250 M. A. Guyot uii the Distribution of Bocks 



der this basin an excellent field for a study of this nature. On 

 the other hand, its double divergency and double outlet, on the 

 east and west, complicate the question, by introducing an 

 element not found in the other basins, and which it is ne- 

 cessary to be particulai'ly careful in taking into account. I 

 shall first notice bi'iefly the principal species of rocks which 

 distinguish the basin of the Rhone, then examine what is the 

 mode of association and distribution peculiar to each of them,. 

 CharQcterisiic species. — The rocks which I consider as truly 

 characteristic of this basin, without belonging to species very 

 distinct, nevertheless form everywhere groups identical with 

 themselves, and perfectly recognisable. They are essentially 

 the following : — 



1. A species of granite, or, if the term bo preferred, talcose 

 syenite, of a yellowish-green colour, composed of a talcose, 

 chloriteous, and most frequently slaty mass, intermixed with 

 numerous crystals of quartz, felspar, and amphibole, and 

 sprinkled here and there with very small crystals of sphene. 

 Pretty frequently it affects the structure of gneiss, or even 

 slate ; in the latter case, the quartz and amphibole, and 

 even the felspar, gradually disappear, and the rock passes 

 into a sort of chloriteovis slate. This rock is the talcose 

 granite of M. De Charpenticr, the sphenitic rock of M. De 

 Buch ; I shall designate it by the single word Arkesine, a name 

 which M. Jurine has given to a rock vei-y analogous, of which 

 I have found some specimens in the collection deposited in 

 the museum of Geneva. 



2. A species of gneiss, very rich in white felspar imper- 

 fectly crystallised, with broken or undulated scales of chlorite, 

 of a fine light green, sprinkled with very shining particles, 

 and containing crystals of quartz, few in number, and irre- 

 gulai'ly distributed. By the disappearance of the quartz, 

 which is often wai.ting, this rock passes into a sort of chlori- 

 teous leptinite ; by the diminution of the felspar to a very 

 minute quantity, and the predominance of the chlorite, it ap- 

 proaches to a simple chloriteous slate. I shall call it chlori- 

 teous gneiss. 



3. Chlorites, of a light or dark bluish-green, usually slaty, 

 appearing as if regularly pricked with a great number 

 of granules of white or yellowish felspar, of very variable 



