340 THE GEOLOGY OF THE ALPS. [XIV. 



portions of the older crystalline schists, which, in cases where 

 the former are destitute of pebbles, makes it impossible to dis- 

 tinguish between the two. Necker, in like manner, asserted 

 that it was impossible to draw a line of demarcation, and was 

 hence led to assert a passage from the one to the other. The 

 same close resemblance was noticed by De Saussure, and is testi- 

 fied to by De Mortillet and by Sismonda, who says of the feld- 

 spathic sandstone (gres) near St. Jean in Maurienne, that " un- 

 less we take care we run the risk of being deceived, and of 

 confounding it with gneiss " ; while elsewhere similar rocks 

 assume the aspect of granite from the predominance in them of 

 feldspar. Hence it has happened that observers like Dolo- 

 mieu and Bakewell placed the anthracites of the Alps in the 

 mica-slate formation, and that Berger described as a " veined 

 granite " the Aiguille des Posettes, which, according to Favre, 

 consists of nearly vertical beds of carboniferous sediments. 

 In illustration of this condition of things, Favre cites the 

 observation of Boulanger, according to whom the triassic sand- 

 stones of the department of Allier are made up of quartz, feld- 

 spar, and mica, so united as to give rise to a sandstone which 

 would be taken for a primitive rock but for the occasional pres- 

 ence of a rolled pebble of granite.* The paste of this Valor- 

 sine conglomerate, which seems identical with certain of the 

 enclosed pebbles, appears, according to Favre, to have undergone 

 a certain rearrangement, so that the beds of these " pretendus 

 schists cristallins " of the carboniferous are with difficulty dis- 

 tinguished from the " vrais schists cristallins " upon which they 

 rest unconformably. I insist the more upon these details, be- 

 cause in the earlier notice of Favre's investigations I erroneously 

 represented him as including in the carboniferous a great mass 

 of the older crystalline schists. 



In this connection we may cite the observation of Sedgwick, 

 who cites similar cases of recomposed rocks in Scotland, " which 

 it ia not always possible to distinguish from the parent rock," 

 and remarks that " a mechanical rock may appear highly crys- 



* See Favre, Terrains liassique et keuperien, etc. (1859), pp. 78, 79, to 

 which, in this work, he refers the reader for further explanation on this point. 



