18 DR. STAMP AND MB. WOOLDRIDGE ON THE [vol. lxxix, 



(3) The Succession of the Strata. 



The succession may be tabulated generally thus : — 



(7. Intrusion.) 

 6. Black Slates, cleaved and almost unfossiliferous. 



f fossiliferous ashy shales. 

 5. The Upper Ashes \ fine banded ashes. 



[ coarse ashes. 

 4. Hardened mudstones, with a band of ashy limestone 



(weathering to ' rottenstone ') in the upper part. 

 3. The spilites and spilite-breccias. 

 2. Hardened sediments, with fossiliferous mudstones. 



1. The Lower Ashes { ° oa f e breccia (politic). 

 I fairly coarse ashes. 



Even in the short distance of 3 miles, from one end of the igneous 

 mass to the other, the succession shows considerable variation. This 

 is particularly the case with the ashes in their degree of coarseness, 

 and with the spilites. 



The whole series (except the higher parts of 6) may be described 

 as of Grlenkiln-Hartf ell age, being on the same horizon 

 as the Dicranograptus Shales of South Wales. 



{a) The Lower Ashes and Breccias. 



These beds are only seen in the deep transverse valley of the 

 Nant Cerdin. The base is not visible, and the lowest strata seen — 

 well exposed in the bed of the stream, at the point A marked on 

 the map (PI. II) — consist of compact, fairly coarse ashes. The 

 rock is pale greenish-grey, and contains whitish angular fragments. 

 Occasional darker veins, which are so arranged as to simulate the 

 divisions between spilite-' pillows,' are also seen. A closer exami- 

 nation, however, reveals no lithological difference, beyond a slight 

 deepening in colour of the matrix. 



Succeeding the ashes is a coarse breccia, which forms rugged, 

 almost vertical cliffs (the Craig Castell of Murchison), some 80 or 

 90 feet high, on the northern side of the stream. The rock has a 

 remarkable and distinctive appearance, since the brecciated frag- 

 ments weather white while the matrix remains black. The whole 

 rock seems to have been silicifiecl, and tends to break with an even 

 or semi-conchoidal fracture. It is well jointed, 1 and the presence of 

 east-and-west joints, together with a slight southward pitch, have 

 produced the steep cliffs. Unweathered portions are of a uniform 

 black or dark grey. The rock bears a close resemblance to certain 

 rhyolitic flow-breccias described from other parts of Wales : as, for 

 example, Conway. 2 On the other hand, features such as banding, 

 observable in one fragment, cannot be traced in the next, and 

 therefore the rock is not comparable with a shatter-breccia. On 

 -the whole, the Llanwrtyd breccia seems to agree closely with the 

 flow-brecciated (or auto-brecciated) lavas described by Mr. J. P. N. 

 'Green from the Lake District. 3 



1 Murchison states that the rock breaks up into slender four-sided columns. 



2 G. L. Elles, ' The Relation of the Ordovician & Silurian Rocks of Conway 

 (North Wales) ' Q. J. G. S. vol. lxv (1909) p. 169. 



3 ' The Vulcanicity of the Lake District ' Proc. Geol. Assoc, vol. xxx (1919) 

 j>. 153. 



