﻿156 MR. W. G. FEAKN SIDES ON THE [May I9IO, 



The cave of Ogof-ddu was worn out by the sea along a 3-foot 

 band of well-banded blue and grey rock, which forms the base of 

 this mass. 



The corresponding sea-cliff section on the east of the anticline is 

 at Trwyn-cae-iago, east of Borth Harbour. The lowest beds seen 

 are the clinking blue rocks of Craig-y-don. Splintery rocks (first 

 without spots, and then well spotted) follow, and have yielded a 

 good many specimens of Niobe and Psilocephalus. The blocky 

 spotted grey rocks of the Obolus Band follow, and determine the 

 edge and highest part of the cliff-bound promontory, being them- 

 selves overlain by the banded, well-jointed, ill-cleaved beds, which 

 at the entrance to Maenofferen Wharf underlie the Dictyonema 

 Band. Here, too, the total thickness is about 200 feet. 



The Tyn-llan exposures behind Penmorfa Church were known to 

 Homfray and Salter as a locality for Niobe and Psilocephalus as 

 long ago as 1860. Again, it is the spotted splintery beds some 60 

 feet from the base which contain the most abundant trilobites. 

 The best exposure is a small crag in the field immediately to the east 

 of Coed-yr-Eglwys, and between that wood and the road by Wern 

 Lodge to Carnarvon. The stream-section along the wood side is 

 also productive, but is difficult to work. 



The fossils usually found are Niobe homfrayi, Psilocephalus inno- 

 tatus, and Hyrnenocaris vermicauda. With these I have obtained 

 some small sheared specimens of a Shumardia, an Agnostus, 

 and a Theca. Acrotreta, LingideUa lepis, and a big Lingulella 

 very like L. davisii, but pointed in front and rectangular behind, 

 occur just below the trilobitc-bed, and are abundant in the massive 

 beds a little lower down the field. 



The Obolus Beds make a small feature, which crosses the W r era- 

 Carnarvon road just below the old tramroad bridge, and continues 

 down the east side of that road to the corner of Coed-tyn-llan. 

 This set of beds is very hard and cherty : by its spotting and 

 white weathering it is easy to follow and map in detail, and 

 so affords a fine datum-line from which to measure up 50 feet, 

 and thus arrive at the exposures of the fossil band with Dictyonema. 

 By this indicator stratum, Niobe Beds have been mapped con- 

 tinuously from the Portmadoc-Criccieth road, south of Wern by 

 Cefn-cyfanedd, round the southern slopes of Moel-y-gest, to the 

 north-and-south fault through Llanerch, and again by way of 

 Moelfra, through the woods of Parc-y-Borth, to the Trwyn-cae-iago 

 section described above. The same bed is also useful as a datum- 

 line across the moorland aud between Ystumllyn and Ogof-ddu. 



The outcrop of the Niobe Beds is generally rough and rocky. 



(2) The Dictyonema Band. 



Characteristic beyond all other beds known in the Ynyscynhaiarn 

 district is the belt of strata, 15 to 20 feet thick, which contains 

 Dictyonema sociale. This band wherever exposed is abundantly 

 fossiliferous, and certain of its bedding-planes are completely covered 



