﻿176 MR. W. 0. FEARNSIDES ON THE [May I9IO, 



leave open the question of an upper limit. In this connexion it is 

 interesting to find that Sedgwick x records a Homalonotus bisulcatus 

 above the dolerite of Pant-Ifan. In similar rock from the waste 

 heaps of Portreuddyn, Mr. J. G. Black in 1909 turned out a 

 Trinucleus, and I an Ogygia ; and in the cliffs of Prenteg, which, as 

 I believe, are a continuation of the rocks of Pen-yr-allt, various 

 ill-preserved specimens of Orthis (0. elegantula, 0. actonii, 0. calli- 

 gramma) are not rare. 



Such indications as we have, suggest therefore, that the grey 

 and banded upper slates and their associated andesites belong to a 

 high Llandeilian or a low Caradocian (Snowdonian) horizon, and 

 this the further tracing of the andesites to a source upon the 

 Hoelwyn seems to confirm. 



The sediments of the Ynyscynhaiarn country having now been 

 described, there remain the rocks which are intrusive into these, 

 and the structures which have been impressed upon the country by 

 post-Ordovician earth-movements. These I shall consider in what 

 was probably the chronological order of their development, and shall 

 give the evidence for this chronology as I describe the phenomena. 



IX. The Folding. 



The most outstanding characteristic of the geological map of 

 Ynyscynhaiarn is the ring-like arrangement of the Lingula Flags 

 and Tremadoc rocks about a centre which lies among the sand- 

 hills half a mile east of Craig-ddu and midway between Borth and 

 Criccieth (see PI. XVI, sections C & D). Prom this centre, an anti- 

 cline, with its axis pitching at about 1 in 6, ranges N. 5° E. through 

 the marshes of Llyn Ystumllyn, and onwards until it is lost beneath 

 the Drift at Gwern-ddwyryd, half a mile beyond the village of 

 Pentrefelin. From this axis the rocks dip away eastwards at about 

 10° or 20°. To the westward the dip is steeper; but this, in part, 

 is due to the later (parasitic) sharp folding which has developed 

 irregularly along this western side. 



Half a mile east of the main anticline the arrangement of the 

 strike-lines between Treflys Church and Coed-y-cefn, south of Wern, 

 shows a syncline which ranges along a curving line considerably 

 ■east of north, and dies away north-eastwards. The anticline which 

 corresponds to this follows the hollow from Bron-y-foel to Morfa- 

 bychan, and runs more nearly north and south. 



From some points of view, the southward thrown fault-mass upon 

 which Borth stands may be regarded as the syncline that corresponds 

 to the Ynyscynhaiarn anticline ; but the evidence before us is not 

 sufficient to decide whether the Llanerch displacement, which 

 bounds it on the west, is earlier or later than the cleavage. The 

 manner in which the dolerite mass of Capel Siloam crosses the 

 Llanerch Fault at Ynys Gyngar Farm seems to show that that 

 dolerite is more recent than the fault. 



1 Quart. Journ. Geol. Soc. vol. iii (1847) p. 141. 



