NO. 31 GEOLOGICAL STRUCTURE OF THE ALPS — WILLIS 9 



striibel. It is found in the summit and is traced down the southern 

 slope to the valley of the Rhone. 



This statement rests upon Prof. Lugeon's own observations. He 

 has traced this overthrust from the Rhone valley over the Wild- 

 striibel to the Prealpes near Lenk, and has identified the overthrust 

 mass as a nappe de recouvrement having its " racine " in the Rhone 

 valley, its "carapace" on the summit of the Wildstriibel, and its "tete" 

 near Lenk. We may accept the identification of the several sections 

 as parts of a whole structure, for it rests upon that detailed strati- 

 graphic and structural study which Prof. Lugeon has pursued with 

 minute care. But, misled by the erroneous assumption that thrust- 

 ing proceeded from one direction only. Prof. Lugeon has, I believe, 

 mistaken the relations of the parts to the whole. The so-called 

 " racine " is the tete and the " tete " is the racine, if we can apply 

 these terms at all. The great major thrust from the northwest can 

 be recognized from Lenk to the base of the Wildstriibel and from 

 the summit of the Wildstriibel to the Rhone valley. It is dislocated 

 between the base and the summit of the Wildstriibel by one younger 

 thrust from the southeast, and is finally cut off by another south of the 

 Rhone. 



Having been obliged by my observations to recognize the preceding 

 statement of the structure of the Prealpes and Hautes-Alpes as cor- 

 rect, and feeling sure that the concept of superimposed " pli-nappes " 

 or '' nappes de recouvrement " should be replaced by an explanation 

 based on intersecting major thrusts, minor thrusts, and folds, I 

 ventured to consider the supposed sequence of pli-nappes which 

 Prof. Lugeon has described as forming the Hautes-Alpes from the 

 Rhone valley eastward. He distinguished the " pli de Morcles," the 

 "pli de Diablerets," and the "pli de V/ildhorn-Wildstriibel," and 

 others, in the order named from west to east, and showed that the pli 

 de Morcles pitches under that of the Diablerets, the latter pitches 

 under the pli-nappe of the Wildstriibel, and so forth, each folded and 

 overthrust mass disappearing under the one to the east of it. I 

 accept this observed structure, a portion of which I had the pleasure 

 of seeing with Prof. Lugeon in an excursion to the Diablerets. Ac- 

 cording to the current interpretation, which I caimot accept, each 

 of these pli-nappes developed as a distinct overthrust, one after the 

 other, each older one having a " racine " south of the next younger, 

 and each having been thrust over the preceding. They would thus 

 have been piled up, as shown in various hypothetical sections, in such, 

 for instance, as profiles I to IV of the Geologic de la Suisse, by Dr. 



