Granitic Rocks of Monl Blanc. 119 



Let us now return to the granitic rocks of Mont Blanc^ 



It has, like our talcose felspathic schists, felspar and talc 

 for its principal constituent parts. The talc in it is most 

 frequently of a dark green, and of the same green colour 

 common to chlorite ; it forms small veins which have always 

 a slightly fibrous texture. The rock constantly has a greater 

 or less tendency to the schistose texture ; it sometimes even 

 becomes decidedly laminated, and what is very remarkable, 

 the specimens collected at the summit of Mont Blanc afford 

 this character. There is a difference only in an inverse re- 

 lation between the constituent parts. The felspar predo- 

 minates in the granitic rocks ; the talc in the schistose. 

 But this difTerence in the proportions, which to the mine- 

 ralogist is very great, is only essential to the geologist (as I 

 have above stated) when it is joined to differences of po- 

 sition, formation, &c. as between greenstone and syenite.* 



Quartz is nevertheless found also in the granitic rock 

 of Mont Blanc ; but I would recall to mind that it is some- 

 times met with in the talcose felspathic schists. It is in truth 

 rare in it; but it is any thing but constant in the granitic 

 rock of Mont Blanc, and it occurs rather in knots or small 

 nodules, scattered or irregularly grouped, than in crystal- 

 line grains spread uniformly through the mass, as is seen in 

 the true granites. There are even varieties in which it is 

 altogether wanting, and they are rather numerous. 



The two mineralogical differences that I have noticed 

 cannot therefore have any influence on the determination of 

 the geological notions that ought to be entertained of the 



* I have cited this example because it is most known. It may never- 

 theless be with reason objected, that tliese two rocks are sometimes 

 found united in the same formation ; but it is not the less true, that each 

 gives peculiar characters to tlie formations where it predominates. 



Note by the Translator. — The syenite and greenstone of the trap ridge 

 extending from Bolton Beacon to Benton Castle, in Pembrokeshire, form 

 parts of the same mass, as also in the trap district of Gouldtorp Road, 

 in St. Bride's Bay, (Pembrokeshire) ; the trap in both instances hau 

 apparently greatly disturbed the coal measures and carboniferous lime- 

 stone. For an excellent account of the syenites, greenstones and othei; 

 overlying rocks, see Dr. MacCiiUoch's classification of rocks. 



