14 PEOP. T. G. EOIS^NEY ON SOME EESULTS OF 



of these beds, being about a couple of feet. Of the others I did not 

 keep any record, as the exact thickness was immaterial. I have no 

 doubt a mass of granite is very near (5), but it had not been 

 exposed in the quarry ; there ^vas, however, a vein visible by the 

 roadside at a distance of about 10 yards. 



Of these specimens Xo. 1 at once reminds us of the banded slate 

 already described. There are the same alternate bands of dark 

 and light rock, but with this difference, that the former has become 

 a solid, faintly-fohated mass, the latter, also solid, exhibits a foliation 

 due to the presence of small wavy laminae of rather silvery mica. 

 The larger as well as the smaller mineral layers are waved and 

 crumpled, and exhibit in places indubitable remains of a strain- 

 slip cleavage ; but its surfaces have become soldered together, 

 though sometimes the fracture of the rock indicates that they are 

 still planes of imperfect coherence. IS'o one who examines the 

 rock in the field or even, I think, in a hand specimen, can for a 

 moment doubt that here we have the normal Morlaix rock, after it 

 had undergone a certain series of mineral changes. A portion of 

 No. 3 is almost identical with jSTo. 1, but one of its dark layers 

 obviously contains andalusite or a kindred mineral. 



Under the microscope the lighter-coloured part of these rocks 

 consists mainly of the following minerals : — quartz, two micas, one 

 colourless, the other varying from pale olive-colour to a fairly deep 

 brown inclining to olive, a graniilar or slightly fibrous mineral, 

 occurring in rather cloudy patches, and black granular spots, rods 

 or plates, which are probably sometimes iron-oxide, sometimes 

 graphite. The quartz and the mica form (as one sees in the hand 

 specimen) alternating bands, commonly from about •02" to -04" 

 thick (PI. II. fig. 2). In short, at the first glance, the slides cut 

 from this rock resemble those from an ordinary fine-grained mica- 

 schist, such for instance as we obtain at Holyhead in Anglesey. 

 The quartz occurs in grains, rather irregular in outline and vari- 

 able in size, the larger rarely exceeding -03" in diameter. The 

 majority, however, are only about -002", and most of the grains 

 are near the one size or the other. The quartz is clear and 

 seems free from enclosures other than mineral, but little flakes 

 of mica (colourless or nearly so) are interspersed, and perhaps 

 intrude into the smaller grains and are included in the larger. 

 Thus, one of the quartz bands, when regarded by ordinary trans- 

 mitted light, appears to be rather thickly sown with tiny flakes, 

 rods or granules of minerals, chieflj' mica ; sometimes, however, a 

 grain is also outlined by flakes of mica. This mineral, when it 

 occurs in the bands, is very commonly in flakes about "01" long. 

 There are clearly two distinct varieties — one colourless, but giving 

 brilliant tints with the polarizing apparatus, no doubt a hydrous 

 soda- or potash-mica ; the other olive-brown rather than umber or 

 sienna-brown, no doubt a ferro-magnesian mica. The variations 

 in colour very probably are partly due to subsequent mineral 

 change. The fibrous mineral occurs associated with the mica in 

 cloudy patches (in IS^o. 1 only). At the first sight it rather 



