412 E. E. L. Dixon — The Gavarnie Overthnist. 



of ordinary granite on the Carte Geol. Det. It was examined by us 

 during a ti'a verse from Bielsa to Aragnouet, and was found to be 

 a normal biotite - granite of j&ne or coarse grain, generally quite 

 massive, but here and there, as at Bielsa itself, foliated as though 

 by differential movement when partly, consolidated. Its aureole, both 

 in the Gela valley, as described by Bresson, and in the Pinara valley, 

 as observed during our ascent to the Port de Bielsa, evidently belongs 

 to the second of the two types found by Lacroix in the Pyrenees 

 and characterised respectively by hornstones and mica-schist-like 

 rocks.' The actual contact was not seen, but at a short distance 

 mica-schists were observed, so closely resembling the Gavarnie schists, 

 at least in outward appearance, that no specimens were collected for 

 comparison. But it was soon found that such rocks extended for 

 a comparatively short distance from the granite, probably less than 

 400 m., giving place gradually to spotted phyllites, which in turn 

 shaded away into spotted slates. At the Port de Bielsa, at a distance 

 of 800 m. from the granite boundary as mapped by Bresson, the 

 spotting itself had become so slight as to be negligible, and unaltered 

 Palaeozoic slates were reached. In fact, it became obvious that 

 a normal aureole had been crossed. Specimens were collected from 

 the intermediate zone of spotted phyllites (191) and from the outer 

 zone of spotted slates (192), which have been microscopically examined 

 by Mr. George Barrow for comparison with the Gavarnie rocks (see 

 p. 423). The above estimate of the width of the aureole agrees with 

 the width in the Gela valley as shown in the Carte Geol. Det. by 

 Bresson, and does not appear to be exceptionally narrow. 



In the Gavarnie and Heas Valleys the igneous and metamorphic 

 rocks have a completely different relationship, being inextricably 

 mingled to form a complex which shows no progressive change in any 

 direction. This fact has long been recognised, and with the exception 

 of a few igneous veins they are coloured as ' granitised ' schists and 

 quartzites, gneiss, and mica-schists on the Carte Geol. Det. The 

 metamorphism in fact is regional ; the rocks it has affected, as exposed 

 in the two valleys with their tributai'ies, are completely altered 

 throughout, i.e. for a length of II kilom. and a breadth of 9 kilom. 

 (Fig. 2). 



The complex was examined at various places, and the following 

 notes, though referring primarily to the rocks exposed along the 

 torrent which descends the eastern side of the Heas Valley at Heas, 

 may be regarded as descriptive in general of the whole outcrop. The 

 sedimentary rocks vary from mica- to quartz-schists, according as they 

 were originally argillaceous or arenaceous, their banding, which 

 probably repi'esents the original bedding, being now strongly contorted. 

 A specimen (No. 190) of similarly banded schist from the Gavarnie 

 district, showing both quartzose and micaceous parts, is described 

 by Mr. Barrow on p. 422. In that neighbourhood it was noticed that 

 biotite had developed along both the banding and the foliation of 

 some of the schists, and that crystalline limestones wei'e mingled 



1 Lacroix, " Le Granite des PjTenees et ses Phenomenes de Contact " ("2™"' raeni.) : 

 Bull. Carte Geol. Fr., tome xi (1900), No. 71, p. 64. 



