1854.] 



SHARPE — STRUCTURE OF MONT BLANC. 



21 



of one great operation, of which we must carefully study the effects, 

 before we can hope to learn its causes or nature. 



Let us now turn to the line of valleys, which, under the names of 

 the Allee Blanche and the Val Ferret, bounds the eastern side of the 

 Mont Blanc range. The western side of the Alle'e Blanche is in a 

 great measure masked by the enormous moraines of the great glaciers 

 which descend from Mont Blanc, and the various interesting phseno- 

 mena connected with these somewhat distracted my attention from 

 the geology of the valley ; but I sketched the section shown in 

 fig. 2, on the west or Mont Blanc side of the valley, a little above 



Fig. 2. 



Slaty limestone 



Slate. 



Calcareous 

 conglomerate. 



Slate. 



90" 85° 80" 70" 



the Lac de Combal. Slates of various characters rest conformably 

 on a bed of calcareous conglomerate, in which the cleavage is very 

 obscure ; this rests on a thick formation of slaty limestone, with 

 mica lying on the planes of cleavage. The beds all dip conformably 

 to the S.E. at about the angle of 20°, and are consequently resting 

 upon the gneiss of Mont Blanc. The cleavage strikes N. 25° E., 

 dipping near the mountain towards the E. 25° S. at high angles, but 

 is vertical in the limestone at the side of the valley ; this is on the 

 line of the western axis of vertical foliation of the gneiss of Mont 

 Blanc, and connects that line with the vertical cleavage of the Col de 

 la Seigne*. These beds doubtless belong to the Jurassic series of the 

 Col du Bonhomme, and may owe their more metamorphic character 

 to their proximity to the gneiss ; they cross the valley near the 

 chapel ; they are probably separated from the gneiss by a metamor- 

 phic siliceous slate, which is seen on the west side of the valley below 

 the Glacier de 1' Allee Blanche. 



I examined the Piedmontese Val Ferret rather more in detail ; the 

 position of the rocks on the north side of the Col Ferret is shown in 

 Sect. 3, PI. I. A thick bed of quartz rock rests upon the gneiss at 



* Saussure, § 845, mentions two pyramidal hills of a similar micaceous lime- 

 stone near the head of the Allee Blanche with highly inclined beds ; he doubtless 

 mistook the cleavage planes for the stratification. 



