86 A DESCRIPTION OF THE [Ch. V. 



extensive districts, and not in insulated and subordinate beds, 

 as those already noticed. This series is found well developed 

 in the Alps and in Corsica; and to these we shall for the 

 present confine our remarks. 



The rock of Mont Blanc was for a long time considered as 

 a talcose variety of granite. M. Jurine, regarding it as dis- 

 tinct from true granite, gave it the name of protogine ; which 

 term has been adopted by Brongniart, and other French 

 geologists. M. Brochant*, after an extensive examination 

 of this part of the Alps, has also concluded that the granitic 

 rock of Mont Blanc is not true granite; but only an 

 extreme variety (more crystalline, and containing a greater 

 quantity of felspar,) of a talcose rock, with which it is asso- 

 ciated ; and which is very abundant in the adjacent moun- 

 tains. This talcose formation extends from Mont Rosa to 

 St. Bernard, and to Mont Blanc, and even to Mont Cenis, 

 and, very probably, far beyond these limits ; and its most 

 crystalline or granitic variety composes all the highest peaks 

 along this tract. We will not, in this place, stop to enquire 

 whether this rock is the equivalent of granite, and, therefore, 

 referrible to the protogine of the granitic group ; or whether 

 it is a member of the schistose group, and best distinguished 

 by the name of felspathic talc-rock, conferred on it by Bro- 

 chant : but, availing ourselves of the observations of this dis- 

 tinguished geologist, we will endeavour to describe this 

 interesting formation. 



The principal and prevailing rock of this series is a talc- 

 schist. Mica-slate is also found in this part of the Alps, and 

 in similar associations, exhibiting frequent passages from mica 

 into talc, from mica- slate into talc-schist, and even into 

 chlorite-schist. The talcose rocks, however, are much more 

 abundant than the micaceous rocks; and thus it is, that 

 micaceous limestones are more rarely met with than the 

 talcose limestones : and this predominance of talc is more 



* Annales des Mines, tome 4. p. 283. 



