144 PEOP. T. G. BONNET ON THE MICEOSCOPIC 



tween the conglomerates and the agglomerates is more strongly 

 marked in South than in North Wales, on the other hand it con- 

 firms the impression that there is no unconformity between the Caer- 

 narvon and Bangor beds, as it appeared to me that the upper beds 

 of the Dimetian at St. David's {e.g. at Lower Moor, on the side of 

 the valley running down from the cathedral) were distinctly brec- 

 ciated, and so formed a passage into the more marked agglomerates 

 of the Pebidian, while all the discordances involving conflicting 

 strikes and dips were along valleys, in some part of which there 

 were undoubted faults. Moreover it seemed to me that the green 

 beds in the Dimetian were all dykes. The microscopic examination 

 of the Lower-Moor rock by Professor Bonnejr leads him to the con- 

 clusion that it is a product of denudation (No. V.) ; and two of 

 the green beds, Nos. I. & II., he would refer to altered basalts. 



On the whole, then, it seems that we have an old volcanic series 

 of remarkafr'y persistent character in North and South Wales (and 

 probably we shall find, beyond the borders, that subdivisions can 

 be made out in it, though as yet no break has been proved in the 

 series), and that the base of the Cambrian consists of a strong con- 

 glomerate and grit, between which and the underlying series there 

 is great probability of an unconformity. 



This is only working out the details of the classification pub- 

 lished by Prof. Sedgwick thirty-five years ago. 



Appendix. 



JS/ote on the Microscopic Structure of some Welsh Rocks. 

 By Professor T. G. Bonnet, M.A., F.G.S. 



No. I. Dyke in Dimetian ; east of PorthlisTcy, St. David's. — The 

 ground-mass of this rock consists of crystals of plagioclase, now 

 greatly kaolinized, or replaced by secondary products, together with 

 viridite, which is partly the normal isotropic mineral, partly the 

 serpentinous form, showing, with crossed Nicols, a fibrous structure 

 of a dull blue colour. This probably replaces an augitic constituent. 

 There is a considerable quantity of magnetite, some calcite, visible, 

 with probably much more finely disseminated and one or two crystals 

 of a deep brown isotropic mineral, resembling picotite or a dark- 

 coloured garnet. The rock, then, is a diabase. 



No. II. Band in Dimetian; Quarry by Church School, St. David's. — 

 An ill-defined ground-mass of minute plagioclase crystals (or their 

 pseudomorphs) with dusty patches, resembling kaolin, and much 

 fibrous viridite. In this numerous small grains of magnetite are 

 scattered, with larger crystals, much altered, often more or less re- 

 placed by viridite : some certainly are plagioclase; but others may 

 have been augite. Plagioclase, magnetite, and a little (?) apatite 

 arc now the only unaltered minerals. The crystals show a slight 

 parallel structure ; and some are broken, the fragments occasionally 

 remaining near together, as though the rock, when almost solid, had 

 been subjected to a strain or pressure. At present it must be called 



