E. E. L. Dixon — The Gavarnie Overthrust. 411 



Trournouse, Silurian slates thrust over the Cretaceous (as shown in 

 PL XVIII, Fig. 2) contain abundant chiastolite, and though not in 

 their original position they doubtless, he says (p. 162), come from the 

 deep-seated margin of the schists. But he attaches the greatest 

 weight to the reported occurrence, within the granite-plexus, of 

 masses of unaltered slates and quartzites, as well as the schists, in one 

 of which at la Prade de Saint-Jean, between the village and the 

 cirque of Gavarnie, Calymene Tristani is said to have been found 

 (pp. 39 and 162) by the elder Frossard.^ He further points to the 

 close petrological affinity of the Gavarnie-Heas complex to that of 

 Caillaouas, which, though at some distance to the east, is connected 

 with the other by the belt of metamoi'phism previously mentioned, 

 and in the neighbourhood of which Devonian rocks themselves have 

 been metamorphosed (p. 163). As this suggests that the Gavarnie- 

 Heas granite itself is post-Devonian, and as throughout the district 

 he finds little evidence of large earth - movements prior to the 

 Hercynian folding, such as elsewhere precede pre - Carboniferous 

 intrusions, but on the other hand can prove that the Hercynian 

 mountain-building was followed hereabouts by the irruption of granite 

 masses into various formations, he concludes that the Gavarnie-Heas 

 and the Caillaouas granites are themselves Hercynian (p. 169). 



His evidence does not, however, appear to be conclusive. The increase 

 in intensity of the metamorphic belt westward is not necessarilj' due 

 to our approaching the Gavarnie complex, but is sufficiently explained 

 by the fact, unknown to Bresson when he published in 1903 the work 

 to which we have been referring, that granite is present in the higher 

 part of the Gela Valley, where, as stated above, the metamorphism 

 of the belt becomes very intense. It is shown on the Luz sheet of 

 the Carte Geol. Det., published in 1906, and appears to have been 

 foiind by Bresson himself, as it is included in that part of the sheet 

 surveyed by him. It is represented (see Fig. 2) as a westward 

 continuation of the Bielsa granite, which lies to the south of the 

 belt. The belt itself is due to granite, apparently an offshoot of the 

 Bielsa mass, which just reaches the surface at the Pic de Lustou. 

 We must ascertain, therefore, whether the Gavarnie granite may be 

 regarded as a continuation still further westward of the Bielsa mass, 

 and with that view we shall now compare the rocks of the two places, 

 as their relationship is not directly determinable. 



The Bielsa granite appears as a large, uniform, and coherent mass 



1 The original record does not state very clearly the exact source of this specimen, 

 and on that account the conclusion drawn frona it has appeared to be of limited value, 

 but fortunately the specimen itself is still in existence, and has been examined by 

 Stuart-Menteath, who has questioned the identification in his " Pyrenean Geology," 

 and, more recently, by Bresson. The former has kindly supplied the following 

 extract from Bresson's remarks in the recently published final part of the Bull. 

 See. Geol. France, tome vi, which contains an account of an excursion of the 

 Society to Gavarnie, and which has appeared since these notes were written : — 

 "Mais I'examen de ce fossile a montre qu'il provient, d'apres ses caracteres et 

 d'apres la composition de la gangue, du Coblenzien du pic de Mourgat, dont las 

 eboulis recouvrent les pentes de la Prade et qu'il appartient au genre Phacops 

 {Phaeops aff. Potieri, Bayle)." In view of these views of Stuart-Menteath and 

 Bresson the fossil evidently has no bearing on the question of the age of the schists. 



