188 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Mar. 5, 



mountains, — one down the Linth, through, the area now occupied by 

 the Lake of Zurich (No. 4), another down the Upper Eeuss, across 

 the area in which lie the Lakes of Lucerne, Zug, and others (No. 3), 

 and a third down the valley of the Aar to Berne, through the country 

 that now contains the Lakes of Brienz and Thun (No. 2). Accord- 

 ing to this view (the result of the researches of the best Swiss 

 geologists), the greater part of the Swiss Miocene area lay deep under 

 ice*, and I am inclined to think that the country between the great 

 old glaciers of the Eeuss, Aar, and Rhone was much more covered 

 with ice than any map shows, the whole helping to swell the pro- 

 digious glacier of the Rhone that abutted on the Jura. 



Connexion between Tarns and Glaciers. — In ' The Old Glaciers of 

 North Wales ' I have shown that in all glacier- countries, whether 

 past or present, there is an intimate connexion between tarns and 

 glaciers. Some of these are dammed by old moraines f, but the 

 greater number lie in rock-basins, formed by the grinding of glacier- 

 ice as it passed across the country, whether in valleys, on rough table- 

 lands, or on the watersheds of passes. These lakes and pools are of all 

 sizes, from a few yards in width, lying amid the mammiUations of the 

 roches moutonnees, to several miles in diameter. Sometimes in the 

 convolutions of the strata (conjoined with preglacial denudation 

 subsequent to the contortion of the beds), softer parts of the country 

 may have been scooped out, leaving a hollow surrounded by a frame- 

 work of harder rock ; but perhaps more generally they were formed 

 by the greater thickness and weight, and consequently proportionally 

 greater grinding pressure, of glacier-ice on particular areas, due to 

 accidents to which it is now often difficult or impossible to find the 

 clue. Trifling as this phenomenon at first sight may seem, I yet believe 

 the manner of the formation of these lakes is of much importance to 

 the right understanding of the glacial theory, whether taken in con- 

 nexion with the great extension of extinct glaciers in recognized 

 glacier-regions, or, further, when viewed on a general continental 

 scale ; for the theory of the glacial origin of many rock-basins must, I 

 feel convinced, be extended much beyond such mountain- districts as 

 Switzerland, Wales, and the Highlands of Scotland, where they first 

 attracted my attention $. 



Origin of the Great Alpine Lakes. Subject stated, — From the con- 

 sideration of the origin of mountain-lakes and tarns, the question 

 easily arises, — ^What are the causes that have operated in the formation 

 of the great lakes of Switzerland, such as those of Geneva, Zurich, 

 and Constance, and, south of the Alps, of Maggiore, Lugano, Como, 



* The limits of the northern glaciers on the Map (PI. YIII.) are chiefly given 

 from a MS. map compiled by M. Morlot. Those on the south are taken from a 

 map by M. de Mortillet. Both were lent me by Sir Charles Lyell. 



t Quart. Journ. Geol. Soc. 1851, vol. viii. p. 371 ; and ' Old Glaciers of Iforth 

 Wales.' 



X It is not to be supposed that I attribute the origin of all rock-basins to 

 glacial action. Many lie in the craters of extinct volcanos, some, no doubt, in 

 areas of special subsidence, and others may be due to causes of which I know 

 nothing. I now confine my remarks to certain lakes common in all highly 

 glaciated regions such as I know. 



