George Barroiv — Rocks from the Oavarnie District. 423 



to. The first (191), collected about half-way between the Eiver Pinara 

 and Port du Bielsa, is a very fine spotted schist or phyllite. Like the 

 specimens from the older series already described, this was subjected 

 to considerable deformation before heating, and is now a very fine 

 schist largely composed of quartz grains and small crystals of white 

 and brown mica. In this, as a matrix, are set a number of large 

 patches of shimmer aggregate material, almost certainly decomposed 

 andalusite. (Photo, PL XIX, Fig. 2.) 



The second (192), from Port de Bielsa, is a fine phyllite composed 

 of small flakes of biotite, with some muscovite and chlorite, wrapping 

 round lenticles of quartz and chlorite. The latter are reconstructed 

 segregations, formed when the rock was crushed, anterior to the 

 heating. The fine texture of these rocks (190 and 191) at once 

 separates them from the former group ; in the Archaean rocks it would 

 be practically impossible to find such large patches of andalusite 

 associated with such a fine groundmass. 



Rocks of very fine texture and well foliated do occur on the 

 'outer margins of ciystallization ' of the old Archaean areas, but they 

 contain no such minerals as andalusite. It is necessary to go some 

 distance into these areas, in the direction of increasing crystallization, 

 before such minerals are met with, and by this time the texture of 

 the groundmass has invariably become far coarser than that of 191. 



I-^ 



K 



12 3 4 5 6 

 Irtcrc^ising Coarseness 



The lines X, Y, Z show the coarseness of the texture for a given 

 temperature in — 



X ^ = post-Torridon aureoles of thermo-metamorphism. 

 Y = areas of newer Archaean ,, ,, 



Z = areas of older Archaean ,, ,, 



The distinctive feature of the two groups is best shown by the 

 size of grain of the quartz-felspar material in the two specimens 190 

 and 191, which is brought out by the photographs of sections of them 

 (PL XIX, Figs. 1 and 2). The texture seen in Fig. 1 may occasionally 

 be met with close to the actual margin of a great coherent mass of 



^ The flattened termination of X is intended to show the rapid increase of texture 

 just at the margin of a big intrusion. 



