414 E. E. L. Dixon — The Gavarnie Overthrust. 



of that granite. T5ut here I feci compelled to differ from him on 

 a question of fact, for knowing that the age of the schists was said to 

 be Ordovician I carefully examined many exposures in the (Javarnie 

 and Heas Valleys, with the open mind of an enquirer, without 

 discovering any evidence anywhere of unaltered sediments. The 

 shaliest-looking schist (No. 188, p. 422) was found under the microscope 

 to be as thoroughly metamorphosed as any. And in the neighbour- 

 hood of the bridge near la Prade de St. Jean, whence Bresson recorded 

 reddish and greenish * schistes,' almost ixnaltered, associated with 

 grey quartzites, the whole resembling the Ordovician beds of the Gela 

 (op. cit., pp. 39, 162, 182), every exposure of sedimentary rock showed, 

 as Mr, Stuart - Menteath pointed out, merely mica- and quartz- 

 schists. In fact, so complete had been the metamorphism in the rocks 

 that were seen that the occurrence close by of comparatively unaltered 

 Ordovician sediments, if unquestionable, would point either to extensive 

 faulting or to the metamorphism of the schists being pre-Ordovician. 

 But it is so far from being unquestionable in view of the inference 

 that palaeontological confirmation of the Palaeozoic age of any part of 

 the schists is lacking, which may be drawn from Bresson's statement 

 about the Calymene, previously quoted, that we may say that no 

 conclusions, one way or the other, should be based on it. 



Bresson next points with truth to the close resemblance of the 

 Gavarnie granite to the Caillaouas granite as indicative of a genetic 

 connection, but the post-Devonian age of the second is not so certain. 

 It is based on the occurrence of altered fossiliferous Devonian, at 

 no great distance from the granite, but Bresson's most recent work, 

 published on the Luz sheet, shows that the altered rocks form part of 

 the metamorphosed belt previously mentioned, and are much nearer 

 the granite which crops out at the Pic de Lustou in the middle of 

 the belt than the Caillaouas granite, so that their alteration is, at 

 least, as likely to be due to granite connected with the Bielsa mass, 

 which no one doubts is Hercynian, as to the Caillaouas granite. 



Finally, although there is a lack of such direct evidence of powerful 

 pre -Carboniferous earth-movements as is found in connexion with 

 pre-Hercynian granite intrusions in other districts, it is certain that 

 the folding of the Gavarnie schists preceded their thermal meta- 

 morphism, and that the latter itself was earlier than at least one 

 period of shearing. This period is attested by the crushing, which in 

 places has I'educed the rocks, contact-minerals and all, to mylonite. 

 As it is probably Hercynian from the rarity of later disturbances in 

 these schists, the thermal metamorphism and original folding of the 

 schists maj^ well be pre-Hercynian. 



Thus we see that Bresson's evidence that the age of the Gavarnie 

 schists is Palajozoic and their metamorphism Hercynian appears to be 

 inadequate and to leave the question open to solution from other 

 considerations, such as those brought forward in the following pages by 

 Mr. Barrow on petrological grounds. But at the same time light may 

 also be thrown on the problem by stratigraphical evidence, on the lines 

 of a comparative study of the neighbouring granitic and metamorj)hic 

 rocks. Materials for a preliminary comparison are already to hand in 

 the admirable work of Bresson (pp. 139-197), already so extensively 



