1854.] SHARPE STRUCTURE OF MONT BLANC. 23 



towards the gneiss, coinciding in direction with the dip of the 

 planes of foliation of the gneiss itself, has led our distinguished 

 countryman to a belief in the actual superposition of the gneiss over 

 the slates. 



The same Section, No. 4, PI. I., shows the position of the beds of the 

 southern extremity of the Montague de la Saxe, as they are seen on 

 the road from Entreves to Courmayeur. The upper beds consist of 

 metamorphic semi-crystalline slate, dipping E.S.E. 50°, and resting 

 conformably on a series of black slates with the same dip. The 

 mineral waters of La Saxe rise at the junction of these two slate for- 

 mations. Prof. J. Forbes describes these beds as granite resting 

 upon limestone, p. 211, and his section, p. 210, shows a thick mass 

 of granite overlying limestone at an angle of 50°. I did not observe 

 any calcareous beds in the lower slates, nor are they mentioned by 

 Saussure, who describes the beds in detail, § 881. But M. Studer 

 has observed them to consist of black slate and limestone, vol. i. 

 pp. 1/3 and 383, and Section, p. 175. The upper series, however, 

 is undoubtedly a slate, with distinct bedding and cleavage, both con- 

 formable to those of the beds below. M. Studer terms it Feldspath- 

 Schiefer ; Saussure calls it une roche feuilletee, quartz et micaf. 



The last-mentioned beds abut against a mass of rock of a more 

 crystalline character, which must, I think, be considered gneiss ; it 

 forms a low hill, reaching from the village of La Saxe to Courmayeur. 

 The foliation of this mass is vertical, but ill-defined ; the cleavage of 

 the slates between La Saxe and Val Ferret dips E. 40° S., at angles 

 diminishing from 85° to 75°, as we recede from the gneiss. Thus 

 the planes of cleavage and the beds both form an anticlinal with a 

 common axis on the line of the Val Ferret, but with different degrees 

 of inclination, and the foliation of the gneiss of the eastern side of 

 the chain of Mont Blanc forms part of the same anticlinal arrange- 

 ment, showing us that the elevation of the beds of Val Ferret to 

 their present position was contemporary with the elevation of Mont 

 Blanc ; for any subsequent elevation of the beds would have disturbed 

 the symmetry of the cleavage planes, as has been the case in the 

 valley of Chamounix. 



Listead of crossing the Col Ferret and thus keeping close round 

 the chain of Mont Blanc, I turned down the valley of Aosta and 

 returned to Martigny by the Great St. Bernard. Sect. 4, PL L, shows 

 the position of the rocks as far as Aosta, but being drawn from the 

 road, the western portion was taken on the north side of the valley, 

 the eastern end beyond the bridge of Escutira on the south side. 



beds at Pra Sec in the Val Ferret. Memoria sui Terreni stratificati delle Alpi, 

 p. 12. 



* Saussure adds below, § 881, " Voila done des roches regardees comme primi- 

 tives, qui reposent sur un genre de pierre unanimement regardee comme secon- 

 daire. Ces denominations de primitives et de secondaires sont-elles fautives, ou 

 bien cette superposition monstrueuse des roches primitives sur les secondaires 

 serait-elle I'effet d'un bouleversement ? C'est, ce que je n'oserais point encore 

 decider." We escape from the horns of this dilemma by answering that the upper 

 rock is not primitive but secondary. 



