﻿166 ME. W. G. FEAENSIDES ON THE [May I9IO, 



(3) The Criccieth Felsites. 



Overlying the black slates of Moel Ednyfed and of the valley east 

 of Ystum-cegid, as far south as the village of Dolbenmaen, is a 

 great mass of fine-grained, cream-colonred or pink, felsitic rock with 

 small porphyritic crystals of felspar, and generally without visible 

 quartz. This rock is either a set of lava-flows or of sills, and, from 

 the character of the platy and columnar jointing and the general 

 absence of associated ashy beds, I incline to the latter view. There 

 is, however, practically no evidence of inetamorphism along the 

 junction ; and hence, while suggesting that the rock is one of the 

 Lleyn rhyolites, which were intruded at the time of the Snowdonian 

 volcanic activity, I would leave the matter open. The fine colum- 

 nar rhyolite of Dinas and the sea-cliffs of Criccieth Castle form the 

 seaward continuation of the same rock-mass. It is noticed by 

 Mr. Harker 1 as one of those rocks which is probably intrusive; 

 but, so far as I can find its exposures, they show no stratigraphical 

 relations except with the Drift. At Ystum-cegid- bellaf the 

 rhyolite mass is seen to be overlain by andesitic-rhyolite ashes and 

 agglomerate, which it seems to cut transgressively. 



VIII. The Teemadoc Country. 



In the consideration of the Cambrian and Tremadoc rocks it was 

 found possible to make out a stratigraphical succession from the 

 evidences of the accidental dip-sections which circumstances have 

 provided, and to carry on that succession unchanged around the 

 whole Ynyscynhaiarn riog. The Ordovician rocks are not so simple, 

 and the Ordovician succession about Tremadoc appears very unlike 

 the slate-rock series near Criccieth, which I have just described. 

 This difference was long ago recognized by Sharpe, who, in 1846, 2 

 remarked that the Tremadoc district with its greenstones belongs 

 to Snowdonia ; but, except in so far as Ramsay 3 and after him 

 Harker 4 have noted that Tremadoc lies within the westerly exten- 

 sion of the Lower Ordovician volcanic products, while Criccieth is 

 outside, the matter has received little comment. 



When one begins to measure a horizontal section along the dip, 

 say from Tyddyn-dicwm to the base of the Snowdonian rhyolites of 

 Ynys-wen or Moel-ddu across this Tremadoc country, one is im- 

 pressed by the apparent tremendous thickness of the rocks traversed. 

 Such a section measured directly seems to show about 4000 feet 

 of slate-rock between Tremadoc and the base of the Snowdonian 

 rhyolites. In the old days, when the Tyddyn-dicwm rocks with 

 their iron-ores were thought to be ' Tremadoc Slate,' a thickness 

 of 4000 feet to represent the Tremadoc, Arenig, and Llandeilo series 

 might be received with little comment ; but, when Salter in 1857 

 had shown that the rocks at Tyddyn-dicwm contained graptolites, 



1 ' Bala Volcanic Series of Caernarvonshire ' (Sedgwick Prize Essay) 1889, 

 p. 12. 



2 Quart. Journ. Geol. Soc. vol. ii, p. 303. 



3 Mem. Geol. Surv. vol. iii (1866) chapt. xi. 



4 ' Bala Volcanic Series of Caernarvonshire ' 1889, § vi. 



