Granitic Rocks of Mont Blanc. 117 



M. Brongniart has described one variety under the name of 

 porphyritic gneiss (gneiss porphyroide). 



I conceive however that this rock deserves to be consi- 

 sdered alone, at least geologically, because it is so frequently 

 met with, and on account of the uniformity observable in its 

 principal characters. 



I shall endeavour to notice them, and shew the passages 

 uniting the different varieties. 



The principal and prominent base of this rock is a schis- 

 tose talc, commonly of a green colour, between leek green 

 and meadow green, analogous to that of chlorite, as I have 

 remarked for ike talcose sciiists in general. 



The texture is almost always a little fibrous, and the 

 fracture schistose ; but the lamina; are scarcely ever so thin 

 as in the true mica slates. 



The felspar is uniformly disseminated In It in crystals of 

 most commonly, one millimetre [^ of an in.] in length ; 

 somfetlmes of two or three centimetres [about ^ in.] in 

 the varieties resembling gneiss (rock of Cervin in the Taren- 

 taise) ; sometimes very small and scarcely discernible j 

 small white points are solely seen at the surface or rather at 

 the edges of the rock, and the felspar is only recognized in 

 it by the passages that are met with in the same block into 

 other varieties where the crystals are visible. 



Quartz very rarely exists in it, and most frequently ap- 

 pears to be entirely wanting ; when it is visible. It Is very 

 irregularly disseminated in small grains grouped together. 



Hornblende does not shew itself in it ; at least neither 

 prisms nor laminated needles of that substance are met with ; 

 but It Is very certainly sometimes intimately mixed In it. 

 Rocks decidedly of hornblende are seen associated in the 

 same mass with the talcose felspathic schist, and insensible 

 passages may be traced between these two rocks. 



These and many other passages give reason for conjec- 

 turing that it is sometimes to an intimate mixture of horn- 

 blende, sometimes to an intimate mixture of quartz, and 

 perhaps also of felspar itself, that the differences of hard- 



