﻿Vol. 66.~\ TEEMADOC SLATES OF SOUTH-EAST CAENAEVONSHIEE. 155 



show that even the minutest variations of lithology, in Dolgelly 

 rocks, are constant over several miles of county. 



The Bron-y-foel slate-quarry was worked in the lower part of 

 the Black Band, and afforded a black, closely-laminated slate, which 

 is too soft and full of pyrite to enter into serious competition with 

 the slates of Ffestiniog. 



The Black Band, as the least resistant rock of the district, is 

 usually weathered into a hollow, and deeply covered with soil. Its 

 outcrop is therefore marked by a succession of arable fields and 

 smooth meadows, with copses and plantations along the stream- 

 courses. Its outcrop can also usually be recognized by the flaky 

 black slates and the ochreous springs which come out at intervals 

 along its course. 



VI. The Teemadoc Seeies. 



(1) The Tynllan or Niobe Beds. 



Besting upon the Black Band, and lying between it and the 

 horizon of abundant Dictyonema, come the Niobe Beds. These are 

 a series of fine-grained, silky grey slates, and with the Black Band 

 preserve their characteristics unchanged round the Ynyscynhaiam 

 anticline, from Ogof-ddu to Borth. 



The section at Ogof-ddu is a striking one. The highest beds 

 adjoin the Dictyonema Band, while the lowest beds are continuous 

 with the Black-Band section just described. The section is as 

 follows : — 



Thickness in feet. 

 Rough, dark leaden-grey slates with some whitish partings and 

 white-weathering pyritous spots. Ill-cleaved, and breaking 

 into long tile-like fragments, with splintery ends, weathering 



grey or rusting to a rather bright tan colour 50 



Hard, blocky, blue to grey-blue beds with pyritous spottings, in 

 bands about 4 inches apart, uncleared. This series is bedded 

 in l-to3-f'oot bands, most massive in the higher part, and often 

 showing conchoidal fracture. Obolus is found on certain 

 bedding-planes of these blocky rocks, which weather blue or 

 drab, and do not rust. The middle beds are often sufficiently 

 rough-grained to be worthy of the name of grit, but generally 



the beds are hard mudstones 40 



Splintery or shivery black-weathering shales with an earthy 

 fracture. The topmost 12 feet are passage-beds to the more 

 massive grey -weathering bed above. The lower beds are very 

 thinly bedded, and when far weathered turn brown and flake 



into short flat splinters 40 



Splintery, smoothly-breaking blue beds without pyritous nodules. 

 They either bleach or rust to a foxy-brown colour during 



weathering 20 



Hard, clinking blue rock, very massive, and showing cleavage- 

 joints only. It breaks conchoidaily, but has some slight 

 banding. It contains Acrotreta, and rusts thickly. This rock 

 forms the outstanding crag through which the railway-cutting 

 at Ogof-ddu has been made , 40 



Total 190 



