106 FroccetUuys of I ltd Uui/ul Sccieti/. 



appendage to the main Alpine cliain, which occupies a considerable 

 poition of the old province of Daiiphine, and the modern depart- 

 ments of the Hantes Alpes and Iserc. It is bounded, roughly, by 

 the rivers Arc and Isere on the north, and by the Durance and the 

 Drac in other directions. Its nucleus is esssntially granitic, against 

 which sedimentary deposits of limestone, of different ages, and espc- 

 <Mally of lias and chalk, repose in highly elevated or contorted strata ; 

 and 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 l)ighest 

 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 

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

 measures 13,4G0 English feet. The ravines by which the chain is 

 intersected have a coi responding depth and ruggedness, so that the 

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

 with perpetual ice and snow, and present, beside?, more continuous 

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

 land. The author shortly described several journeys made through 

 the central pait 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 tiie structure of these mountains can be ob- 

 tained. 



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

 the geology of the Montagnes d' Oisans, and by the admirable map 

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

 the observations of the first named distinguishsd geologist, especially 

 as refers to the phenomena visible at the contract 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 crateriform nature of this elevation, as 

 indicated by a qna-qud-vcrsal dip of the stratified rocks round a cen- 

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



