﻿162 MR. W. G. FEARNSIDES ON THE [May I9IO, 



with many trilobites. Their coarse-textured needle-slates form the 

 country-rock into which the Tu-hwn't-i'r-bwlch sill of porphyritic 

 dolerite is intruded, and over much of their outcrop to the east of 

 Hoel-y-gest they are altered by that intrusion. 



The old quarry at Tyddyn-llwyn is opened in the beds immediately 

 overlying the Portmadoc Flags, and is just within the metamorphic 

 aureole. Here the slates exhibit no needle-cleavage, but break 

 rather along the bedding, showing a fracture with a silky sheen. 

 At the top of the quarry (nearest the intrusion) some of the beds 

 become pale and spotted almost to a desmoisite, and the coarser 

 bands come away in massive flagstones of first-class quality. The 

 pyrite which at Garth gives rise to very rusty weathering is here con- 

 centrated into sporadic bands of small, well-formed, cubic crystals. 



The Tu-hwn't-i'r-bwlch quarry on the north side of the sill, and 

 close to the Criccieth-Portmadoc road, is also partty within the zone 

 ■of alteration. There, too, the rock has little cleavage, but the shape 

 >of the fossils gives ample proof of great distortion. The rock 

 quarried is of a blue colour, and comes away in massive slabs along 



• certain more gritty bedding-planes. The bedding-planes are gene- 

 rally silky, and in the deepest part of the quarry some of the beds 



•show the beginnings of a ' desmoisitic' spotting. Fossils, abundant 



• when the quarry was less advanced, are now difficult to obtain at 

 ' Tu-hwn't-i'r-bwlch. The finding of a Hohmetopus and a Macro- 

 (Cystella at the entrance to the quarry persuades me that the fossil 



band of Penmorfa is not far above. 



On the hill-side overlooking Ystumllyn (near Criccieth) finely- 

 bedded blue-grey slates overlie the grit-belt. From these I have 

 obtained a few specimens of AsapJiellus and a Cheirurus, but the 

 shivering and cleavage are so intense that fossil-collecting is a slow 

 process. From the banded slates in the field north of the Criccieth 

 road, next but one east of Tan-y-rhiwiau, I have recently obtained 

 the tail of a Dicellocephalus, and by it am further confirmed in my 

 identification of the Penmorfa Beds in this district. 



(6) The Garth Hill Beds. 



Overlying the Penmorfa fossil-band are the thinly-bedded, rusty, 

 'flaggy slates containing Angelina, which from the time of Salter's 

 survey of 'the region extending from Tremadoc to Fiestiniog' in 

 1853 l have been known as uppermost Tremadoc Slates. 

 These have. their most open exposure along the slopes of Garth Hill 

 near Minffordd on the south side of the Afon Glaslyn, and outside 

 •our district ; but, as they form a dip-slope, and are richly fossiliferous 

 along Garth Terrace above Portmadoc, the name taken from the 

 famous collecting-ground across the river will also serve here. 



Perhaps the most accessible exposure is the cliff at the edge of 

 the alluvium or morfa south of Penmorfa village. The section 



1 Mem. Geol. Surv. vol. iii (1866) p. 308. 



