder 

 clay was allowed to gather, and hollows of smaller or larger dimensions 

 were scooped out, when the nature of the ground was favorable to that end. 

 Farther on, where the grinding power exerted by the ice was less, thick 

 boulder clay frequently accumulated, and subglacial and interglacial beds 

 were often preserved. In hilly regions there will be roches mouionnees 

 and lakes, but little till ; cliffs and escarpments will yield large fragments; 

 on open lowlands, where the ice has thinned by spreading and its motion 

 has slackened, much till will be deposited, and near its margin it will pro- 

 duce very little disturbance. (Prehistoric Europe, 1881, 288, 289.) 



Helland writes (freely translated and condensed) : The glaciated area 

 consists of two parts; the region of erosion, and the region of deposition 

 of the great ice-streams. The first includes Norway, Sweden and Finland, 

 where bare glaciated rocks and rock-basins occupy much of the surface ; 

 the second is the North European plain, where the rocks, seldom striated, 

 are generally hidden by glacial deposits. (Deutsch. Geol. Ges. Zft. xxxi, 

 1879, 99). 



A. Penck adopts these views and says that land-ice accumulates under its 

 peripheral parts, the material eroded from near its source. (Vergletscherung 

 der Deutschen Alpen, Leipzig, 1882, 199, 98.) 



The following examples will illustrate these general statements. 



B. 5. The Alps. In the upper valleys of Switzerland, I find no 

 account of old gravels, nor is there in these elevated districts, 



