426 MK. J. PARKINSON ON THE GEOLOGY OF [Aug. I9O3, 



larger flakes of the last-named mineral. Evidence of contact- 

 metamorphism is, however, shown by the presence of shadowy 

 patches, rather more opaque than their surroundings, and pro- 

 ducing little effect between the two nicols. Associated with these 

 spots are numerous much smaller ones x of an orange-yellow colour, 

 which exhibit no definite microscopical characters, and do not 

 appear distinctly marked off from the larger spots when they are 

 included by them. There can be little doubt that these represent 

 incipient staurolite. 



By the stream-side near Higher Trefrew (north of the word 

 Slaughterb ridge on the 1-inch map) a greatly-crushed rock of 

 igneous origin has been worked in a few shallow pits, but as it is 

 surrounded by moorland its relation to the slates has not been 

 made out. A thin section shows the remnants of felspar, secondary 

 biotite, flakes of a green mica, and a mosaic of crushed quartz 

 and felspar. The biotite is of a rich brown, aggregated patchily 

 in groups, members of which were occasionally strong enough 

 to form boldly across the foliation. The mineral suggests by its 

 appearance that it underwent pressure, which was followed by some 

 mineral revival. The original rock, whether pyroclastic or not, was 

 of an acid composition. 



The Upper and Lower Blue-Black Slates. 



These may be subdivided into two types. The first are soft and 

 not banded ; in the second well-marked laminae are conspicuous, and 

 the rocks are sufficiently hard to resist a knife-blade. Both appa- 

 rently contain carbonaceous particles. The second type occurs 

 characteristically above the Volcanic Series, but the first are found 

 on that horizon at Tregulland, at Davidstow, in cutting ~No. 90 of the 

 London & South-Western Railway, and elsewhere. The brittle rocks 

 of the second type consist almost entirely of subangular or irregular 

 quartz-grains 2 ; these form a fine mosaic, through which are scat- 

 tered carbonaceous particles and minute flakes of greenish-white 

 mica. Although in a different crystalline condition, these rocks 

 recall some of the radiolarian cherts from the Lower Culm. 3 



The Greenstones (Epidiorites) . 



In a small pit near Stone Cross, west of St. Clether, is quarried 

 a peculiar rock which answers to the ' epidiorites ' of Giimbel. 

 Augite is entirely absent, the ferromagnesian constituent is an 

 actinolitic hornblende, the crystals of which are arranged in clumps 

 and tufts. The felspar shows no definite structure, but is, on the 

 contrary, blotched by clouds of kaolin, and between crossed nicols is 

 cryptocrystalline. Large patches of ilmenite — usually replaced by 



1 Averaging "002 inch across ; the larger patches are about "015 inch in 

 length. 



2 Eeferred to by Mr. W. M. Hutchings as ' very fine-grained quartzites,' 

 Geol. Mag. 1889, p. 220. 



3 For an opportunity of examining a large number of sections of these rocks- 

 I am indebted to the kindness of Mr. Howard Fox, F.G.S. 



