TIIK CAMBIUAN SIIBIES IN N.W. CAEBNARVONSHIRi:. 



33^ 



with the diagram given. Mr. ]ilake regards the section afforded by 

 this as a " decisive " proof of theageofthefelsite. According to his 

 description, junctions with grit and slate are here exhibited at the 

 base of the hiva-fiow, and the sedimentaries show contact-alteration, 

 which proves them to be older than the felsite. Though I carefully 

 examined the section on more than one occasion, I failed to find 

 any evidence in favour of Mr. Blake's view ; so far as I could see, 

 there is no slate in the quarry, and if there be any grit it is not 

 older than the felsite. This is what I found. A greenstone dyke 

 has been almost quarried out, and the felstone is left as a kind of 

 miniature cliff overlooking the valley. Part of the greenstone is 

 shown in Mr. Blake's plan ; but its northward prolongation should 

 not be represented as adjoining a band of grit, for it runs continu- 

 ously in contact with the felsite ; and the greenstone should not be 

 limited on the plan to the northern end of the quarry, for it can be 

 traced, by means of small bosses clinging to the base of the scarp 

 of felsite, southward as far as the road, and it is seen even beyond. 

 The high boundary-wall at this end is constructed mainly of green- 

 stone blocks, doubtless obtained from the quarry, and two small 

 masses still remain in situ built into the wall as buttresses * (figs. 2 

 and 3). The greenstone normally is an ordinary, rather coarse 



Fig. 3. — Quarry at Bryn Ef ail ;' face of cliff seen from lower ground. 



If.N.W. S.8.E. 



G. Greenstone. 



dolerite, with ophitic structure, but it becomes fine-grained for the 

 space of about a foot from the felsite, and the band ^ inch wide 

 along the boundary is a dirty " sahlband," or an edge still partially 

 tachylytic. This greenstone (or diabase), in general structure and 

 composition, agrees with those which are so common over all the 

 North-Wales district ; thus it would have been rather curious if it had 

 belonged to a distinct and very much earlier period. The character 

 of the edge, however, proves beyond doubt that it is intrusive in the 

 felsite, so it may be of the same age as similar masses elsewhere. 

 As Mr. Blake's description of his slides appeared to be very minute, 

 I was careful to collect the most dubious-looking specimens for 

 microscopic examination. I took most of these at intervals along 

 the boundary of the felsite, searching for an example of the so-called 



* The greenstone may have extended to the edge of the ground, which for- 

 merly sloped to the alluvial plain at the south of the quarry ; if, however, a 

 small boss of j^laty felsite, which occurs near the north-west of the present 

 ' lower ground,' may be taken to mark the farther limit of the greenslone, the 

 dyke at that part could not have been more than some twenty yards in width. 



