198' pkoOeedings of the geological society. [Mar. 5, 



The Lake of Constance. — The Lake of Constance, the largest sheet 

 of water in Switzerland, is about 50 miles in length, by about 15 

 in breadth at its broadest part. It is entirely surrounded by Mio- 

 cene strata, often considerably disturbed, and forming great hills to- 

 wards the S.E., which in a remarkable manner evince all the signs 

 of long- continued erosion by running water, — conveying the impres- 

 sion that chiefly by that means all the deep valleys of the district 

 have been worn since the close of the Miocene epoch. This lake lies 

 1298 feet above the sea ; and, its depth being 912 feet, its bottom is 

 only 386 feet above the sea. The falls of the E-hine are 1247 feet 

 above the sea ; and the lake therefore lies in an unmistakeable rock- 

 basin, the whole of which was once overflowed by the deep and broad- 

 spreading glacier of the Tipper Ehine valleys (No. 5 of the Map), 

 which stretched far northward beyond the lake into Baden and 

 "Wurtemberg. Being of greatest thickness where it entered the region 

 of the lake, by its enormous weight and grinding power it scooped 

 out, in the soft rocks below, the wide hollow now filled with water. 



The Italian Lakes. — If we now turn to the Italian side of the 

 Alps, we shall find the same phenomena prevailing in the Lakes of 

 Maggiore, Lugano, and Como, the only important lakes I have 

 yet had an opportunity of seeing south of the great chain. To 

 each of these the same reasoning applies, modified only in detail ; 

 and I shall therefore briefly pass them over. 



The most westerly, the Lago Maggiore, lies in a winding valley, 

 40 miles long, excavated in gneissic and Jurassic rocks, which rise 

 on either side in lofty mountains. The surface of the lake is 685 

 feet above the level of the sea, and near the Borromean Islands it 

 has the enormous depth of 2625 feet ; so that its bottom is ] 940 

 feet lower than the sea-level. It must, therefore, be enclosed aU 

 round by rocks, unless we suppose the narrow passage at Arena, 

 near its outlet, to be as deep as its deepest part, or that the alluvial 

 deposits of the Ticino and the Po are more than 1940 feet deep — 

 an assumption no one is likely to make. 



Of all the Alpine lakes, that of Lugano is the most irregular in 

 form, — in the language of M. Desor, stretching its arms like a 

 great polyp among the mountains in all directions *. Its surface is 

 938 feet above the level of the sea, and its depth 515 feet. Its 

 bottom is therefore only 410 feet above the sea-level, and the shape 

 of the surrounding ground renders it impossible to believe that it is 

 not entirely surrounded by rocks. 



The Lake of Como, the hoUow of which has been scooped out gene- 



* See memoirs "De la Physionomie des Lacs Suisses" (extrait de la 'Eevue 

 Suisse,' 1860) and " Quelques Considerations sur la Classification des Lacs, a 

 propos des bassins du revers meridional des Alpes," by E. Desor. The opinions 

 of M. Desor and my own do not agfee^ On the question of the origin of 

 the lake-basins of the Alps. His yiews are well expounded in the above- 

 named memoirs. It was in conversation with my friend, in 1860, that I first 

 proposed what I consider the tru.e solution of the question, and to this conver- 

 sation I presume he alludes in the latter memoir, p. 13, — " On a pretendu que 

 les lacs etaient I'eiFei de I'afFouillement des glaciers qui auraient laboure le sol sur 

 lequel ils s'avan9aient," &c. 



