16 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [NoV. 15, 



of slate of different colours and consistency, all of which are flattened 

 between the planes of cleavage, showing the pressure to which the 

 rock has been subjected in a direction perpendicular to the cleavage 

 planes*. 



Owing to the great accumulation of fragments which have fallen 

 from the mountains and form a steep talus on both sides of the valley, 

 the secondary rocks of the valley of Chamounix are difficult of exa- 

 mination ; indeed in many parts there are no secondary beds visible, 

 the detritus of the valley extending up to the base of the crystalline 

 rocks. I had not time to ascend the various ravines which offer a 

 chance of detecting the secondary formations exposed ; but in the 

 localities which I examined, I saw nothing to justify the idea that the 

 gneiss really overlies the secondary beds ; nor do the descriptions 

 published by preceding observers justify any such conclusion. I am 

 persuaded that the notion has arisen from an approach to conformity 

 in the dip of the foliation of the gneiss of Mont Blanc, along the side 

 of the valley, with that of the beds of stratified rocks, which for the 

 most part dip at various angles towards the chain of Mont Blanc. 

 But this apparent conformity is accidental : the folia of the gneiss 

 owe their position to a deep-seated agency of the nature of which we 

 are ignorant, which produced an arrangement of wonderful symmetry, 

 extending on a uniform plan over a vast area, before the more mo- 

 dern of the beds at Chamounix were formed : the easterly and south- 

 easterly dip of the beds in the valley is due to the circumstance 

 already alluded to, that the chain of Mont Brevent and the Aiguilles 

 Rouges has received a movement of elevation at a later period than 

 Mont Blanc ; and in consequence the beds lying between the two 

 chains are higher on the western side of the valley and dip towards 

 Mont Blanc. 



The interest attaching to the question, whether the crystalline rocks 

 overlie the secondary formations of Chamounix, makes it necessary to 

 examine the statements on the subject in some detail. 



Saussure devotes Chap. 22 to the secondary rocks of Chamounix : 

 he mentions beds of slate, limestone, and gypsum dipping to the S.W. 

 at angles of 28°, 30°, and 45°, and sums up at the end that they are 

 all more modern than the crystalline rocks, adding, " celle du Biolay, 

 § 708, dont les couches sont engagees sous celles de la montagne 

 primitive, semblerait pourtant faire une exception a celle regie." 

 Yet the previous description of the quarry at Biolay here referred to 

 by no means justifies this exception; it is *'les couches sont situees 

 precisement comme celles de la montagne primitive a laquelle elles 

 sont adossees." 



Mr. James Forbes calls attention to two masses of dark grey lime- 



* Saussure in several parts of his Travels, § 841, 848, 850, &c., describes with 

 great minuteness, brecciated slates with araygdaloidal pebbles arranged with their 

 flat sides parallel to the plans des feuillets, and in every instance concludes that 

 thefeuillets indicate the original stratification, and that the rock has been raised 

 from a horizontal to its present vertical position. In the bed at the Col de Balme 

 the bedding cannot be mistaken, as this bed is interposed between others of a 

 totally different character. 



