Dr. R. D. Roberts — The Cambrian in Anglesea, etc. 439 



evidence that the long green crystals are hornblende. In a specimen 

 from the Pitehstone vein on the Corriegills shore, many of the green, 

 prisms are larger than usual, and happen to lie in positions either 

 nearly or quite vertical to the plane of the slice, so that the angle 

 of 00 P may be readily measured ; in all favourable positions it is 

 found to be a very close approximation to 124° 30'. The larger 

 prisms which lie in the plane of the slice resemble crystals of 

 actinolite, and, like them, are often traversed at irregular distances 

 by transverse fractures. The angle of extinction is always small, 

 but varies from 3° or 4° to about 15°. There can be no doubt there- 

 fore that the mineral is hornblende. 



The mineral constitution of the Arran Pitehstone may now be 

 regarded as definitely ascertained. The rock consists of a struc- 

 tureless glass, in which are imbedded the following minerals — two 

 felspars, orthoclase and a triclinic species, bipyramidal quartz, horn- 

 blende, augite and magnetite, all of which occur occasionally in more 

 or less perfect crystalline forms. The glassy matrix is clear and 

 colourless, but the innumerable crystallites of hornblende with which 

 it is crowded impart to hand-specimens their well-known shade of 

 dark glossy green. 



Mason College, Birmingham. 



Ill, — The Basement Beds of the Cambrian in Anglesea and 

 Carnarvonshiee. 



By E. D. Roberts, M.A., D.Sc. (Lend.), F.G.S., 

 Clare College, Cambridge. 



THE following facts were observed during a visit to Anglesea and 

 Carnarvonshire in company with Prof Hughes in the early part 

 of July. They still further bear out and confirm the evidence given 

 in a short communication to the Geological Magazine for May, 

 1881, bearing upon the position of the Twt Hill conglomerate. 

 Among the arguments brought forward against the Cambrian age of 

 that conglomerate, much stress has been laid upon the absence from 

 it of felsite pebbles, which are so characteristic of the Cambrian 

 conglomerate in the neighbourhood of Bangor. 



In the former paper I stated that fossiliferous Cambrian shales 

 and sandstones had been traced down by Prof. Hughes at two or 

 three points in Anglesea into a quartz conglomerate, lithologically 

 not distinguishable from the Twt Hill conglomerate. We have 

 now traced this conglomerate, which always passes up into brown 

 sandstones that are sometimes fossiliferous, over a considerable area 

 round Llanerchymedd, and found it always resting upon and following 

 the contours of the pre-Cambrian axis. It presents variations both in 

 thickness and in lithological character. In a quarry near the wind- 

 mill half a mile east of Llanerchymedd, bands of congioiuerate, largely 

 made up of felspathic fragments, alternate with bands of pure quartz 

 conglomerate of the Twt Hill type. Half-way between this quarry 

 and Llanerchymedd, in a bye-road on the south side, the conglomerate 

 is seen to be actually divided by a band of shale from ten to twenty 

 feet thick ; the conglomerate above and below the shales being made 



