362 E. JS. L. Dixon — The Gavarnie Overthrust, 



Trias, which in its turn rests with the strongest possible unconformity^ 

 on a wide, even platform of far older rocks, notably crystalline schists 

 and granite. But on the northern side the formations and structures 

 are as varied as the surface-features, including, as they do, a practically 

 complete sequence of stratified rocks, pierced by igneous intrusions 

 of all kinds, and thrown into apparently hopeless confusion by in- 

 numerable faults and folds. Under these circumstances it is especially 

 interesting to find that, according to Bresson's views, near the frontier 

 itself, in the Gavarnie district, the northern side overlaps the 

 southern. Exposures in deep valleys in that neighbourhood show 

 clearly, as we shall see, that a wide sheet made up of a complex of 

 Palaeozoic rocks, which, by various characteristics, are linked with 

 those forming the northern zone, has been forced southward, by 

 intense tangential pressure, for several miles over the southern plateau. 

 In its progress it has cleared away almost the whole of the beds 

 which once rested on the underlying, apparently immovable platform 

 of crystalline rocks, but has left still in place, and almost unaffected, 

 a thin layer, generally 10-15 m. thick, of the basement-beds of the 

 Hippurite-limestone, but has crumpled up to a prodigious extent 

 higher horizons in the region immediately beyond where it came to 

 rest. In somewhat the same way a snow-plough leaves a thin layer 

 of snow on the ground and heaves iip what lies immediately in front. 

 The analogy is the more complete, as the overthrust mass appears 

 to have come to a standstill with its front still buried deeply, and 

 possibly even somewhat overhung by the beds opposing its advance. 

 (See Fig. 1.) The basal platform has not, however, entirely escaped 

 distortion, for several sharp 'tucks' along the top testify to the 

 intensity of the compression it has undergone. One of these, shown 

 diagrammatically beneath the words ' Middle Devonian ' on Fig. 1 , 

 has been described and figured by Bresson (op. cit., pp. 247-8, pi. iv, 

 fig. 2) and Carez (op. cit., pi. viii, fig. 1), whilst another, discovered 

 by Mr. Stuart-Menteath, will be mentioned later. 



In some such way as this we may interpret Bresson's conclusion 

 that a sheet of Palaeozoic rocks has been thrust from the north over 

 the Hipimrite-limestone. That conclusion is based on the following 

 evidence. 



The valleys in which the structure is exposed, those of the Gela, 

 Heas, and Pau on the French side, and various branches of the 

 Pinede valley on the Spanish side, are shown on Fig. 2, reduced from 

 the French Geological Survey map," together with the rocks of the 

 basal platform, the Trias and Cretaceous above, and the Palaeozoic 

 complex which is supposed to have been thrust over all. 



The relationships of the outcrop of the Trias and Cretaceous to 

 that of the Palaeozoics and to the course of the valleys throughout the 

 area are seen at once from the map and PI. XYIII, Fig. 1, which 

 illustrates the essential features of almost any part of the district. It 

 represents the Gavarnie (Pau) valley, just above Gavarnie, as seen 



' As will be seen later, Carez -would probably regard one or both of these 

 junctions as overthrusts. 



■ Carte Geol. Dot. 251, i'euille de Luz, 1906. 



