ON MANOD AND THE MOELWYNS. 373 



As ropffirds tho oripjinal nature of tlicse rocks, it is difficult to 

 speak with certainty. Tlio wliole series, from the granite to the 

 Arcnig agglomerates, is described on the Geological-Survey Map as 

 "• Felspatho-calcareous, flinty and slaty ashes, often talcose ' * ; and 

 this description is retained in the later editions, in which, owing to 

 the insertion of the Garth Grit, the greater thickness of them is seen 

 to belong to the Tremadoc group. On the other hand, in the later 

 editions of the memoir, Sir A. llamsay still holds the opinion that 

 ashes do not occur earlier than the Arenig period, and states that on 

 re-examining these rocks he concluded they were only the ordinary 

 sediments of the district greatly altered f. Considering the dif- 

 ference between them and altered slates, it seems to us more natural 

 to regard them as having been mainly composed of fine ash, or of 

 mixed ashy and sandy materials. Such a conclusion would be in 

 harmony with the result of observations on Cuder IdrisJ. 



The generalization that the Lower-Silurian igneous rocks are 

 confined to the Arenig period does not seem to be tenable in detail. 

 Just as the ejection of ashes continued in the earlier Llandeilo 

 period, so there seems no reason to supj)ose that the process 

 may not have commenced in Tremadoc times. It may be objected 

 that if these rocks are traced round the south of Moelwyn towards 

 the west, they pass into the ordinary slates of the Llanfrothen area ; 

 but seeing that the point at which they disappear is the point at 

 which the whole agglomerate-mass above also thins out entirely, 

 the fact is rather in favour of our view than against it. It must, 

 however, be confessed that microscopic study gives no evidence of 

 an originally ashy composition. 



(b) 21ie Garth Grit, which on paloeontological evidence has been 

 regarded as forming the base of the Arenig series, is found below 

 Moelwyn Mawr at a height of 1300 feet, and, though repeatedly 

 displaced by minor faults, can be traced northwards towards Blaenau. 

 The main mass is about 13 feet thick, but for several feet below 

 there are thin bands of grit among the schistose rocks, showing 

 that the change of conditions was not altogether a sudden one. 

 Microscopic examination shows the grit to be made up of large 

 quartz-grains with a slight admixture of felspar, and a little green 

 mica between the grains. The appearance of the quartz suggests 

 its derivation from granitic rocks, though not of necessity directly ; 

 indeed, it seems most probable that it is derived from the older 

 Cambrian grits. 



Like the rocks above and below, the grit has been altered by 

 metamorphism. It is much more compact than in the locality 

 where it was first observed ; so much so that it might fairly be 

 termed a quartzite, and its appearance under the microscope would 

 give it a greater right to that name than is the case with other 

 Paheozoic grits, such as that of the Stiper Stones. 



Above the white grit there is in parts a thick band of a dark- 



* Ramsay, ' Geology of North Wales' (1881), pp. 25, 88, &c. 



t Ibid. p. 87. 



\ Cole and Jennings, Quart. Journ. Geol. Soc. vol. xlv. (1889) p. 436. 



