﻿614 ME. W. G. EEARNSIDES ON THE GEOLOGY OE [Aug. I905, 



The best exposure in the district is the stream-section in the 

 lower part of the Trinant Yalley ; but that in the small stream 

 which comes down from Moel Llyfnant to the Beudy-uchaf of 

 Bendre Blaen-lliw is also quite good. The Drift south of Tai 

 Herion, too, affords a good collecting-ground, and the specimens of 

 Peltura which it contains in abundance are the only evidence for 

 the completion of the Lingula-Yl'dg succession to the east of the 

 great North- an d-South Fault. 



The Niobe-Be&s [17]. 



Towards the top the beds of the Black Band lose their silky 

 character, and become cleaved mudstones rather than slates. Pel- 

 tura is no longer abundant, but occasional examples of Niobe 

 Homfrayi, Salt., and Psilocephalus innotatus, Salt., take its place. 

 Passing up, the amount of pyrites rapidly diminishes, and through 

 dark-blue the sediments assume a rather light leaden-grey colour. 

 Weathered and joint-surfaces are no longer thickly coated with 

 scaly rust, but are merely filmed over with it, and often appear 

 iridescent. 



The character of the sediment also alters, and the main mass 

 of the Niobe-Beds is not unlike that of the lower part of the 

 Parabolina-Be&s. The jointing, however, is quite different, and is 

 rather like that of the well-known ' needle-slate ' rocks of Port- 

 madoc, which belong to the same horizon. Judging from casual 

 specimens, obtained from washouts in the heather-covered moorland, 

 the Niobe-Beds must occupy a good deal of ground on the north 

 and west of Moel Llyfnant, but exposures are not encouraging. 

 The best exposure is undoubtedly the stream-section along the 

 eastern branch of the Trinant Valley, which runs along the strike 

 of these beds for more than half a mile. This section well shows 

 the relation between the beds and a series of sill-like intrusions of 

 granular andesite or dolerite, which characterize this horizon right 

 across the district. Similar beds and intrusions are well seen in 

 the old road-section west of Nant-ddu, in the railway-cutting north 

 of Y Waun, which is east of the great North-and-South Fault, and 

 again for about a mile to the west and north-west of Hendre 

 Blaen-lliw. 



The Dictyonema-Band [16 pars]. 



Overlying the iWo6<?-Beds is the Dictyonema-Band, which (most 

 unfortunately) is even more imperfectly exposed than they are. 

 Dictyonema has been found abundantly in certain small boulders 

 in the Drift of the Tryweryn Yalley and elsewhere, but is only seen 

 in situ at one locality. Fortunately this spot, a few yards west 

 of Nant-ddu, adjoins one of the few sections from which Niobe 

 has been obtained, and it is perfectly clear that there, as ever 

 in the Portmadoc district, the Piety onema -Band overlies the 

 Niobe-Beds, and has nothing whatever to do with the Black Band 

 below. On this evidence, I would class the Welsh 



