﻿160 ME. W. G. FEAKNSIDES ON THE [May I9IO, 



bands, but bedding-planes with actual discontinuity come in at 

 intervals of from 2 to 6 feet, and the quarry is mostly worked by 

 4-foot shot-holes along the direction of cleavage and at right angles 

 to these. 



Cone-in-cone ironstone nodules, sometimes spreading into beds, 

 occur at irregular intervals through the quarry, and sporadic pyrite- 

 crystals or white-weathering nodules are not uncommon. In 

 weathering the sulphides leach out to the surface, which becomes 

 first black, and then bleaches without much rusting. The heading 

 joints are very clean cut, but only occur at long intervals, and 

 when the weathering eventually breaks up the rock into small 

 pieces it splits into long, rod-like, prismatic masses which have their 

 greatest elongation along the direction of the cleavage. 



The best stone is provided by the more felspathic beds, which 

 occur at the southern end of the quarry, and belong to the basal 

 beds ; but stone has been got all along the wharf-side, and a thickness 

 of some 180 or 200 feet of strata has been worked over. Upwards 

 above this the texture becomes finer and the rock more pyritous, 

 and though some of its beds are still massive and flaggy, the bulk 

 of it passes into a splintery rusty-weathering needle-slate. 



A similar succession can be confirmed along the road by Garth 

 Terrace to Bron-y-garth, and also along the main road to Borth-y- 

 gest. In the quarry opposite the end of Ffordd Horfa-bychan, the 

 lower gritty beds are worked for slabs and building-stone. At 

 Pen-y-clogwyn, and at the cliff above the gate to Morfa Lodge, 

 from this same road, the gritty beds make a fine exposure, and 

 from it can be traced continuously to their abutment upon the 

 Llanerch Fault, at the eastern end of Moel-y-gest. West of this 

 fault the Portmadoc Beds are much broken by strike-faulting, and, 

 although I have mapped the gritty beds, I have not been able 

 always to distinguish the needle-slate above from the Moelygest 

 Beds below them, and the grit outcrop accordingly appears as a set 

 of discontinuous augen in a slaty matrix. 



The grit of the augen which includes the two farms of Tyddyn- 

 dicwm (isaf and uchaf) has long been known. One of its grits 

 is massive and almost a quartzite, and has been taken by some to 

 represent the basal Arenig grit. It is, however, only 4 feet thick, 

 and the flags above it contain Tremadoc trilobites in fair abundance. 

 The gritty augen on the Criccieth side of the outcrop contain 

 coarser and even stronger grits. The slates among them are much 

 crushed and very ill exposed, but the quartz-cemented felspathic 

 grits generally protrude through the scanty soil and can be readily 

 discovered. One of these cropping out along the 300-foot contour, 

 a quarter o£ a mile south of the farm of Mynydd-ddu, is 16 feet 

 thick and almost massive throughout. Below the 100-foot contour, 

 100 yards north of Caer-dyni, a whole bundle of similar grits is 

 exposed. 



The fossils of the Portmadoc Flags are few and ill preserved. 

 Occasional tails and broken fragments of Asajphellus can be got by 

 breaking the cone-in-cone ironstone nodules along their centres, 



