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



which they rest ; since the higher the authority on which an error 

 is promulgated, the more pernicious is it to the progress of science. 



On a recent visit to the Alps I devoted ten days to the environs 

 of Mont Blanc, to examine the accuracy of the views above stated, 

 and to observe the relations of the foliation of the crystalline rocks 

 to the cleavage of the surrounding stratified slates. The time at my 

 disposal only allowed me to take a hasty view of the principal phse- 

 nomena ; but here, as throughout Switzerland, the higher mountains 

 have long been especial objects of study, and require little re-exami- 

 nation, and it is in the valleys and on the lower flanks of the hills 

 that observations are wanted to classify and work out the position of 

 the secondary rocks. The following pages contain the results of my 

 observations, which, though very incomplete, will help to bring the 

 geology of this interesting district into harmony with our general 

 experience. 



The structure of the great chain of Mont Blanc is well seen from 

 the Mer de Glace ; by ascending as far as the Jardin we obtain a 

 section of more than three-fourths of the chain, which may be com- 

 pleted by visiting its eastern flank in the Val Ferret ; the direction 

 and dip of the rocks which form the intervening ridge being distinctly 

 visible from both sides. Saussare has given minute descriptions of 

 many parts of the chain ; Forbes, Studer, and others describe other 

 portions, so that very little remains unknown. 



The mineral character of the rocks has been well described* ; they 

 consist for the most part of a talcose gneiss, usually containing both 

 mica and talc, which towards the centre of the chain is so slightly 

 foliated as to resemble granite, while on its flanks the more marked 

 foliation brings it to the condition of talc-schist or mica-schist. The 

 less foliated portions have been called Granit veine, Alpine granite, and 

 Protogine ; but there is no natural line to be drawn between these 

 and the more schistose varieties, and a passage may be traced by in- 

 sensible gradations from the schist to the more massive and granitic 

 rock of the centre f. 



Instead of the simple fan-shaped arrangement of the foliation, with 

 one vertical axis, which has previously been attributed to the gneiss 

 of Mont Blanc, I found two nearly parallel lines of vertical foliation 

 running through the whole chain, separated by a narrow, steep anti- 

 clinal axis ; on the Mer de Glace these lines are about a mile and a 

 half apart, but they diverge a little in their course both to the north 

 and south. Ascending the Montanvert from the valley of Cha- 

 mounix, the foliation when first seen dips E. 50°, becomes gradually 

 steeper as we ascend till it reaches E. 80° above the little inn ; on 

 the Mer de Glace, between the Montanvert and Trelaporte^, the direc- 



* Saussure, § 677 ; Studer, vol. i. p. 168 ; Bakewell, Tarentaise, vol. ii. p. 22. 



t There is an exception at the angle on the Mer de Glace, where a projecting 

 mass of granite is distinctly separated from the surrounding gneiss. 



+ These points will be found in Professor J. Forbes's Map of the Mer de Glace 

 accompanying bis " Travels through the Alps of Savoy." It is necessary to warn 

 the reader that the engraver of that map has laid the line of True North to the 

 west instead of the east of the magnetic north. 



