1854.] SHARPE STRUCTURE OF MONT BLANC. 15 



regularity both of the bedding and cleavage in that neighbourhood * . 

 These rocks, and others of similar character occurring in the same 

 chain near Valorsine, are treated by M. Studer, p. 161 and 162, as 

 parts of the crystalline axis of the chain ; but their disturbing the 

 regularity of the cleavage planes proves their eruption to be after the 

 lamination had been completed. Now the foliation of the Aiguilles 

 Rouges corresponds so exactly to that of Mont Blanc, as to leave no 

 doubt of these two chains having been formed contemporaneously ; 

 and we thus learn that Mont Brevent and the Aiguilles Rouges have 

 been subjected to disturbing influences at a later period than Mont 

 Blanc ; an observation of which the full importance will be seen 

 in reference to the beds in the intervening valley of Chamounix, to 

 which we will next proceed. 



The section No, 3, PI. I. shows roughly the position of the beds at 

 the head of the valley of Chamounix on the southern side of the Col 

 de Balme : the rock which rests on the gneiss of the chain of Mont 

 Blanc, at the foot of the Aiguille de Tour, is a metamorphic slate, con- 

 taining nodules of quartz set in a semi-crystalline mass. These 

 nodules appear not to be true pebbles, but to owe their form, in some 

 degree at least, to metamorphic action ; but this is a point of great 

 difficulty. The rock reminded me of the slaty crystalline grits of 

 Cumberland. The bedding is distinct, dipping N.W. 40° to 50°, and 

 the cleavage, equally distinct, dips E. 35°, S. 70° to 80°, in conformity 

 with the foliation of the gneiss on which it rests f. 



Upon the metamorphic slate rests a great series of black and dark 

 brown slates dipping 30° to the westward ; but on the east side of the 

 Col de Balme the same slates dip 50° to the E.N.E., and on the west 

 side of the Col they dip 30° to the W.S.W., and further on they are 

 thrown into some confusion. The cleavage of the slates forms a 

 regular anticlinal axis at the Col de Balme, and a synclinal axis a 

 little east of the Col ; further east it forms another anticlinal, which 

 must be the continuation of the anticlinal axis of the Aiguilles Rouges, 

 mentioned above. At this line the cleavage is thrown into great 

 confusion, which may be seen in some of the little ravines on the de- 

 scent from the Col de Balme towards La Tour : here again we see 

 proofs of the disturbance caused by a more recent elevation of the 

 Aiguilles Rouges, by which the rocks on the western side of the 

 valley are more affected than those on the eastern side. One of the 

 beds seen near the village of La Tour deserves especial notice ; it is a 

 purple slate slightly micaceous, brecciated, with pebbles or masses 



* The rocks at the Hospice of the Great St. Bernard and of the Val d'Entremont, 

 near St. Pierre, and also the intrusive rocks in the Val d'Aosta, east and west of 

 Livrogne, appeared to me to be of a similar character to the above : they also 

 strike a little W. of N,, and disturb both the bedding and cleavage of their respec- 

 tive neighbourhoods : see Studer, vol. i. p. 205. 



t Saussure, § 552, describes a rock near Valorsine as " une espece de granit 

 veine, parseme de noeuds de quartz lenticulaires, poses de plat entre les feuillets 

 de la pierre et parallelement a eux." This is probably similar to the metamorphic 

 slate on the flank of the Aiguille de Tour. Similar rocks -occur in many parts of 

 this district in contact with the gneiss, and their resemblance to gneiss has often 

 proved a source of confusion. 



