﻿Yol. 6 1.] THE GEOLOGY OF ARENIG EAWR AND MOEL LLYPNANT. 639 



the mountain. He had visited the district with llamsay and Aveline 

 when the difficulty of fixing a base first became acute, and as the 

 grit so well seen between Portmadoc and Garthbarn seemed always 

 to occur where the paloeontological change from Tremadoc to Arenig 

 set in, it was taken as the base. This was founded chiefly upon 

 J. W. Salter's palaeontological work, for in those days they had no 

 detailed succession based upon the graptolites, such as Miss Elles 

 had now established. The chief difficulty about this base was, that 

 although at Portmadoc the grit rested upon shales which contained 

 Angelina within an inch of its base, at Garthbarn the grit was 

 divided by a fossiliferous shale, and farther on towards Tan-y-bwlch 

 it was represented by several lenticular beds of grit, so that some 

 grits were well up in the Arenig, while the overlapping beds of the 

 Arenio- crept one over the other on to various parts of the uncon- 

 formably-underlying Tremadoc. At the top of the Arenig also there 

 was a difficulty as to the best place to draw the upper boundary, 

 because the beds which cropped out farther west towards Bala (and 

 had always been taken as Bala) reappeared on the top of Arenig 

 Mountain. As, however, the Arenig passed up into the overlying 

 Bala, this was only a small matter of convenience of correlation. 

 The question was asked, where then was the Llandeilo ? He pointed 

 out that across the middle of the island there was evidence of an 

 east-and-west movement, which had not had much effect on the 

 beds up to the top of the Tremadoc, but had resulted in a different 

 stratigraphical and palaeontological facies in the Arenig, Bala, and 

 Silurian. In the south, a number of limestones occurred in the 

 Silurian, without which the divisions would never have been drawn 

 where they were ; while the muddy and sandy sediment of the 

 northern area differed much from that of the south. So in the 

 Arenig and Bala Series a number of volcanic rocks occurred in the 

 northern area, showing that the conditions were different there 

 from those which prevailed in the south. The Llandeilo group was 

 not absent owing to any unconformity or contemporary erosion, but 

 simply because the conditions affecting sedimentation and life were 

 different, and sufficient data were not yet available to determine 

 which deposits were being laid down in the Arenig area when the 

 Llandeilo Elags of Caermarthenshire were being formed. 



Mr. P. Lake drew attention to the similarity of the lower part 

 of the Author's sequence and that of the Dolgelly district, de- 

 scribed by Prof. Reynolds and himself. Up to the Diclyonema-Beds 

 the correspondence was precise, excepting for the fact that the 

 Mobe-Be&s had not yet been recognized at Dolgelly, probably on 

 account of the poorness of the exposures. But the Bellerojyhon- and 

 higher beds were apparently absent at Dolgelly, or were perhaps 

 represented in part by the volcanic beds which appear a short distance 

 above the Dictyonema-~Ban&. The most remarkable feature, how- 

 ever, was that at Dolgelly, as in the Arenig district, the Tremadoc 

 Beds were covered un conformably by laccolitic masses or sheets of 

 doleritic rock. Prof. Reynolds and the speaker had inferred that 

 these had been intruded along an unconformity ; but, as the overlying 



