CHAP. xv. BY THE EROSIVE ACTION OF GLACIERS. .317 



miocene ages which preceded the glacial period there was 

 ample time for the slow erosion by water of all the principal 

 hydrographical basins of the Alps, and the sites of all the 

 great lakes coincide, as Professor Eamsay truly says, with 

 these great lines of drainage. The lake-cavities do not lie in 

 synclinal troughs, following the strike and foldings of the 

 strata, but often, as the same geologist remarks, cross them 

 at high angles ; nor are they due to rents or gaping fissures, 

 although these, with other accidents connected with the 

 disturbing movements of the Alps, may sometimes have 

 determined originally the direction of the valleys. The 

 conformity of the lake-basins to the principal watercourses is 

 explicable if we assume them to have resulted from inequali 

 ties in the upward and downward movements of the whole 

 country in post-pliocene times, after the valleys were eroded. 

 We know that in Sweden the rate of the rise of the land 

 is far from uniform, being only a few inches in a century 

 near Stockholm, while north of it, and beyond Grefle, it 

 amounts to as many feet in the same number of years. Let 

 us suppose, with Charpentier, that the Alps gained in height 

 several thousand feet at the time when the intense cold of 

 the glacial period was coming on. This gradual rise would 

 be an era of aqueous erosion, and of the deepening, widening, 

 and lengthening of the valleys. It is very improbable that 

 the elevation would be everywhere identical in quantity, but 

 if it was never in excess in the outskirts as compared to the 

 central region or crest of the chain, it would not give rise to 

 lakes. When, however, the period of upheaval was followed 

 by one of gradual subsidence, the movement not being every 

 where strictly uniform, lake-basins would be formed where- 

 ever the rate of depression was in excess in the upper country. 

 Let the region, for example, near the head waters of the great 

 rivers sink at the rate of from four to six feet per century, 

 while only half as much subsidence occurs towards the cir- 



