392 MR. H. H. TH0MA3 AXD PROF. 0. T. JOXES ON THE [Sept. igi2, 



sandstones which are at times difficult to distinguish from some of 

 the Pebidian tuffs. Bedding-planes are usually not well-developed ; 

 near Welsh Hook some of the coarser varieties exhibit false- 

 bedding. 



In the area to the west of Brimaston the rocks of this group are 

 somewhat coarser in texture, and weather light or dark brown 

 with dark small spots. 



(3) The red shales, although only a thin band, are remarkably 

 persistent and uniform in character, and they formed a useful 

 horizon in mapping the ground. In the middle of the band they 

 are soft smooth shales of a rather vivid dark red or maroon colour ; 

 but they take on a dull purple tint in the upper and lower 

 portions. 



They are well-exposed to the north-west of Welsh-Hook Bridge,, 

 and in a hollow overlooking Cwm-bach, south-west of Pen-y-cwm. 



At the last-named place they yielded fossils to one of us and to- 

 Mr. J. Pringle, notably a large species of Leperditia ; similar shales 

 in Crow-cwm Wood, to the south-east of Llethr. yielded the same 

 form. 



A thin band of pale tuff traverses these red shales, and is 

 exposed in the little hollow above Cwm-bach. It consists almost 

 entirely of well -shaped crystals of plagioclase-felspar, and represents 

 the only trace of volcanic activity discovered within the Cambrian 

 System. 



(4) The purple sandstone group consists of well-bedded, fine- 

 grained, micaceous sandstones, which are generally felspathic and 

 of a dull purple-red or purple-grey colour. 



Occasional bands of green chloritic sandstone are present in the 

 series, especially on the coast to the south-west of Pen-y-cwm. 



The best exposures are near Welsh Hook, near Trefgarn-Owen,. 

 on the coast north of Cwm-mawr, and at Eoch Bridge. 



These sandstones have been quarried extensively for building 

 purposes. 



(5) Succeeding the purple sandstones, in the neighbourhood of 

 Welsh Hook, is a thin group of green micaceous and quartzitic 

 sandstones, some bands of which are somewhat coarsely grained. 

 Similar beds succeed purple sandstones on the south side of the 

 Cleddau, south-east of Musland ; and also on the east side of 

 Brandy Brook, near Rhindaston Mountain. 



They appear to form a normal upward continuation of the 

 underlying group, although they have not been observed elsewhere, 

 the reason being that the upper portion of the purple sandstone- 

 group is faulted out in all the other localities. 



On the eastern side of the valley, near Welsh Hook, they are 

 invaded by a dark-green spotted doleritic rock which appears to. 

 follow the dip and strike of the surrounding beds. 



