410 E, E. L. Dixon — The Gavarnie Overthrud. 



abundance of secondary minerals, such as biotite, garnet, cordierite, 

 silliminate, etc., with, here and there, ci'ystalline limestones, the 

 whole injected with a plexus of plutonic rocks ; they evidentlj', as 

 MM. Michel Levy and Lacroix have already stated,' represent 

 a sedimentary series which has been profoundly modified by thermal 

 metamoi-phism. However, the rocks which ci'op out over the whole 

 area enclosed by the Cretaceous of the above valleys (Fig. 2) show no 

 general diminution of metamorphism in any direction to guide us 

 to the unaltered originals ; they have, in fact, been completely 

 recrystallised throughout, and are so abundantly and intricately 

 veined with gneisses and granites that it is often difficult to say 

 whether the heterogeneous resultant is a mass of granite crowded 

 with enclosures or a mass of sediments crowded with veins. 



The resemblance of this sedimentary and igneous complex to the 

 fundamental rocks of other parts of the world led the older writers, 

 Ramond and De Charpentier amongst others, to refer them to the 

 ' primitive ' series (our Archsean), at that time regarded as part of the 

 original crust or as crystalline sediments of a primaeval sea. Bresson, 

 however, points out that Levy and Lacroix have adduced evidence of 

 the production of these schists from normal sediments, and goes on 

 (op. cit., pp. 39, 160-163, 167-169, 181-184, 194-197) to give 

 reasons for regarding the age of the sediments in this case as 

 Ordovician, and of the granites which have altered them as Hercynian 

 (post-Carboniferous, but pre-Triassic), though he states at the same 

 time (p. 197) that both groups differ in no respect from Archaean 

 rocks. The evidence he cites is as follows. The rocks in question, 

 though occurring as an inlier in the Cretaceous without visible 

 connection with neighbouring Palaeozoic outcrops, lie in the course of 

 a belt of metamorphism which he has traced in a westerly direction to 

 within a short distance of them, through an area composed entirely 

 of Palaeozoic rocks (see Fig. 2). This metamorphism is generally 

 characterised by considerable reconstruction of the sediments, e.g., 

 the production of secondary mica, sillimanite, and large chiastolites, 

 and appears to increase in intensity westward towards the Gela, where 

 the rocks (Ordovician slates) become highly micaceous and crowded 

 with chiastolite, and pass into leptynolites. At the same time a great 

 number of veins of quartz, pegmatites, and ' micro-gran ulites' [quartz- 

 felsites] appear. Hereabouts the sedimentary and igneous rocks form 

 the basement-platform of the Cretaceous, beneath which they dis- 

 appear westward. At a short distance in that direction the platform 

 reappears in the Heas and Pau valleys, where, however, it is composed 

 of the rocks whose age is in question. The relationship of the latter 

 to the metamorphic rocks of the Gela cannot, therefore, be determined 

 directly, as the change occurs below the Cretaceous, but the relative 

 positions and degrees of metamorphism of the two series of altered 

 rocks has suggested to Bresson that those in the Heas and Gavarnie 

 valleys are continuous beneath the Cretaceous with those to the cast, 

 a conclusion which is supported, in his opinion, by observations in the 

 valleys themselves. Tlius, in several places, such as the Cirque de 



1 Bres.'on, op. cit,, p. 160. 



