﻿Yol. 66.~\ TKEMADOC SLATES OF SOUTH-EAST CARNARVONSHIRE. 159 



bridge. There the joints are clean cnt, and the rocks shiver rather 

 than break under the hammer. They are overlain by the more 

 thinly-bedded grey slate, which not infrequently contains Acrotreta. 

 The great laccolite of Moel-y-gest follows approximately the bedding 

 of the paler slates above the line of ironstone concretions, and about 

 150 feet above the Dictyonema Band ; and the small sill quarried in 

 the old railway above Penmorfa is at a similar horizon. Every- 

 where in the vicinity of these intrusions the rocks have lost their 

 cleavage, and, being hardened almost to recrystallization, have had 

 their bedding and flaggy character rendered very prominent. 



Among the upper portion of the Moelygest Beds I have found no 

 trace of fossils, and this despite a long-continued search over the 

 open dip-slopes on the north of Moel-y-gest. 



The Geological Survey Memoir (vol. iii, 1866, pp. 251, etc.) records 

 fossils from the northern slopes of Moel-y-gest, but their exact 

 localities are now lost. In the Memoir it is also stated (pp. 69 & 

 256) that all the localities for Lower Tremadoc fossils (Niobe and 

 Psiloceplialus) occur along a line about 100 feet above the Dictyo- 

 nema Band ; but with Homf ray I agree in regarding this statement 

 as due to a mistaken conception of the position of Dictyonema. 



On the hill-side between Tyddyn-dicwm-uchaf and the Carnarvon 

 turnpike there are many exposures of the Moelygest Beds, but, owing 

 to strike-faulting, I have not made out the tangle of their succession. 

 At Ty'n-y-coed, and along the bank above Llwyn-derw, I have col- 

 lected specimens of an Orthis, very like the Orthis christiance figured 

 by Moberg, 1 and these I think must belong to the banded lower 

 portion of the Moelygest Beds. 



The exposures in the rough pastures of Caer-dyni near Criccieth 

 show slates of similar type, too crushed and slickensided for suc- 

 cessful fossil-hunting. The pale bands which here occur about the 

 middle of the series are associated with gritty or felspathic bandings 

 and partings. 



(4) The Portmadoc Flags. 



The flaggy beds which overlie the rusty bladed slates of the 

 highest Moelygest Beds are quarried along the wharf-side at 

 Portmadoc, and as they have provided the stone of which the town 

 is built, I suggest for them the name ' Portmadoc Flags.' 



The change from slates to the more massive and coarser beds, 

 which have been termed ' flags/ takes place rapidly, and is asso- 

 ciated with the appearance of a more massive and regular arrange- 

 ment of the jointing, which makes the quarrying of building-blocks 

 possible. The rock quarried is a minutely-banded blue-grey stone, 

 with ten or twelve flag-like laminae to each inch, and with prominent 

 pale, and somewhat coarser, felspathic bands every few inches. By 

 reason of a rude cleavage, the rock does not split along all these 



1 ' Ceratopyye Begionen ' Med. fran Lunds Geol. Faltklub, ser. B, No. 2 

 1906, pi. ii. 



Q.J.G.S. No. 262. m 



