10 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 56 



Schardt/ The total altitude would then have been not far from 

 12,000 meters. 



Were this view correct, there must be a notable difference in age 

 between the successive overthrusts, and particularly between the earli- 

 est and latest of the series. Since they have been exposed to erosion, 

 and by hypothesis some have been exposedlongerthan others, weshould 

 expect the older masses to be more eroded then the younger ; as, for 

 instance, in the case of older and younger volcanoes of the Canary 

 or Hawaiian Islands. Such is not the case in the Alps. The Hautes- 

 Alpes constitute a physiographic unit, although they are structur- 

 ally complex. Considering them for the moment also as a structural 

 unit, and recalling the great recent thrust plane up which they have 

 advanced as a whole from the southeast, we may recognize that the 

 altitude of the chain above the Prealpes is due to the rise on the 

 inclined thrust plane, while the superb scarp which they present to 

 the northwest and north is the eft'ect of consequent erosion on the 

 front of the rising mountain mass. This scarp is of uniform physio- 

 graphic age from one end of the chain to the other, it being every- 

 where in a stage of extreme youth. 



Having studied a similar scarp and structure in the Lewis range of 

 Montana, where Pre-Cambrian rocks were overthrust on Cretaceous, 

 probably during the Miocene, I judge by comparing the effects of 

 erosion in the two mountain ranges that the elevation of the Hautes- 

 Alpes by advance on the thrust plane occurred not earlier than the 

 Pliocene and possibly in the early Quaternary. However this may be, 

 the scarp is very young, and uniformly young from end to end. 



At certain points a lesser scarp branches from the great front of the 

 range and turns back into the heights. As the front faces north by 

 west, these lesser ones face westerly. They also are physiograph- 

 ically young. Indeed, they are distinguishable from the great scarp 

 only by their branching off and having less altitude. Each of these 

 minor scarps is the western face of a segment of the range which Prof. 

 Lugeon has described as a distinct pli-nappe. The cliffs of the Dia- 

 blerets thus overlook the segment which is called the pli de Morcles, 

 Each minor scarp is based on a thrust plane, dipping easterly, just as 

 the major scarp is based on the major thrust dipping southeasterly. 

 Each minor scarp is related to its minor thrust as the major scarp is 

 related to the major thrust. They all form one system and are of one 

 and the same age. 



' Schardt, H. : Geologic de la Suisse, article extrait de " La Suisse " ; 

 Publications de la Dictionnaire Geographique de la Suisse, Neuchatel, 1908, 

 pis. 22-23. 



