360 E. E. L. Bixon—The Gavaniie Overthrust, 



Bresson/ as another example of overtlirusting, and named by him 

 the " nappe de recouvrement de Gedre-Gavarnie," after two villages 

 in the neighbourhood. I have the less hesitation in doing this because 

 the phenomena, considering their importance and their setting amidst 

 the magnificent scenery of the lofty central parts of the range, appear 

 to have received less attention from English geologists than they 

 deserve. For these reasons also, other questions in Pyrenean geology 

 are afterwards discussed in the light of some recent observations. 



In so far as the account relates to the overthrust, it is merely that 

 part of Bresson's work which I was able, in the short time available, 

 to verify three years ago when my friend, Mr. MacAlister, to whom 

 I am greatly indebted for help in preparing these notes, and myself 

 had the pleasure of visiting the ground under the guidance of 

 Mr. Stuart-Menteath. It is difficult to express adequately our 

 gratitude to Mr. Stuart-Menteath for his kindness in putting himself 

 at our disposal, strangers as we were, and though we cannot follow him 

 in some of his conclusions, to him are due many of the observations on 

 which this account is based." Finally, it should be mentioned, Carez 

 has recently described and illustrated by some admirable photogravures 

 the geology of the whole district in a work ' which will be referred to 

 again in the sequel. 



The Franco-Spanish frontier in the Gavarnie district (Hautes- 

 Pyrenees) follows the chief watershed of the range, and is an 

 important boundary, geographically, geologically, and politically. To 

 the south, under a semi-tropical sun, extends a great area of plateaux, 

 gashed by winding, canon-like gorges ; to the north lies a zone of 

 peak and pinnacle, hanging-valley and cirque, which, though narrower, 

 stretches far beyond the horizon, along which it has the appearance of 

 an angry sea turned to stone. The more immediate cause of this 

 difference has been the great rainfall on the northern side, both now 

 and in the past, when the valleys on that side were occupied by 

 glaciers, but at the same time the geological difference is as profound 

 as the geographical. 



The southern plateau-blocks have been carved out of gently- 

 inclined beds, thousands of feet thick in all, chiefly Cretaceous and 

 Eocene. Near the frontier the former system is seen to commence 

 "with Campanian Hippurite-limestone, correlative with the zone of 

 Beleynnitella mucronata of the Chalk, and to overlie, with apparent 

 conformity, a comparatively thin representative of the Permian or 



' "Etudes sur les formations anciennes des Hautes- et Basses-Pyrenees " : Bull. 

 Carte Geol. France, tome xiv (1903), No. 93, pp. 238-68, pi. iv, maps. 



"^ Mr. Stuart- Menteath's discoveries in this part of the Pyrenees have provided 

 a starting-point for much of the more recent work ; an idea of the development of 

 his own views during the controversy over the Gavarnie rocks may best be obtained 

 by referring to his book on "Pyrenean Geology" (parts 1-8, London, 1903-7), 

 especiallv to the following pages : — Ft. 3, pp. 14-15 ; pt. 4, p. 26 ; pt. 5, pp. 5-7, 

 17; pt. "6, pp. 7, 12, 1.5-17, 26, 29, 30 ; pts. 7 and 8, pp. 2, 25-26. In the same 

 work he deals also with the general question of charriar/es on lines to which we shall 

 have occasion to refer later. He has increased my indebtedness to hmi by checking 

 the statements of fact made in this paper, and by adding an appendix. 



^ " Geologic des Pyrenees Fran9aises " : Mem. Carte Geol. France, fasc. ii (1904). 



