me 



it not unfrequently happens, that the dislocation of strata has been 

 so great, that the gneiss or granite rocks are superimposed upon the 

 secondary formations. 



The granitic mountains of Oisans, which are amongst the highest 

 of the second order of European chains, attain a greater elevation at 

 their culminating point, the !Mont Pelvoux, than any of the Alps 

 between Mont Blanc and the Mediterranean. Even Mont Iseran 

 and Monte Viso are surpassed in height by this summit, which 

 measures 13,468 English feet. The ravines by which the chain is 

 intersected have a corresponding depth and ruggedness, so that the 

 cols, or passages from one valley to another, are generally covered 

 with perpetual ice and snow, and present, besides, more continuous 

 and inaccessible precipices than are common in any part of Switzer- 

 land. The author shortly described several joui-neys made through 

 the central part of this district, in which it became necessary to 

 cross cols of above 10,000 feet in height, from whence alone an in- 

 timate knowledge of the structure of these mountains can be ob- 

 tained. 



Guided by the interesting memoir of M. Elie de Beaumont, on 

 the geology of the Montagues cV Oisans, and by the admirable map 

 of Bourcet, he was enabled, in a great many particulars, to verify 

 the observations of the first named distinguished geologist, especially 

 as refers to the phenomena visible at the contact of the calcareous 

 and granitic rocks, which left no doubt on the author's mind that 

 the superposition of the latter to the former is undeniably true. No 

 more can it be doubted, that, as M. E. de Beaumont affirms, we 

 have here evidence of the extensive elevation of previously deposited 

 sedimentary rocks, probably by the appearance from below of the 

 granite itself. Professor Forbes feels some hesitation in admitting, 

 with M. de Beaumont, the crateri/orm nature of this elevation, as 

 indicated by a qud-qua-versal dip of the stratified rocks round a cen- 

 tral point in the neighbourhood of the Mont Pelvoux, and by the 

 radiation of the vallies from that centre. He considers that the 

 observations of the great French geologist, when analyzed, as well 

 as his own, rather point to an anti-clinal axis passing through the 

 point in question, and prolonged in a NN.W. and SS.E. direction ; 

 accompanied, however, with various minor lines or centres of dislo- 

 cation, especially that which elevated the mountain of Grande Rousse 

 to the northward, of which the geology has been ably described by 

 M. Dausse. The interfei-ence of this elevation with the previous 



