152 ON SOME PRE-CAMBRIAN ROCKS IN CAERNARVONSHIRE. 



well shown at St. David's. With the former of these I would associate 

 the so-called syenitic ridge (granitoid rock) at Caernarvon ; and with 

 the latter the altered beds which rest directly on the syenitic ridge 

 towards Bangor, and the series chiefly described in this paper to the 

 south and north of Llyn Padarn and Moel Tryfaen. The prevailing- 

 characters in this last series indicate the metamorphism of a Pre- 

 Cambrian volcanic group of ashes and breccias rather than of true 

 sedimentary beds, the result of denudation and alteration only ; and 

 this, as is shown more fully in the following paper *, is also the 

 character of the larger part of the Pebidian group at St. David's. 



This proof of the extension of Pre-Cambrian rocks into and under* 

 the North Welsh area is highly interesting, and likely, I hope, to 

 lead to researches in other areas where the Lower Cambrian rocks 

 are exposed. The rough materials of which the Lower Cambrian 

 rocks are for the most part made up indicate the presence of land I 

 near at hand when they were deposited ; and the almost invariable 

 difference to be observed in these formations, when seen on a large 

 scale, in the one being a highly metamorphosed series and the other 

 but slightly changed, ought greatly to aid in this research. 



Note on a Bock-specimen from the Centre of the so-called Por- 

 phyritic Mass to the East of Tal-y-sarn. By T. Davies, Esq., 



E.G.S. 



A microscopical examination of this rock discloses a very fine- 

 grained ground-mass containing evenly distributed quartz grains, and 

 a felspar in very small, dull, opaque, crystalline grains. 



In thin sections, under the microscope, the quartz is found to be 

 more abundant than at first was evident ; indeed it forms a very 

 considerable part of the mass. It is very angular in outline, and 

 appears to be fragmentary, the rounded crystalline grains with in- 

 clusions of the ground-mass, so characteristic of the quartz-felsites 

 and porphyries, being but sparsely represented. The felsitic ground- 

 mass is very uneven in texture, has very little action on polarized 

 light, and is only occasionally to be observed folding in bands around 

 a crystal of felspar or quartz. In some parts the quartz grains are 

 so numerous and closely aggregated as to afford but little space for 

 the ground-mass, and bear a close resemblance to some of the more 

 quartzose members of the so called " ashes " of North Wales. The 

 felspar, like the quartz, is very fragmentary and much kaolinized. 

 Some altered magnetite is present. 



The rock is evidently a breccia in which both the fragments 

 and the cement are of the same mineral nature ; it is, in fact, a 

 quartz-felsite breccia. 



* This paper, with the title " Additional Notes on the Dimetian and Pebidian 

 Eocks of St. David's," will appear in the next Number of the Journal. 



