﻿Vol. 6 1.] ARENIG FAWR AND MOEL LLYFNANT. 625 



come. The forms taken by this colour-staining are most fantastic, 

 and in strata with the bedding-planes undisturbed may show what 

 appears to be a most advanced type of folding or overfaulting. A 

 characteristic mass of pisolitic iron-ore, like those of Tremadoc and 

 Cader Idris, appears along a crush-belt in these shales, and is 

 exposed in the western branch of the Filltirgerig stream, south of 

 Maen Grugog, but the field- evidence is not sufficient to show 

 whether the mass is really a contemporaneous bed or a fault-rock 

 deposit. 



Of fossils, Biplograptus foliaceus, Murch., var., Biplograptus 

 angustifolius, Hall, and Bicellograptus moffatensis, Carr., are the 

 only species definitely identifiable. Unfortunately, although these 

 strongly suggest the higher part of the zone of Bidymograptus 

 Murchisoni, they only make us long for more, and do not definitely 

 determine the horizon. With them occur several forms of 

 Bicranograptus and Climacograptus, also a small species of lamelli- 

 branch and a very small Obolella. 



The upper part of the series is again mixed with coarse ashy 

 material, and the passage into the Upper Ashes is quite gradual. 



The distribution of the Daerfawr Shales on the map is most 

 curious. In part, at least, this is due to the great variations in 

 thickness mentioned above, and may, therefore, depend to some 

 extent upon the irregularities of the original surface of deposition. 

 Largely, however, it is determined by peculiarities of surface-contour, 

 and by the adjustment cross-folds which affected the rigid beds 

 above and below it during the production of the cleavage of the 

 district. The Daerfawr Shales are usually well-exposed. Good 

 sections are afforded all along the base of the Daerfawr precipice. 

 Fossils have been obtained immediately west of Clogwyn-y-Fran, 

 close to Arenig Railway-station, all along the screes under Craig- 

 yr-Hyrddod, and on the south side of the Amnodd-wen Fault, 

 where the shales are finely colour-banded. The great spread of 

 Y Merdd is not well exposed. 



The Upper Ashes of Arenig. 



The Upper Ashes are not sufficiently fresh to be of surpassing 

 petrological interest. They are the most resistant rocks in the 

 whole series, and determine all the higher summits, precipices, and 

 climbing-ground in the neighbourhood. Dipping with the other 

 rocks gently to the eastward, they have a very wide outcrop along 

 the eastern slopes of the hills, and at first sight appear to be much 

 thicker than they really are. Lower, Middle, and Upper divisions 

 have been tentatively distinguished in the mapping, but there are 

 no very sudden changes in character of the material, and what may 

 appear to be good mappable lines at one place usually lapse into a 

 maze of similarities within a very few hundred yards along the 

 outcrop. 



Q. J. G. S. No. 243. 2 x 



