1875.] 129 [Shaler. 



the condition of any rock lake-basin during the time when it was 

 deeply covered with ice, and melting from pressure was taking place 

 therein. This basin would be the seat of much more movement than 

 the other parts of the glacier's base; the change in the condition of 

 the water from solid to fluid would inevitably lead to a certain waste 

 of the ice at this point, to a continued tumbling in of the ice from 

 above, and to an incessant sliding of ice from the sides; these 

 changes would, on account of the frequent alterations of the strains 

 arising from the formation and breaking of arches over the area of 

 melting, occur with a certain paroxysmal force. These frequent 

 accidents in the glacial mass, together with the movement of the 

 water driving before it sand and pebbles, would necessarily add to 

 the erosion of the point where they occurred. For every increase in 

 the depth of the excavation, there would be a proportional increase 

 in the intensity of the melting, arising mainly from the deepening of 

 the ice-section; but also, though in a comparatively small degree, 

 from the greater heat in the bottom of the deepened pit, caused by 

 its approach to the central heat. To this we may safely attribute 

 the singular depth of many of the lake-basins within the Fjord Zone, 

 the deeper they become the greater the forces leading to their deep- 

 ening. The limit to this increase of the intensity of the deepening 

 forces would be found in the formation of a pit on the surface of the 

 ice just above the basin. The independence of movement in the 

 bottom and upper parts of the glacier sheet would prevent the forma- 

 tion of a depression on the surface of the ice, until the area of the 

 basin grew quite large. The important fact that all glacial lake- 

 basins excavated in solid rock have their greatest length in the direc- 

 tion in which the ice stream moved, shows us that there was some 

 necessary connection between the movement and the formation of 

 the basin ; this can be accounted for from the fact that the stream of 

 water made fluid by the action of pressure, would necessarily flow off 

 in the direction of the border of the ice sheet, while the principal 

 supply of ice must come from the direction in which it was thickest. 

 These two actions, arising from the entrance of the ice and its exit 

 from the basin, may well account for the elongation of these lake-ba- 

 sins in the direction of the ice movement. 



The advantage of this view over that which seeks to explain the 

 erosion of those basins by the grinding of the ice alone, is, I think, 

 manifest. The difficulty with the latter view is to account for the 

 rise of the ice from the basin after its descent into it. The shearing 



o 

 PROCEEDINGS B. S. N. H. — VOL. XVIII. 9 NOVEMBER, 1875. 



