﻿164 ME. W. G. FEARNSIDES ON THE [May I9IO, 



VII. The Aeenig and Llandeilo Seeies. 

 (1) The Basal Grit. 



Upon the Garth Hill Beds at Ynys-towyn rests a strong, massive, 

 quartzose pea-grit which is supposed to overlie the Tremadoc beds 

 un conformably. 



The base of the grit rests upon a rough surface, which shows 

 erosion -pockets some few inches deep and a foot or two wide, but I 

 am unable to prove any continuous discordance in the dip. The 

 lowest grit is clean, compact, and well-cemented. It builds a bank 

 almost 20 feet thick, which is massive or irregularly jointed through- 

 out. In the midst of this bank a pebble-bed with quartz-pebbles 

 up to an inch in diameter occurs, and along with the quartz-pebbles 

 occasional flakes of shale, which also may have been pebbles, are 

 observed, but do not break the massive character of the grit. 



Above the basal grit-bank small shale-pebbles abound, and flaggy 

 partings break the second 20 feet into irregular 1 to 3-foot banks 

 not quite so coarse-grained as the basal mass. The upper 30 feet 

 are again massive, but, except in the topmost bed, the dip-slopes of 

 which are washed by the waters of the harbour, there are no 

 evident pebbles. 



The cementing material of the grit is almost wholly quartz in 

 crystalline continuity with the quartz-grains, but in some places 

 adjoining the more vertical joints interstitial iron pyrites serves the 

 same purpose. Some of the quartz-grains are quite hyaline and 

 blue, and small quantities of various fresh felspars, including micro- 

 cline, are not infrequent. 



As to the exact age of this grit, we know little. The grit 

 itself seems to contain no fossils ; but, by analogy with the grit of 

 Garth Hill, Minffordd, 1 and elsewhere, it is generally considered to 

 be the original local base of the Ordovician. 



On the Criccieth side of the district here described, from Tan-y- 

 rhiwiau by the eastern ridge of Moel-bach and Pen-ystumllyn Farm 

 to Pen-mynydd-du and Bryn Braich-y-saint, a more or less massive 

 felspathic grit much crushed by faulting may be traced continuously, 

 and seems to separate Tremadoc from Ordovician slates all the way. 

 As at Ynys-towyn, it is rich in hyaline quartz, and is often very 

 pyritous. Generally the crushing is extreme, and, although slaty 

 beds are usually interstratified with the grit at many localities, they 

 may be crushed in, and no information as to the succession can be 

 obtained. 



Below the first gate across the cart-track, 75 yards north-east 

 from Tan-y-rhiwiau, an exposure of rusty, fine-grained, gritty rocks 

 forms a passage series between the normal grits and slates, and 

 overlies the basal grit. This rock closely resembles some beds of 

 the Calymene 2 Ashes of Arenig or the flag series 3 of Manod-bach, 

 and like them contains occasional examples of Obolella plumbea and 

 Ogygia selivynii. .From a loose block adjoining this exposure I 



1 Presid. Address, Quart. Journ. Geol. Soc. vol. xix (1863) p. xxxviii. 



2 Quart. Journ. Geoi. Soc. vol. lxi (1905) p. 619. 



3 Mem. Geol. Surv. vol. iii (1866) p. 60. 



