﻿Vol. 6 1.] AEENIG FAWK AND MOEL LLYFXANT. 621 



suggestive of the JWesuretus-Be&a of South Wales. The fossiliferous 

 character of these beds has long been known, and it was from 

 them, at localities north and south-east of Tai Herion, that the 

 officers of the Geological Survejr obtained all their truly-Arenig 

 specimens. A better collecting-ground is afforded by the slopes of 

 Llechwedd Erwent, and by those between that hill and Foel Boeth. 

 The highly-fossiliferous band is not more than 10 or 12 feet thick, 

 but is rendered very evident by the streaky character of the rock. 

 The outcrop of the belt runs roughly parallel to the contours of the 

 hill, from the northern dolerite-intrusion of Llechwedd Erwent to 

 the pass which goes over to the LI afar Valley, and then turns 

 southward following round the slopes of Foel Boeth, and is fossili- 

 ferous all the way, 



The Filltirgerig or Hirundo-Beds [9] . 



Becoming still more streaky, the Erwent Beds pass gradually, 

 and finally rather suddenly, into the dark, blocky, calcareous shales 

 of the Hirundo-Beds. These, too, are as a rule slightly ashy, and 

 in the upper part include one or two well-defined bands of tuff. 

 In the main, however, they are of detrital origin, and contain a 

 great deal of remarkably-angular chips of quartz, together with 

 some very well-preserved flakes and shreds of colourless mica 

 (4645). The more shaly beds are well bedded, and show rapid 

 alternations of sandy, shaly, and micaceous material, as in the 

 corresponding beds of Pont Seiont. Fossils are not especially 

 abundant, but graptolites, where found, are usually preserved in 

 pyrites and stand out in full relief. In shaly beds Didymogrciptus 

 hirundo, Salt., is most frequent, and with it small forms referred 

 to D. nitidus, Hall, or D. patulus, Hall, have been found. There 

 are also a good many ill-preserved Tetragraptids, including Tetra- 

 graptus serra, Brongn., and T. reclinatus, Elles & Wood ; and a 

 single example of Didymogrciptus gibber ulus, Nich., has been found. 

 These are associated with imperfect fragments of Asaphid trilobites 

 and of JSglina. In the very streaky beds at the base of the series 

 Obolella plumbea, Salt., continues to be abundant, and with it has 

 been found a fine slab of Azygograptus suecicus, Tbg. 



When traced across the district these beds undergo considerable 

 variation. In the south the streaky character is more or less 

 constant throughout the series, but in the north the middle part 

 becomes less calcareous and more shaly, and at the top there 

 appear two well-defined ash-beds, which in the extreme north attain 

 a thickness of quite 20 feet. Under the microscope it is seen that 

 these ashes agree in composition with the andesites of the main Lower 

 Yolcanic Series of Arenig (4646 &4647). The lower band contains 

 numerous well-rounded grains of quartz and certain ' nummulite '- 

 like pebbles of friable shale which agree exactly in character with 

 the masses described by Prof. Grenville A. J. Cole l as occurring 



1 Geol. Mag. 1890, p. 449. 



