252 THE POPULAR SCIENCE MONTHLY. 



In connection with this subject may be noticed the many cases 

 in which Alpine valleys present indications of having been greatly 

 deepened by glacial erosion, although, owing either to the slope 

 of the ground or the uniformity of the ice action, no lake has 

 been produced. In some valleys, as in that of Lauterbrunnen, 

 the trough between the vertical rock walls was probably partly 

 formed before the Ice age, but was greatly deepened by glacial 

 erosion, the result being that the tributary streams have not since 

 had time to evacuate ravines of equal depth with the main val- 

 ley, and therefore form a series of cascades over the lateral preci- 

 pices, of which the Staubbach is the finest example. In many 

 other cases, however, the side streams have cut wonderfully nar- 

 row gorges by which they enter the main vally. This work was 

 probably begun by a subglacial stream, and the action of the 

 atmosphere being shut out by the superincumbent ice and all va- 

 riation of temperature avoided, the torrent cut for itself a very 

 narrow groove, sometimes with overhanging sides, as it found 

 layers of somewhat softer rock to eat away ; and the upper sur- 

 face of the rock being ground smooth by the ice, the atmosphere 

 has had little effect since, and the gorge, while deepened below, 

 has remained as restricted above as when first eroded. Such are 

 the gorges of the Trient, Leuk, Pfaffers, and many others well 

 known to Alpine tourists. I am not aware whether such ex- 

 tremely narrow winding gorges, often only two or three feet be- 

 tween the rock walls, are to be found in countries which have 

 never been glaciated. I do not myself remember reading of any, 

 though, of course, tremendously deep ravines are common, but 

 these are of quite a different character. Should it be found that 

 these extremely narrow rock-walled gorges are peculiar to gla- 

 ciated districts they will afford us a means of estimating the 

 amount of glacial erosion in valleys where no lake basins have 

 been formed. 



THE LAKE or GENEVA AS A TEST OF THE RIVAL THEORIES. 

 When I recently began to study this question anew, I was in- 

 clined to think that the largest and deepest of the Alpine lakes, 

 such as Geneva, Constance, Lago Maggiore, and Lago di Garda, 

 might perhaps have originated from a combination of earth move- 

 ments with ice erosion. But on further consideration it appears 

 that all the characteristic features of erosion are present in these 

 as fully as in the smaller lakes. They are situated in the largest 

 river valleys or in positions of greatest concentration of the gla- 

 cier streams; their contours and outlines are those of eroded 

 basins ; while all the difficulties in the way of an origin by earth 

 movements are as prominent in their case as in that of any other 

 of the lakes. I will therefore discuss, first, some of the chief ob- 

 jections to the erosion theory as applied to the above-named lake, 



