12 



PEOP. T. G-. BOimEy ON SOME EEST7LT8 OE 



bands, and the direction of the folds is practically the same in eachy 

 making an angle of about 70° with the boundary of the band. 



We have, in short, in the Morlaix district, those satiny slates, 

 approaching phyllites, which are commonly found in regions where 

 suitable materials have been exposed to intense pressure, as, for 

 instance, parts of the Ardennes, Devon, and the Alps. 



On examining the rock under the microscope (PI. II. fig. 1), we 

 find the grey bands mainly composed of granules of quartz, and flakes 

 of a mica varying from almost colourless to a pale yellow-brown 

 or buff tint, among which are occasionally scattered granules of 

 darker colour, sometimes associated in clusters. The quartz grains 

 are not very definite in outline ; they occasionally exceed -OOi" in 

 diameter, but are more commonly about that size or a little less. 

 Usually they are free from mineral enclosures, but sometimes 

 envelop a tiny flake of mica. Xear the edge, or where one of the 

 iabove-mentioned darker films occurs, the mica flakes have a distinct 

 tendency to be parallel with the surfaces of the layer ; but generally 

 it is difficult to detect any deflnite orientation. The occasional 

 granules appear to be sometimes ferruginous or earthy matter, 

 sometimes carbonaceous ; in one or two cases they resemble specks 

 of epidote *. The mica flakes, more especially where they begin to 

 dominate, are occasionally quite "002" in length, but commonly 

 they are about '001" long, and often much less. 



The dark bands consist chiefly of flakes of light-coloured mica 

 with granules of earthy and carbonaceous material, sometimes so 

 thickly interspersed that it is difficult to obtain a perfectly clear 

 definition of the constituents, and with occasional specks of clear 

 quartz, which here and there predominate. This, of course, occurs 

 where the black band is interrupted by a sandy film, just as the sandy 

 bands are interrupted by black films. The cleavage-planes are defined 

 by rude dark Imes. Careful study shows that the micaceous 

 constituent is arranged in a series of wavelets, the general 

 surface of which is at a high angle with the cleavage-planes, 

 so that the latter structure is a strain-slip cleavage t- 



A specimen taken from a small quarry south of the town (on the 

 Huelgoat road) shows the two last-named structures still better. 

 Here the sandy bands are wanting. The dark rock exhibits yet 

 more distinctly this " incipient foliation," as we may term it ; 

 the surfaces are waved, and there is frequently a very distinct 

 strain-slip cleavage, making, with their general direction, angles 



* Here, however, as in all the instances to follow, I hare not endeavoured to 

 make a precise investigation into the exact nature of these less deflnite substances, 

 for two reasons. One, that my eyes always feel the effects of a prolonged use 

 of hiah-power lenses ; the other that my purpose is to endeavour to follow up 

 the history of the rock, and for that the identification of the less defijiite 

 constituents is immaterial. Moreover an excellent study of the effects of 

 contact metamorphism in Brittany already exists in an elaborate paper by Dr. 

 Barrois, " Sur le granite de Eostrenen, ses apophyses et ses contactes " (Ann. de 

 la Soc. Geol. du Jford, t. xii. p. 1). I have therefore, as far as possible, confined 

 myself to details which appeared to have a distinct " historical" significance, 



t As stated in my Presidential Address (Quart. Journ. Geol. Soc. vol. xhi. 

 Proc. p. 95) I use this phrase as the equivalent of " AusweichungscHvage." 



