170 RELATION OF LAKES TO GLACIAL ACTION. [Cn. XII. 



said of most of the longitudinal and transverse valleys of every moun- 

 tain chain. Mr. Jukes has well observed that lake-basins are by no 

 means caused by rents gaping or widening in their higher extremities ; 

 and he adds that where fissures have been examined by miners in the 

 interior of the earth, whether the rocks have been shifted or not, they 

 are usually only a few feet wide, and even when traced for more than 

 1000 feet in a vertical direction, they preserve a remarkable uniformity 

 in width. Nor are valleys and lake-basins the result of engulfment or 

 the swallowing up in subterranean abysses of masses once at or near 

 the surface. Had this been the case, we should not find, as we now 

 do, the same strata often continuous from side to side, at the upper and 

 lower ends of the lake. It is evident that the materials which once 

 filled the basin have been gradually removed, so that older formations 

 are now exposed to view on the bottom. It may be said of the par- 

 ticular masses of rock now constituting the sides of such cavities, as 

 we may affirm of valleys in general, that they were never nearer each 

 other than they are at present. The only question, then, to be discussed 

 is, whether the denuding cause was ice or running water — a glacier or 

 a river. 



At the foot of every cataract we see that the water has formed a 

 deep circular pool. In like manner it is suggested that ice, descend- 

 ing a precipice or steep slope, and rubbing off sand and stones from 

 the surface of the same, may, when it reaches the bottom and presses 

 on it with its whole weight, so grind down and wear away the rock, 

 as to scoop out one of those cavities called tarns. But if we admit 

 such a process as matter of speculation, we must at the same time 

 suppose that after it has worked out a cavity it loses all power to ex- 

 tend the same, being wholly unable to cut a gorge through the barrier 

 forming the lower margin of the tarn at the point where the discharge 

 of ice formerly took place, and where a stream now issues. This di- 

 minished force of erosion wherever the ice has to ascend a slope, or to 

 move horizontally, seems adverse to the hypothesis advanced by 

 Professor Eamsay of the formation of lakes of considerable length 

 and depth by glaciers. Yet the advocates of the origin of lakes by 

 ice-action do not hesitate to appeal to the same causation to account 

 for the largest Swiss and Italian lakes at the northern and southern 

 foot of the Alps, such as those of Geneva, Como, and Lago Maggiore, 

 which vary from twenty to nearly fifty miles in length, and in depth 

 from 500 to 2000 feet, and more. 



In speculating on such a mode of origin, we feel greatly the want 

 of positive data, which might enable us to test the actual power of a 

 glacier to scoop cavities out of a floor of subjacent rock. It may, 

 however, be remarked, that where opportunities are enjoyed of seeing 

 part of a valley from which a glacier has retreated in historical times, 

 no basin-shaped hollows are conspicuous. Domeshaped protuberances, 

 the " roches moutonnees" before described, are frequent ; but the con- 

 verse of them, or cup and saucer-shaped cavities, are wanting. Every- 



