﻿616 MR. W. G. EEARNSIDES ON THE GEOLOGY OF [A-Ug. 1905, 



to be built up of flattened lenses of the coarser, closely set 

 in a matrix of the finer material. Such rock, when compelled 

 to break along the bedding, affords surfaces which are either 

 nodular, or crossed somewhat irregularly by ripples of short wave- 

 length. Curiously enough, such ripples always appear to have a 

 trend which is almost parallel to the strike of the bedding-planes, 

 which here makes a considerable angle with that of the cleavage. 



Apart from the ripple-bedding, the Asaphellus-Fl&gs are much more 

 massive than any of the other divisions of the Tremadoc ; and this 

 character has so enabled them to withstand the agents of denuda- 

 tion, that they stand up as a visible feature right across the district. 

 The same property has also caused them to be considerably quarried 

 for wall-building by the makers of the roads. Fossils are not 

 superabundant, but a good many very large specimens of Asaphellus 

 Homfrayi, Salt., of broad type have been found in the alternations 

 of the coarse and fine layers of the ripple-bedded flags ; and it is 

 noticeable that all such specimens are found with the dorsal side 

 upwards, and are filled with coarser and covered by finer material. 

 One or two enrolled specimens have been found, and these too are 

 filled with coarse, and embedded in fine, material. 



The best and most accessible exposure is that afforded by the 

 road-side quarry immediately west of the Nant-Rhosddu bridge at 

 Tai Herion. The waterfall just below that bridge is determined 

 by the same beds, and the old quarries west of the Amnodd-wen 

 road, near Hafotty Filltirgerig, are in similar though slightly- 

 higher beds. Other sections are also seen at Yr Orsedd, and a 

 curious piece of folding below Ceunant-y-gareg-ddu allows them to 

 appear as inliers in the gorge there. The sections west of Moel 

 Llyfnant are not so satisfactory, and have not proved to be very 

 fossiliferous. The whole series is not more than 80 or 100 feet 

 thick. It appears to correspond with the gritty shales of Penmorfa 

 Post-Office, although unfortunately no Dikelocephalus has as yet 

 been obtained here. 



The Amnodd or Shumardia-Shales [14]. 



Above the AsapTiellus-Fl&gs the beds gradually become much 

 softer, and pass into a series of fossiliferous blue-grey mudstones. 

 Except when baked by the intrusive dolerite- sills, which are 

 common at this horizon, these beds are much softer than any of the 

 other beds of the district ; and so they usually occur in hollows 

 and boggy places in the moors. Though sheared and considerably 

 distorted, they rarely show any definite cleavage. Close to the 

 dolerite-sills they are often much hardened, but break readily along 

 a series of close-set quadrangular joints. In such places certain beds 

 take on a very characteristic concretionary or shrinkage-jointing, 

 and weather out into a mass of ovoidal lumps and curved splinters 

 rather after the fashion of a massive basalt. When such con- 

 cretions are broken across during the process of weathering, one 

 finds the successive rings beautifully and differentially iron-stained, 



