1854.] SHARPE — STRUCTURE OF MONT BLANC. 13 



tion of the foliation changes to N. 15° E., N. 25° E. and N. 30° E., 

 its inclination rising from a dip of E. 25° S. 75° to the perpendicular 

 at Trelaporte, where it strikes N. 30° E. ; to the eastward of this 

 spot it dips W. 30° N. 85°, then 80°, again changes to E. 30° S. 80°, 

 thus forming an anticlinal : it again reaches the perpendicular at the 

 Couvercle, with the strike of N. 30° E. : from this point to the east- 

 ward the dip is W. 30° N., the inclination gradually diminishing till 

 it reaches 60° on the west side of the Val Ferret. In the intervening 

 ridge which separates the Mer de Glace from the Val Ferret, inclu- 

 ding the Geant, the Col de Geant, the two Jorasses and the Aiguille de 

 Lechaud, the foliation dips between 75° and 80° (see PL I. Sections, 

 Nos. 3 and 4) . There is therefore a narrow anticlinal axis in the centre 

 of the chain, with half an arch on each side of it ; and the other parts 

 of these arches must be looked for on the other sides of the Val 

 Ferret and Allee Blanche, and of the valley of Chamounix. 



Let us now follow the direction of the two hues of vertical foliation 

 just mentioned. The western line, which is seen at Trelaporte on 

 the Mer de Glace, runs S. 30° W. through the Aiguille des Charmoz ; 

 if continued in the same direction it would pass through the highest 

 point of Mont Blanc ; on the side towards Chamounix the summit 

 is entirely covered with snow, but on the eastern side the rock is less 

 concealed, and it appears, when seen from the Val Ferret, to be 

 composed of vertical masses : we carry the same line through the 

 Aiguilles de Blaitiere, du Plan, and du Midi, in all of which Saussure 

 informs us (chap, xviii.) that the foliation is vertical with a strike of 

 S. 35° W. For a short distance further south my information fails 

 me, but on nearly the same line we find the Jurassic rocks of the 

 Col du Bonhomme intersected by a vertical cleavage striking S. 

 25° W. 



The same line of vertical foliation may be traced in the same manner 

 northward from the Mer de Glace ; it runs N. 30° E. through the 

 Aiguille du Dru, and N. 35° E. on the western side of the Aiguille 

 du Tour ; from that point I can only carry it on conjecturally to 

 meet a line of vertical cleavage striking N. 30° E. through the slate 

 a little west of Sembranchier in the valley of the Drance. 



I have not followed the eastern line of vertical foliation to the 

 northward, but I can point out its course for some distance south- 

 ward ; from the Mer de Glace we can see that it runs S. 30° W. from 

 the Aiguille du Moine to the Couvercle ; from the southern point of 

 the Tacul it passes S. 25° W. through La Tour Rondeandthe second 

 Flambeau to the Vierge : Saussure describes Mont Brogha behind 

 the Glacier de Miage, as composed of vertical mica-schist, striking 

 N.E. and N.N.E. (§ 891), and farther on I found a slaty limestone 

 in the Allee Blanche, above the Lac de Combal, intersected bv a 

 vertical cleavage striking S. 35° W., with plates of mica on the planes 

 of cleavage (see PL L Sect. 7), and the Jurassic rocks of the Col de la 

 Seigne are vertically cleaved in the same direction on the continuation 

 of the same line. 



The information relating to the parallel chain of Mont Brevent 

 and the Aiguilles Rouges is less complete ; but I have reason to be- 



