414 Geological Society. 



as to produce open cracks ; on the contrary they are compressed late- 

 rally and crumpled tip into smaller space ; and the uppermost strata, 

 that pressed heavily on the crumpled rocks now visible, would pre- 

 vent the formation of wide open fractures below, — these upper strata, 

 as in North Wales, having, over a great part of the area, been 

 mostly or altogether removed by denudation. Next, lakes of the 

 rock-basin kind do not lie each in an area of special subsidence. 

 If so, for instance, we should require one for the Todten See, one 

 for the Grimsel, one for the ancient lake of the Kirchet, several at 

 the foot of the Siedelhorn, many hundreds close together in Suther- 

 landshire, and thousands in North America. 



If then the lake-basins were formed by none of the above-named 

 causes, the only other agent that has affected the country on a great 

 scale is glacier ice. All the lakes lie directly in the courses of the 

 ancient glaciers. The basin of the Lake of Geneva is 950 French 

 feet deep near its eastern end, and was scooped out by the great 

 glacier of the Rhone, the ice of which, from data supplied by Char- 

 pentier, was, as it issued from the valley, 3550 feet thick to the 

 bottom of the lake. This great weight of ice ground out the hollow 

 of the lake, which gradually shallows towards Geneva, where the 

 glacier thinned and the grinding-power was lessened. Where the 

 same glacier abutted on the Jura, the ice-current was arrested, and 

 it flowed to the N.E. and S.W. ; and where the ice was thickest and 

 heaviest, above the Lake of Neuchatel, it ground out the hollow in 

 which the lake lies. 



The lakes of Thun and Brienz lie in the course of the great Aar 

 glacier, those of Zug and the Four Cantons in that of Altorf, the 

 Lake of Zurich lies in that of the Linth, the Lake of Constance in 

 the course of the prodigious glacier of the Rhine Valleys, the nume- 

 rous little rock-basin lakes near Ivrea in the line of the glacier of 

 the Val d'Aosta, and those of Maggiore, Lugano, and Como in the 

 courses of the two gigantic glacier-areas that drained the moun- 

 tains between Monte Rosa and the Sondrio. 



The sizes of the lakes and their depths were then shown to be, 

 in several cases, proportional to the magnitude of the glaciers that 

 ground out the basins in which they lie, and to the circumstance as 

 to whether the pressure of ice was broadly diffused, or vertical as in 

 narrow valleys. 



Finally, it was shown that rock-basins holding lakes are always 

 exceedingly numerous in and characteristic of all countries that have 

 been extensively glaciated. Lakes are comparatively few in the 

 southern half of North America ; but immediately south and north of 

 the great lakes and the St. Lawrence, the whole country is moutonnie 

 and striated, and is also covered with a prodigious number of rock- 

 basins holding water. The same is the case in the North of Scot- 

 land, the whole area of which has been moulded by ice ; and east of 

 the Scandinavian chain, in another intensely glaciated region, the 

 country is covered by innumerable lakes. 



