OLDEE ROCKS OF BEITTA1STT. 



315 



dark olive-brown mica, with an occasional c^stalline grain of tour- 

 maline and of a mineral which I have not succeeded in identifying*. 

 The quartz and the felspar grains have the subangular or subrotimd 

 outlines which seem to be characteristic of the older gneisses, and 



Pig. 5. — At Roscoff. 



A-B. Lines of mineral banding. 

 C-D. Lines of slight rude cleavage. 



the former are occasionally distinctly elongated, with a slight linear 

 association, and have some appearance of strain-polarization. Among 

 the felspars plagioclastic twinning is seen, but many grains are un- 

 twinned. The mica occurs in well-developed flakes, often about 

 •02 inch long or rather less. They have a fair thickness perpen- 

 dicular to the basal plane, the proportions in the sections being about 

 1 to 4. They lie roughly parallel with the mineral banding. The 

 crystals are well developed, and do not generally appear compressed 

 or crushed at the edges. In the more micaceous layers there is also 

 a marked diminution in the quartz, and plagioclastic twinning is 

 much more common in the felspar. There is also more of the 

 unidentified mineral. This gneiss appears to be very little affected 

 by the intrusion of the porphyritic granite. A slice cut from a spe- 

 cimen taken from within 4 or 5 inches of a junction does not differ 

 appreciably from apparently normal specimens ; and even at a junc- 

 tion where the granite interpenetrates and appears to be fused into 

 the gneiss, the latter in parts of the slide retains its normal character. 

 Yet the granite is fairly coarse and porphyritic to the last, so that 

 the temperature of the surrounding rock must have been high during 

 the solidification of the former. Hence I conclude that the latter 

 had assumed its present mineral condition before the intrusion of 

 the granite. In the case where a faint cleavage is seen crossing the 

 stratification-foliation, both the quartz and the felspar, especially in 

 the larger grains, indicate strains. Many parts of the slide have a 

 peculiar fragmental aspect, not as if there had been absolute crush- 

 ing, but as though the grains had been separated, slightly displaced, 

 and recemented ; indeed, I am almost certain that sometimes two or 



* It occurs sometimes in roundish grains, but sometimes in imperfect prisms, 

 angles about 120° being occasionally shown. In texture and faint bluish tint 

 it a little resembles apatite. It seems to have two imperfect cleavages, nearly, 

 if not quite, at right angles. It encloses rarely brown mica, zircon (?), and 

 cavities, and it shows (also rarely) a brown decomposition-product. It is rather 

 feebly doubly refracting, which adds to the difficulty of determination ; but I 

 think it is not uniaxial. 



