366 E. E. L. Dixon — The Gararnie Orerf/irusf, 



sporadic occurrences of folding in the Cretaceous sheet and the basenieat- 

 phitform below do not militate against Bresson's theory of overthrusting, 

 as they do not explain the occurrence of Palaeozoic rocks in the higher 

 ground, and, in fact, may be regarded as ' tucks ' in the otherwise 

 undistorted floor of the overthrust. 



At some places such as Peyreblanque, a short distance to the south 

 of the scene of PL XVIII, Fig. 1, the Cretaceous sheet, horizontal and 

 yielding abundant hippurites, gives rise to a distinct terrace along the 

 side of the valley, below the higher ground formed of Paloeozoic rocks. 

 But its relations to the latter are most strikingly shown in the neigh- 

 bourhood of the cirque de Troumouse, at the head of the Heas Yalley. 

 There the high, wide ledge which constitutes the floor of the cirque is 

 formed essentially of the approximately horizontal basement-platform of 

 crystalline rocks, in part still covered with a thin, gently undulating 

 veneer of hippurite-limestones, on which, near the steep walls enclosing 

 the cirque on three sides, rests chiastolite-slate. This slate has been 

 proved to be Silurian by Bresson's discovery in it of a characteristic 

 Orthoceras -liraestone (op. cit., p. 246). Its superposition on the 

 hippurite-limestones is equally unquestionable, for in the left bank of 

 the Maillet stream it visibly overlies limestones, which on the opposite 

 side have been bared over a wide, flat area, and contain typical 

 hippurites. The position of affairs may be gathered from PI. XYIII, 

 Fig. 2, in which in the foreground is seen the wide area (its surface 

 parallel to the bedding) of white, hippurite-limestone, whilst behind 

 the figure rises a low, vertical cliff of dark chiastolite-slate, at the 

 top of which the ground recedes at a gentle gradient to the steep bank 

 just visible in the background. This bank is really the foot of the 

 northern slope of the frontier-ridge, the crest of which rises two to 

 three thousand feet above the floor of the cirque, and which is made 

 up entirely of various Palaeozoic formations, including, near the Port 

 de la Canaou as already mentioned, Lower Devonian to a great extent. 

 Nevertheless, on crossing by the 'port' into the valley of Hount-Sainte, 

 it is found that, on the southern side also, these old rocks rest on a thin 

 sheet of hippurite-limestone, at a level which is not greatly different 

 from that on the northern side, four kilometres distant. When we 

 consider further that the limestone on both sides is but slightly inclined 

 and is underlain by an even basement-platform of far older rocks, from 

 which it is separated on the southern side by a thin group merely of 

 Permo-Triassic beds, we are forced to conclude that the platform with 

 its thin covering of hippurite-limestone forms the foundation of the 

 ridge from side to side, and underlies the whole Palaeozoic mass above. 

 This conclusion being precisely similar to that drawn from the evidence 

 of outcrops along the valleys on the French side, we are justified in 

 saying that a complex of Palaeozoic rocks rests on a sheet of Cretaceous 

 limestones, dipping slighth* north, over an area which extends at least 

 from the latitude of Gedre to an irregular line near the frontier, 

 a width of ten kilometres, and from the valley of Saux in the east to 

 beyond that of the Pan in the west, a length of more than 18 kilo- 

 metres. 



As regards the issue between overthrust and recumbent fold, 

 Bresson has shown (op. cit., p. 246, etc.) that both the thin group 



