312 Gerald M. Part— 



in the Llanvirn slates. These are useful in the field as a means of 

 determining the true dip of the highly-cleaved sediments. In their 

 general character they are very similar to the " china-stone ashes " 

 found in the Bifidus Shales in many other parts of Wales, and are 

 probably of similar age. 



(&) Greenway type : West of Rosebush there is an extensive peat 

 and drift-filled depression through which the railway and River 

 Syfynwy run. This is formed in a thick band of " felsite " or soda- 

 rhyolite, which is well exposed at numerous points round the vf estem 

 rim of the depression at Cnwc-y-felin, New Inn (Greenway), and in 

 the Syfynwy valley. Similar rocks are exposed in quarries on 

 Goetty Mountaic, at the Syfynwy Falls, and on Bernard's Well 

 Mountain (between Tufton and Rosebush). 



In the field these rocks are pale in colour, weathering with a grey 

 or whitish crust and break down into a yellow gravelly soil. The 

 weathered blocks frequently show lines of flow. There is an 

 approximately rectangiilar jointing — very well shown in the section 

 at the Syfynwy Falls. 



Fresh specimens are pale bluish- or greenish-grey, fine-grained or 

 even flnaty in texture and containing many small phenocrysts of 

 felspar. The rock sometimes has a " pepper and salt " appearance, 

 due to patches of chlorite disseminated through it. 



The upper surface of this band is exposed in the crags on the east 

 side of the Banc-du, the promment hill west of the main Haverford- 

 west — Cardigan road, a few hundred yards north of New Inn. The 

 rock is here finer-grained, ochreous, and rather soft. The actual 

 junction can be seen in the crags, and shows the irregular upper 

 surface of the flow upon Avhich the slates have been deposited. On 

 the south side of the hill the slates from the immediate neighbourhood 

 of the junction give the impression of being slightly hardened and 

 silicified, but in general there in no evidence of mtrusion, nor have 

 I observed anything comparable to the slate-keratophyre breccias 

 of Longhouse and other localities in the neighbourhood of Aber- 

 castle.^ The margin of the rock at Banc-du is, however, imdoubtedly 

 slightly brecciated, as can be seen in most of the slices of specimens 

 from this locality. 



Under the microscope specimens collected from Syfynwy Falls, 

 Goetty Mountain, Cnwc-y-felin, and several points north and west 

 of Greenway, all proved to belong to the same general type. The 

 rocks are conspicuously porphyritic with abundant small 

 phenocrysts of felspar lying in a fine somewhat " felsitic " ground- 

 mass (Fig. 1). In size the phenocrysts range up to -25 in., but in 

 general are smaller than this, about -04- -08 in. in length. Some 

 which are un twinned, or simple Carlsbad twins, are a soda-orthoclase. 

 The remainder are a plagioclase of similarly low refractive index 

 giving extinction angles on the twin lamellae up to 16°. These 



^ Elsden, op. cit., p. 574 ; Cox, op. cit., p. 307. 



