214 On the Origin of the Alpine Lakes and Valleys. 



Playaux near Vevey, and on the southern at Voirons, east of 

 Geneva. These two mountains are both situated on the borders 

 of the Alpine chain. In order properly to grasp the relation 

 which subsists between the overthrow of the strata along this 

 line and the great depths of the lake, it is necessary to mark on 

 the map the soundings of the lake by De la Beche, the positions 

 of the crests of the Voirons, the Allinges south of Thonon, and 

 of Playaux near Vevey *. 



The principal soundings of the lake placed opposite Meillerie 

 and Evian may likewise (and perhaps it is the most easily 

 effected) be marked on the geological map of Savoy. Then 

 Voirons and Playaux should be joined by a line (but not by a 

 straight line, because the chains on the borders of the lake are 

 curved) drawn through Calvaire (Voirons), Allinges, the point 

 where the Alpine Macigno (M) (Affleure) descends to a level 

 with the bed of the Dranse, at a distance of four kilometres 

 from Thonon and the city of Evian. Such a line as this would 

 terminate towards Playaux, passing over the northern bank of 

 the lake between Corsier and St. Saphorin. 



This course shows pretty nearly the line of the reversal of the 

 strata situated on the flanks of the Alps ; and, presenting a certain 

 parallelism to the denudations of the different rocks traced on 

 my geological map of Savoy, it passes through the midst of the 

 soundings which indicate the greatest depth of the lake. Con- 

 sequently this depth bears a relation to the reversal of the beds. 

 It is in such fractures, I am confident, that the true cause of the 

 origin of these lake-basins is to be found. 



From a summary of these facts it may be concluded — 



1st. That the Lake of Geneva deviates much from the median 

 (central) line of the great glacier or glaciers which extended from 

 the Rhone to the Jura. 



2ndly. That these ancient glaciers not having had the power to 

 remove the older drift below Geneva, have not been able to pro- 

 duce in the lake-basins what is called their excavation {Vajfouille- 

 ment). If they could not scoop out these basins, still less have 

 they excavated the adjacent valleys which terminate in them. 



3rdly. The valleys and the basins of mountain-regions are 

 related to the cause which has given to the mountains their 

 orographical characters, and to the strata their greater or less 

 inclination. 



* This mountain, called Pleyaux or Playaux, is indicated on the 

 Federal map by the name of Pleiades. To facilitate the indication of it on 

 my geological map of Savoy, on which it is not marked, I should say that 

 it lies 6 kilometres from the mouth of the stream which discharges 

 itself into the lake between Vevey and Corsier, and 6^ kilometres from 

 the point of Montreux. 



