part 3] LOWER PALEOZOIC OF ARTHOG— DOLGELLET. 273 



(6) The Lower Acid Series east of the Gwynant. — East 

 of the Grwynant the Lower Acid Series increases considerably in 

 thickness, owing to the much greater development of the rhyolitic 

 ashes. At the same time the slate-hands (the Pont Kings Slates) 

 dwindle away rapidly, and finally disappear altogether, and thus 

 •eventually the series consists entirely of a great thickness of rhyo- 

 litic rocks with practically no slate-intercalations. This aspect of 

 the volcanic series is, therefore, very different from that which 

 obtains west of the Gwynant. 



The Pont Kings Slates, already thinning considerably, dive 

 under the granophyre a little east of Pont Kings itself, never 

 to reappear in the country farther east, although the associated 

 rhyolitic rocks can be followed to and beyond the Aran Valley, 

 a distance of 3 miles. 1 The inference is, therefore, that the slates 

 have died away through attenuation against a volcanic pile. 2 It is 

 true that, owing to the presence of intrusive rocks and faults, the 

 volcanic series is never seen in its entirety east of the Gwynant : 

 that is, neither on Gelli-llwj'd, nor on Mynydd-y-Gader, nor nearer 

 Dolgelley. Accordingly, the evidence for the dying-out of the slates 

 is to some extent incomplete. Still , it is very unlikely that, if 

 really present, they should be concealed over the whole 3 miles to 

 the Afon Aran. Therefore, when account is taken of the observed 

 thinning of the slates and thickening of the volcanic rocks, together 

 with the extent to which the latter rocks are exposed, the absence 

 ■of the slates appears fairly certain. 



Among the rhyolitic rocks east of the Gwynant, examples of all 

 the various types already described as being present west of the 

 river may be found. Also some new types make their appearance. 

 The most prominent of these new rocks are agglomeratic in 

 character. The agglomerates occur at both the top and the bottom 

 of the volcanic series. The fragments are of all sizes up to 1 foot 

 in diameter, and consist of rhyolites as also of vesicular rocks of 

 more andesitic composition. Actual andesitic lava-flows do not 

 occur within the area described, therefore the fragments must have 

 •come from some neighbouring district ; but, in view of their large 

 size, the source of origin could not have been very far distant, and, 

 taking into account the north-eastward thickening of the volcanic 

 series, we are inclined to consider that their point of origin is to be 

 sought towards the Rhobell-Fawr area. Further reference will 

 be made to this question (see p. 316). 



Other rock-types that make their appearance in the volcanic series east of 

 the Gwynant are grits. The grits occur only in thin bands, and are seen but 



1 It is true that slate-bands are recorded by Mr. Lake & Prof. Reynolds 

 (Q. J. G. S. vol. lxviii, 1912, p. 347) as occurring among the ' Ashy Series ' of 

 Mynydd-y-Gader ; but it should be noted that the Lower Acid Series, as defined 

 in the present communication, does not include all the strata described by 

 those authors under the term ' Ashy Series.' The slates mentioned by them 

 are here referred to the Didymograptios-bifidus Beds, or to beds still higher 

 in the stratigraphical succession. 



2 It will be shown later that another and higher slate-band, the Moelyn 

 •Slates, also thins out against the volcanic pile (see p. 274 & fig. 2, p. 277). 



