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



lieve that, like Mont Blanc, it has a narrow anticlinal axis bounded 

 by two lines, along which the planes of fohation and cleavage are 

 vertical, and outside of which these planes dip towards the central 

 axis : the chain consists of gneiss or protogine, overlaid and flanked 

 by a metamorphic semi-crystalhne slate, so that it contains foliated 

 and laminated rocks in contact. But the regularity of its structure 

 has been disturbed by intrusive rocks of more modern date. The 

 following are the details which I collected regarding it. On the west 

 of Martigny the high ridge of mica-schist which intervenes between 

 the Drance and the Trient has an axis of vertical foliation striking 

 N. 30° E. ; this axis may be traced on the west side of the ravine 

 leading from Martigny le Bourg to the Forclaz on the road to Cha- 

 mounix, and it is seen well exposed close to the village of Trient : 

 the river Trient here runs nearly west, and separates the mica-schist 

 just mentioned from the slate of the Col de Balme ; on the south 

 side of the valley we find the cleavage of these slates vertical, with a 

 strike of N. 15° E., which changes on the Piedmontese side of the 

 Col de Balme to N. 45° E., and continues with this latter direction 

 till it meets the vertical foliation of the crystalline rocks of the Aiguilles 

 Rouges. Saussure (§ 642 and 646) informs us that the foliation of 

 Mont Brevent is vertical, with a strike to the magnetic north, or 

 N. 19° W., and a little south-west of Chamounix vertical gneiss form- 

 ing part of the flank of Mont Brevent is seen to strike due N. on the 

 side of the valley ; some miles to the S.W., near Bionnay, a meta- 

 morphic slate has a vertical cleavage striking N. 30° E., but the 

 rocks in the interval between this point and the foot of Mont Brevent 

 are in great confusion. 



M. Studer, p. 162 and 163, states that the structure of the middle 

 of the group of the Aiguilles Rouges is anticlinal ; and Saussure, 

 § 552, mentions the vertical foliation of Mont Loguia, and at § 598 

 and 689, vertical cleavage near Valorsine striking W.N.W. ; com- 

 bining these observations, there is little doubt that the general struc- 

 ture of the chain is that stated above, and that its foliation corre- 

 sponds to the foliation and cleavages of the hills west of Martigny, 

 which is shown in Section 1, PI. I. 



The Arve, after running S.S.W. through the valley of Chamounix, 

 turns round the foot of Mont Brevent to the N.W., and lays open in 

 a deep ravine a large mass of hard dark felspathic rock *, massive in 

 the centre of the mass, but irregularly slaty towards its exterior, 

 which has thrown the neighbouring rocks into great disorder, giving 

 both to the beds and cleavage a strike of about N.W. The foliation 

 of the intrusive rock itself is obscure, but appears to be in concentric 

 curves corresponding to the external form of the mass, a common 

 arrangement in rocks of this class : this rock is seen on both sides of 

 the Arve, and is crossed by the road at the highest point of Les 

 Montees, a little west of the village of Ouches. A smaller mass of a 

 similar rock is seen a little east of Servoz, equally disturbing the 



* Saussure, § 503, describes it as consisting of pierre de come, quartz, and 

 felspar, with very little mica. 



