part 3] lower palaeozoic or arthog-dolgelley. 269* 



describe first the characters and sequence of the rocks as exposed 

 west of the Grwynant, where the series is thinner and its age can 

 be more readily determined by means of the included slate-bands. 



(«) The development Avest of the Grwynant Valley. — 

 West of the CrAvynant the Lower Acid Series consists of three well- 

 marked subdivisions : — 



(iii) ' China-stone ' Ashes, 

 (ii) Pont-Kings Slates, 

 (i) Lower Ashes. 



This sequence remains practically constant, from the Grwynant 

 Valley on the east to the Arthog Valley on the west. But. 

 although the sequence remains the same, the westward thinning of 

 the rhyolitic rocks themselves continues, and accordingly the slate- 

 beds appear to play a much more important part in the west than 

 they do in the east. 



(i) The Lower Ashes. — Massive rhyolitic rocks succeed the 

 arenaceous rocks of the Basement Series quite abruptly. The 

 junction most clearly exposed is the one already mentioned as 

 situate on the banks of the stream flowing from Llynau Crogenen 

 to join the Afon Arthog. 



Among the rhyolitic rocks themselves various lithological t} r pes 

 may be distinguished, the differences being sometimes shaiply 

 marked, while in other cases the varieties seem to grade one into 

 the other. The rocks are all essentially acidic, the variations being 

 due soleky to the variable amounts of pumice-dust and of pyroclastic 

 felspar-crystals that go to make up the strata, or to the admixture 

 of these volcanic products with varying amounts of argillaceous- 

 material. 



One of the most commonly occurring types, especially prominent in the 

 lower and middle portions of the series, forms massive beds made up largely 

 of small broken felspars accompanied by small and scattered quartzes, 

 embedded in a more or less abundant matrix of rhyolitic dust. Such rocks, 

 on weathering, yield rough joint-surfaces which show a rather irregular 

 lamination that may simulate flow-structure. Fractured surfaces are 

 frequently somewhat iron-stained, but joint-surfaces weather white or cream- 

 colour after the usual manner of rhyolitic rocks. Such massive rocks may or 

 may not show the effects of cleavage. Associated with the massive rocks, but 

 not so abundant, are well-cleaved ashes that weather into thin papery leaves. 

 The leaves are strongly iron-stained, although joint- surfaces weather white in 

 the usual manner ; joint-surfaces in these well-cleaved rocks are smooth, not 

 rough as in the massive rocks described above. As regards their composition 

 the better-cleaved rocks consist almost entirely of the finest rhyolitic dust, 

 distinct broken crystals and recognizable fragments being absent ; but the 

 one type grades into the other. 



Another type consists of fine-grained ashes resembling the ' China-stone ' 

 Ashes to be described below (p. 272). These rocks break with a smooth 

 conchoidal fracture, giving sharp-edged splinters in exactly the same manner 

 as flint. They do not weather so deeply as the types described in the pre- 

 ceding paragraph, but eventually they, too, give rise to a white crust similar 

 to that of the other rhyolitic rocks. Fresh surfaces may show a beautiful 

 and regular colour-banding which is especially prominent where the ashes are 



