296 THE ICE AGE AND ITS WORK. 



just as they would do if liowiug iuto a rivei'. Exactly tlie same feature 

 is present in the lower portions of these two lakes, and it is equally a 

 characteristii; of every lake in the lake district, and of all the Swiss 

 aud Italian lakes. On looking- at the maps of any of these lakes one 

 can not but see that the lake surface, not the lake bottom, represents 

 ai)proximately the level of the pre-glacial valley, and tliat the lateral 

 streams and torrents enter the lake in the way they do because they 

 could only erode their channels down to the level of the old valley 

 before the ice overwlielmed it. Of course this rule does not apply to 

 large tributary valleys carrying- separate glaciers, since these would 

 be eroded by the ice almost as deeply as the main valley. - - - 



The Lake of Geneva as a test of the rival theories. — When I recently 

 began to study this question anew I was inclined 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 movements with ice erosion. But on fur- 

 ther consideration it appears that all the (iharacteristic features of ero- 

 sion are present in these as fully as in the smaller lakes. They are sitn- 

 ated in the largest river valleys or in i)ositions of greatest concentra- 

 tion of the glacier streams: their contours aud outlines are those of 

 eroded basins; while all the difiticuities m the way of an origin by 

 earth movements are as jnominent in their case as in that of any 

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

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

 and then consider the only alternative theory that has obtained the 

 accei)tance of modern writers. 



One of the first objections made was that the lake did not lie in the 

 direction of the greatest action of the glacier, which was straight 

 across to the Jura, where the highest erratic blocks are found. This 

 was urged by Sir Charles Lyell immediately after Ivamsay's paper was 

 read, aud as it has (juite recently been put forth by Prof, Bouncy, it 

 would appear to be thought to be a real difificulty. Yet a little consid- 

 eration will show that it has not the slightest weight. No lake was 

 eroded in the line of motion of the central and highest part of the old 

 glacier, because that line was over an elevated and hilly plateau, which 

 is even now from 500 to 1,000 feet above the lake, and was then still 

 higher, since the ice sheet certainly effected some erosion. The great- 

 est amouut of erosion was of course in the broad and nearly level 

 valley of the preglacial Rhone, which followed the great (nirve of the 

 existing lake, and had produced so open a valley beeause the roeks in 

 that direetion icere easily denuded. Objectors iuvariably forget or over- 

 look the indisputable fact that the existence of a broad, o])en, fiat-bot- 

 tomed valley in any part of a river's course proves that the rocks were 

 there either softer, or more friable, or more soluble, or, by some combi- 

 nation of characters, more easily denuded. A number of favorable 

 conditions were combined to render ice erosion easy in such a valley. 



