422 George Barrow — Rocks from the Gavarnie District. 



tough, crystalline rock showing marked contortion, which had clearly 

 been subjected to great mechanical stresses and deformations before 

 heating. These stresses broke up the original sediment into lenticles 

 or phacoids, the length of which suggests the rock was a gritty shale, 

 possibly a banded shale. 



The lenticles are built up of well- crystallized quartz and felspar, 

 the whole thickness of a single lenticle being often filled by one grain 

 of quartz. There are also present patches of irregular-shaped garnet, 

 often bordered by brown mica. These lenticles are separated by films 

 or flaser-ribands of pale fibrous material suggestive of decomposed 

 sillimanite. In this only a few needles of sillimanite can be still 

 recognized ; but where the fine mesh comes into contact with quartz 

 the terminal threads of the former penetrate the quartz, and in this 

 case are at once seen to be sillimanite, which has been protected from 

 decomposition by its coating of quartz. This latter phenomenon is 

 a very characteristic feature of these flaser films of sillimanite. The 

 rock is a sillimanite gneiss. 



The second (188), also from Gavarnie, is a slightly horny -looking, 

 brownish rock, largely built iip of lenticles and films rich in brown 

 mica. The lenticles are of slightly different composition ; one set are 

 largely composed of rich red biotite (contact-biotite) associated with 

 a little sillimanite ; the other set are composed of sillimanite in long- 

 bladed crj-stals arranged parallel to the foliation, and associated with 

 brown mica in somewhat smaller crystals. Both types of lenticles 

 are enveloped by films of fibrous sillimanite, much decomposed, and 

 practically identical with those in the rock last described. The rock 

 is a sillimanite gneiss, and was clearly a shale originally and markedly 

 less siliceous than 187. 



So far as the minerals are concerned, these rocks might be of almost 

 any age ; but experience has shown that if of post-Torridon age the 

 sillimanite-bearing rocks would be restricted to the immediate margin 

 of a large mass of coherent granite, a fact well brought out by 

 Dr. Barrels in his descriptions of the great coherent masses of granite 

 in Brittany, which are substantially of the same age as the great 

 coherent masses referred to by Mr. Dixon. 



The third type (190, from the east side of the Pau Yalley at 

 Gavarnie) was much more siliceous originally and probably a banded 

 veiy fine sandstone, partly calcareous. It is a banded rock, with 

 a fine gneissose structure, the darker bands being composed of greenish- 

 brown hornblende in irregular-shaped patches, disseminated through 

 a groundmass of plagioclase felspar with which a subordinate amount 

 of quartz is associated. The hornblende suggests the original rock 

 was slightly calcareous. The lighter - coloured bands are more 

 quartzose and there is little or no hornblende present, but decomposed 

 biotite is fairly abundant. The structure of both is essentially that 

 of a ' granulite ' produced by the prolonged and intense heating of 

 a somewhat crushed siliceous sediment. In many respects the lighter 

 bands bear a close resemblance to the less common ' plagioclase phases ' 

 of the Moine gneiss. (See Photo, PI. XIX, Fig. 1.) 



Of the rocks occurring within the aureole of contact metamorphism 

 surrounding the large coherent granite of Bielsa, two may be referred 



