GEOLOGY. 255 



our terra jirma, that we can readily imagine that local changes of 

 level in the land have not only greatly affected the breadth of water 

 surface in the lake basin, but have perhaps in some instances pro- 

 duced what we have supposed to be proofs of great and general ele- 

 vations of the water level, which are, in fact, only indications of a 

 local rise of the land. 



THE GLACIAL ORIGIN OF LAKES. 



In a paper recently read before the Geological Society (London), 

 Professor Ramsay gave reasons for considering that the great Alpine 

 lakes, such as Geneva, Zurich, Constance, Maggiore, Lugano, Conio, 

 and others, " do not lie among the strata in basins merely produced 

 by disturbance of the rocks, but in hollows due to denuding agencies 

 that operated long after the complicated foldings of the miocene and 

 other strata were produced." He remarked that none of these lakes 

 lie in simple synclinal troughs, and that in no case of lakes among 

 the Alps is it possible to affirm that we have a synclinal hollow, of 

 which the original uppermost beds remain ; neither do they lie in 

 areas of mere watery erosion. Neither running water nor the still 

 water of lakes can scoop large hollow basins, like those of the lakes, 

 bounded on all sides by rocks. Running water may fill them up, but 

 cannot excavate them. Prof. Ramsay next argued that these lakes 

 do not lie in lines of gaping fracture. A glance shows this, with re- 

 spect to such lakes as those of Geneva, Neuchatel, and Constance; 

 and, reasoning on the nature of the contortion of the strata of the 

 Alps, he contended that, though fractures of the rocks must be com- 

 mon, they need not be gaping fractures. To produce such a mountain 

 chain, the strata are not upheaved and stretched so as to produce open 

 cracks ; on the contrary, they are compressed laterally and crumpled 

 up into smaller space, and the uppermost strata, that pressed heavily 

 on the crumpled rocks now visible, would prevent 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 re- 

 moved by denudation. Next, lakes of the rock-basin kind do not lie 

 in an area of special subsidence. If so, for instance, we should 

 require one for the Todten Sea, one for the Grimsel, one for the 

 ancient lake of the Kirchet, several at the foot of the Siedelhorn, 

 many,. hundreds close together in Sutherlandshire (England), 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 about 1,200 feet thick when it abutted upon the moun- 

 tains, and 2,780 feet thick when it first flowed out upon the plain at 

 the mouth of the valley. Add to this the depth of the lake of 984 

 feet, and the total thickness of the ice must have been 3,764 feet at 

 what is now the eastern part of the lake. " I conceive, then," says 

 Prof. Ramsay, " that this enormous mass of ice, pushing first north- 



