OLDER ROCKS OF BRITTANY. 



313 



quent to the crystallization of its constituents ; but the indications of 

 crushing are not quite so clearly marked as I had anticipated, some 

 of the biotite having escaped with little change. I should conjecture 

 that the rock was originally a rather fine-grained micaceous gneiss, 

 fairly homogeneous in structure, and that the pressure had been 

 oblique to the original foliation. The results of crushing are far 

 more marked in the granite. It has evidently been a coarse-grained 

 rock, composed of quartz, felspar, and mica. Parts of the slide show 

 where the rock has been utterly crushed, and consist of a sort of 

 mosaic of quartz, decomposed felspar, and mica. In other parts 

 large fragments of the original felspar crystals still remain. Where 



Fig. 3. — Small intrusions of Coarse Granite in a Micaceous Gneiss, 

 both showing effects of pressure. Near the Ferry House, Belon 

 River. (Size about 6| by 2 J feet.) 



at first sight there appear fair-sized grains of quartz, application of 

 the nicols shows these to be composite in structure, though often one 

 tint predominates in the constituent grains. Recementation, how- 

 ever, is complete ; but the borders (that is, we may presume, the 

 cementing material) are often differently coloured from the grains 

 on either side. The brown mica forms irregular lenticular bands 

 lying in the general direction of the cleavage. It is rare to find a 

 flake *02 inch long. Commonly the bands are made up of aggregated 

 flakes about -01 inch long or less. Here and there is a fair amount 

 of a white mica ; part may be a true secondary product, but some, I 

 think, is only an alteration form of the biotite. I feel convinced 

 that these streaks of biotite owe their origin to a crushing up, per- 

 haps sometimes to a crushing together, of the large flakes which 

 existed in the unaltered granite. I may remark that both in the 

 mica-schist and in the granite reconsolidation appears rather un- 



