E. E. L. Dixon — The Gavarnie Ovevthrust. 415 



referred to. There he enumerates six granite-masses as outcropping 

 in the area of the Luz sheet, viz., those found at (1) Cauterets, 

 (2) Neouvielle, (3) Borderes, (4) Gavarnie, (5) Pic du Midi de Bigorre, 

 (6) Conques. To these Avill be added the Bielsa and Caillaouas 

 granite-masses, the latter already mentioned, because it lies but a short 

 distance to the east of the Luz sheet and has been carefully described 

 by Lacroix.' 



The first three masses have been shown by Bresson to form a natural 

 group in that they penetrate various Palaeozoic formations, including 

 the Carboniferous, and traverse both anticlines and synclines. It is 

 apparent from his descriptions and those of Lacroix- that they are 

 coherent and closely resemble one another and also the Bielsa mass 

 in those very features in which the latter differs from the Gavarnie 

 granite, and that, as he maintains, they are Hercynian. Masses (4), 

 (5), and (6) he places in a second group, because they have not reached 

 the Carboniferous and because they form the cores of anticlines. In 

 their case, the accounts make it clear that they are linked closely 

 together and with the Caillaouas granite by the same characters which 

 have been mentioned as separating one of them, the Gavarnie granite, 

 from that of Bielsa, i.e. from the first group. They each consist of 

 a complex of granite, gneiss, and mica-schist like that of Gavarnie 

 already described.^ 



In spite, however, of the marked differences between the two groups, 

 Bresson regards the second also as Hercynian, chiefly on account of the 

 altered character of Palaeozoic rocks adjacent to its members. But 

 that it is by no means certain that the latter have produced the 

 alteration, is apparent from his description and that of Lacroix and the 

 subsequent work published on the official map, especially in view of 

 both the frequency of undoubted Hercynian intrusions in the whole 

 area, some of which barely reach the surface, and also the complica- 

 tions arising from the subsequent Tertiary disturbances. 



A point of the greatest importance is the fact that these mixtures of 

 schist and granite form the cores of anticlines in those cases where 

 their relations to Palaeozoic formations are known, and in two instances 

 are surrounded by comparatively uniform belts of Silurian rocks. But 

 whereas Bresson explains this fact by supposing the granites to be 

 deep-seated ' roots ' of intrusions, in my opinion it points strongly to 

 their really being parts of a widespread complex, in age pre-Silurian 

 at least, on which the contiguous rocks have been deposited and which 

 we now see exposed in a few places in large folds by extensive denu- 

 dation. This view is greatly strengthened by their marked contrast 

 with the Hercynian granites in the neighbourhood, all of which in their 

 turn resemble one another in essentials. The whole question, however, 

 is a large and important one, worthy of close attention and fraught 

 with difficulties on both sides. 



' Op. cit., pp. 3-1 et seq. - Op. cit., pp. 45-61. 



^ It is interesting to note that Lacroix draws attention to the fact that in the inner 

 part of the aureole of the Cauterets mass (Hercynian) the felspathised schists remain 

 distinct from the igneous rock injected into them, whereas in the case of the 

 Caillaouas mass they pass insensibly into it through highly micaceous, gneissose 

 granite (pp. 64-5). The second mass thus resembles that of Gavarnie in what is 

 probably a fundamental character. 



