118 M.Brochaut on the 



ness and toughness ought to be attributed, which are obser- 

 vable ia the different varieties of this rock. 

 . There are some that are very difficult to break, and at the 

 same time rather hard : these are near the hornblende 

 rocks ; others are easily broken, and are very hard, they 

 contain grains and veins of quartz ; others, lastly, are tough, 

 as they receive the impression of the hammer without break- 

 ing, and are at the same time very soft, so as to be scratched 

 and cut by the knife, like talcose rocks, and especially like 

 chlorite schist, from which in fact these last varieties only 

 differ in the presence of felspar. 



From this sketch of the characters of these talcose fels- 

 pathic schists, it would appear that rocks very different 

 from each other were united under this name ; when they 

 are observed in place, we are invincibly led to acknowledge 

 this affinity. When considered in collections, varieties are 

 no doubt remarked apparently very distinct, and which it 

 might even be useful to describe separately, though com- 

 posed of the same minerals ; but these mineralogical differ- 

 rences lose the greater part of their importance when they 

 are not joined with those which are geological, especially 

 when we see, and often in the same mass, very frequent in- 

 sensible passages of one of these rocks into the other. 



I have already noticed the places where I have observed 

 these talcose felspathic schists ; if the high valleys of the 

 Alps are passed over, from Mont Rosa to St. Bernard and 

 Mont Blanc, even in part as far as Mont Cenis, and no 

 doubt beyond these limits, a great predominance of the tal- 

 cose rocks cited above will be met witli ; and in the places 

 where they are the best characterized and most abundant, 

 ;will be found serpentine, either pure or mixed with lime- 

 stone, fine grained hornblende rocks, more or less crystalline 

 limestones, chlorite schists often mixed with oxidulated 

 iron, lastly, the rocks I have mentioned under the name of 

 talcose schists, and in the midst of these two rocks different 

 varieties of the talcose felspathic schists I have described, 

 which sometimes form distinct subordinate beds, and are 

 sometimes united to them by insensible gradations. 



