1854.] SHARPE — STRUCTURE OF MONT BLANC. 17 



stone mentioned by Saussure, § 709 and 710, one of which rests on 

 the flank of the Aiguilles Rouges below the Croix de la Flegere, 

 dipping S.E. 70° ; the other, of similar character, is opposite to it on 

 the east side of the valley, forming a little hillock, called the Cute du 

 Figet, between the present and the ancient moraines of the Glacier 

 du Bois : the beds dip S.E. 30°, from which he infers that this lime- 

 stone dips under the gneiss of Mont Blanc (Travels, p. G3). But 

 as the contact of the limestone and gneiss is not seen, the mass being, 

 as stated by Saussure, *' entierement isolee dans le has de la vallee," 

 the conclusion is evidently not drawn from observation. The assei- 

 tion that limestone dips under the granite in the valley of Cha- 

 mounix, is several times repeated by Professor Forbes, but the only 

 points especially mentioned, at pp. 63 and QQ^ do not justify this con- 

 clusion. 



M. Necker had previously asserted in general terms that the talc- 

 schists cover the secondary rocks along the whole valley of Cha- 

 mounix*, but without indicating any precise spots where such super- 

 position was to be seen. 



Considerable information relative to the secondary rocks will be 

 found in a very interesting memoir by M. Favre on the Environs of 

 Chamounix, in the Bibliotheque Universelle de Geneve for April 1848, 

 in which the author announced his discovery of Jurassic and Anthraxi- 

 ferous beds resting on the summit of the Aiguilles Rouges. He 

 states that on the west side of the valley of Chamounix a band of 

 anthraxiferous beds rests on the base of the Aiguilles Rouges, over- 

 laid by Jurassic beds which are seen on the other side of the valley 

 dipping S.E. 30°, adding, "les schistes cristallins paraissent plonger 

 sous les roches de cristaUisation et reposer sur les calcaires dont les 

 couches presentent la meme inclinaison." He concludes with great 

 justice, *' II me semble done que c'est la chaine des Aiguilles Rouges 

 qui a determine le redressement des roches sedimentaires placees 

 dans la vallee de Chamounix. Cette opinion me paraissait d'abord 

 assez extraordinaire, car c'etait annuller jusqu'a un certain point 1' im- 

 portance geognostique de I'enorme chaine protogineuse du Mont 

 Blanc." 



It is evident from these passages that M. Favre has nowhere seen 

 the crystalline schists of Mont Blanc lying upon the sedimentary 

 beds in the manner represented in the section which accompanies his 

 memoir. From seeing the Jurassic beds at the base of the hill dip 

 towards the schists which form its side, at angles occasionally 

 coinciding with those of the dip of the foliation of the schists, the 

 inference has been drawn that the Jurassic beds dip under the 

 schists. 



In my Section No. 4, PI. I., I have shown the position of a mass of 

 gypsum alternating with steatitic clay, which I visited in the Ravine 

 between the Montague de Taconnay and the Montague des Forts, 

 near the spot mentioned by Saussure, § 706 : the beds dip S. 15°, 

 and are entirely free from cleavage. Another quarry of gypsum, a 

 little south of the foot of the Glacier de Taconnay, offered some 



* Etudes Geologiques dans les Alpes, vol. i. p. 138. 

 VOL. XI. — PART 1. C 



