90 REV. EDWIX HILL AXD PKOF. T. G. BOJS'XEI OX THE 



III. Additional iS^otes. 



6. Stable Quarri/^ Bradyate Farlc. — We stated in a former paper 

 that we felt some uncertainty as to the relations of the rocks in this 

 quarry, and further study made us yet more doubtful. So we again 

 examined the quarry i]i 1890, with the followiug result : — 



The mass of the rock, as already stated, is a quartzite ; this also, 

 on the southern side, is clearly intercalated in thin bands in a purplish- 

 brown slate several feet in thickness, exactly as lenticular seams of 

 sand might be intercalated in a mud ; the change from the one to the 

 other being verj' rapid — so distinct indeed that, in a hand-specimen, 

 a pin-point could be placed on the junction of the two rocks. The 

 grains in the quartzite are remarkably well rounded ; this pecu- 

 liarity is most conspicuous in immediate contiguity with the slate, 

 probably because here less secondary quartz has been deposited. 

 The beds are practically vertical, and the average strike is about 

 E. and AV. Our former measurement gave it as a few degrees 

 S. of W. ; our last measurement, at a different place, a very few 

 degrees IS^. of \¥. Towards the northern side there is some 

 appearance of an infolding of slate-bauds ; but, if this be so, we 

 now regard it as local, and think that in the main the pit gives a 

 regular succession. 



The point, however, on which we had become most doubtful was 

 the nature of the rock described in our paper as a " spotted slate," 

 a " pinkish-green felsitic rock with ill-defined dull-green chloritoid 

 (chloritic) spots, the sort of rock that we might expect to result 

 from the re-arrangement of a fine ash or the denudation of a not 

 very acid lava " *. Further study of its microscopic structure sug- 

 gested both doubts as to the rock being altered, as we supposed, by 

 contact-metamorphism, and the possibility that it might be a dyke, 

 consisting of small crystals of felspar and a pyroxenic mineral, em- 

 bedded in a rather basic matrix, which had been much modified by 

 crushing and subsequent mineral change. The following is the 

 result of our observations : — 



The rock — about 4 feet thick — is distinctly cleaved, though less 

 fissile than the rest of the slate in the pit. From this it differs some- 

 what in colour, in the presence of the green spots, and in a more 

 " ashy ■*' texture, so that, if a sediment, it must be formed of different 

 materials. But instead of being, like the slate, continuous with the 

 quartzite, the two rocks separate, and the common surface is siicken- 

 sided, thus indicating the crushing together of distinct and different 

 masses. On the northern side a thin wedge-like piece extends from 

 the main mass into the quartzite. This might be explained as either 

 a lenticular band of mud in sand, or an offshoot from a dyke, but here 

 also is so much disturbance that no definite evidence can be obtained. 

 The microscopic structure is very obscure ; probably there have been 

 many small crystals of felspar, now replaced by earthy matter mixed 



Quart. Journ. Geol. See. vol. xxxiii. (1877) p. 763. 



