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178 



IGNEOUS ROCKS, NORTH WALES. 



Upper 

 Gallery. 



Wall-case 4 



to denudation, that nothing now remains but fragments of 

 great lava streams and beds of ashes, sometimes cropping 

 out and spreading over considerable areas, but in general 

 showing little more than their edges, their major portions 

 being buried under thousands of feet of overlying slates 

 and sandstones. — A. C. Ramsay. 



Catalogue of Specimens by A. C. Ramsay, assisted by 



Hilary Bauerman. 



Cader Idris, Aran, Arenig and Moelwyn Series. 

 1. — Slate, with talcose and felspathic slaty bands, 



consisting of alternations of purple slate, with greenish-grey 

 felspathic sediment. — Y-Graig-wen, Dinas Mowddwy, Me- 

 rionethshire. Map 60, N.W. In this specimen the fel- 

 spathic bands predominate. 



The slaty band from which it is derived occurs in the 

 midst of a great mass of felspathic lava. The rock is 

 affected by cleavage, and the layers of which it is composed 

 being of various degrees of fineness, the cleavage and frac- 

 ture are coarse and irregular. The bedding and cleavage 

 form an angle of about 30°. 



2. — Talcose and felspathic slaty beds, dark greenish- 

 grey, with black layers separating the different kinds of 

 sediment. — Half a-mile from Pant -yr-onen, 1 mile south of 

 Dolgelli, Merionethshire. Map 59, N.E. 



From the upper part of the Lingula flags, composed prin- 

 cipally of consolidated fine felspathic sediment, interstratified 

 with what were originally black muddy layers. The fel- 

 spathic part is possibly chiefly of direct volcanic origin. 

 The rock is cleaved nearly at right angles to the planes of 

 bedding. The outer surface is irregularly weathered, the 

 purer felspathic layers having been most easily decomposed. 



3. — Talcose and felspathic slaty beds. — Near Dol- 

 gelli, Merionethshire. Map 59, N.E. Light greenish- 



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