158 Geological Notes on the 



the valley of the Arve ; but the obvious cause of their forma- 

 tion is so peculiar and extraordinary, as only to support the 

 rule above propounded. Hitherto, as far as I am aware, 

 little stress has been laid upon them, and the principal lacus- 

 trine features have been overlooked. 



They occur at a short distance above Chamouni. Profes- 

 sor Forbes and others have described with great exactness 

 the ancient moraine of the Glacier des Bois, or Mer de Glace, 

 which crosses the vale of the Arve immediately above the 

 village of the Les Tines. It is a huge barrier of rough blocks, 

 mingled with smaller detrital matters, leaving only a narrow 

 passage for the river. No doubt is entertained that the Mer 

 de Glace, in ancient times, instead of merely entering the 

 valley, as it now does, was of such greater volume as to pass 

 across it, and abut against the opposite mountains, under the 

 Croix de la Flegere. The barrier in question was the mo- 

 raine which then skirted its right side. The unavoidable 

 effect of such a blockage of the valley would be to dam up 

 the waters of the infant Arve, and form them into a lake of 

 a height not less than that of the barrier. Such a lake would 

 continue to exist as long as the barrier remained entire. 

 Now, in the mile's space above the barrier there are clear 

 memorials of this lake at three successive stages of elevation, 

 — in three alluvial terraces distinctly traceable along the 

 right side of the valley. We are to presume, that the up- 

 permost of these is the indication of the first height of the 

 water ; that a break of the barrier then took place, covering 

 the lake to the level of the next terrace, and so on. Such 

 features are the more striking, inasmuch as, for many miles 

 below Les Tines, the valley shews scarcely any remains of 

 alluvia above the level of the meadows now in the course of 

 being formed by the river. 



At the village of Argentiere, where these terraces melt 

 into the rise of the vale, another glacier comes in ; and here 

 occurs the second barrier, which originated in precisely the 

 same way, and also caused the formation of a lake, though in 

 this case we can only trace the memorial of one intermediate 

 level of the water, after the first had been reduced. 



