70 Professor Favre's Geological Researches 



sea, according to a mean of twelve of my barometrical obser- 

 vations. It is situate exactly on the boundary of the ci*ys- 

 talline slates and the jm-assic formation. The junction of 

 these two formations can, therefore, be easily determined. 



In order to determine the exact limit of the protogine and 

 crystalline slates, it was necessary for me to make a long 

 expedition on the side of the glacier of Trient. Proceeding 

 on my search, and having no other indication than the direc- 

 tion of the beds, I wandered a little from my route ; and it 

 was not till I had crossed five glaciers that I arrived at it. 

 But I was rewarded for my trouble, by finding near the line 

 of contact five banks or veins of granite, the largest of which 

 was 5 or 6 metres in thickness. They are embedded in the 

 crystalline slate, as well as a vein of Potstone, quite ana- 

 logous to that now dug at Montanvert de Chamounix. This 

 latter is likewise near the junction of the crystalline slates 

 and protogines. De Saussure has described {Voyages, § 661) 

 five banks of granite situate near the Chalets of Blaitiere, 

 not far from the limit of the crystalline slates and proto- 

 gines. Now, there are about 15 kilometres between Blai- 

 tiere and the locality where I discovered the granite veins 

 near the glacier of Trient. So that we may conclude, that 

 the Potstone and granite vein^ are placed as bands parallel 

 to the line of contact of the crystalline slates and the protogine ; 

 and that throughout all this line, the same phenomena present 

 the same appearance, at least on the north-west aspect of the 

 chain of Mont Blanc. I can also affirm, that, on the same 

 declivity, there exist parallel bands of serpentine or Pot- 

 stone ; and I have other proofs of the parallelism with which 

 these rocks are disseminated in the crystalline slates. These 

 I have reserved for a work in which they will be described 

 with more details than I can give here. 



These proofs are the result of a journey on foot to the 

 Aiguille du Midi, in front of the Grand-Mulets, which the 

 badness of the weather compelled me to make on two difFer- 

 eut occasions, and in which I ascended to the height of 2757 

 metres — that is to say, about 100 metres above the point 

 which Saussure reached with much difficulty. (§ 660.) 



I use the words crystalline slates in preference to any 



