﻿YoJ. 6 1.] AKENIG FA WE AND MOEL LLYFNANT. 619 



The Llyfnant or Externus-Flags [12]. 



Upon the Basement Grit or surface of unconformity, as the case 

 may be, comes a distinctly-defined series of well-bedded blue-and- 

 white quartzose flags. These flags are remarkably resistant to the 

 weathering processes, wherefore they determine a prominent feature 

 right across the district, and, being well jointed, give rise to the 

 beautiful stepped or terraced scenery which is so well seen on 

 the eastern flanks of Moel Llyfnant. They are remarkably-well 

 bedded, and, being provided with convenient shaly or micaceous 

 partings, split readily into fine large slabs. In former days these 

 were quarried for flags and hearthstones, which were much sought 

 after down at Bala. Under the microscope the bedding becomes even 

 more apparent, and it is seen that calcite and shreds of detrifal 

 mica make up quite as large a proportion of the finer bands as do 

 the actual quartz-grains. There is also a great deal of unresolvable 

 muddy paste and some secondary quartz. The quartz-grains are 

 much more angular than those of the underlying grit, and are often 

 also curiously ragged at the edges (4642). 



Although so flaggy, the Llyfnant Beds are yet quite fossiliferous, 

 and eertain of their bands have yielded very numerous graptolites of 

 the extensiform type. The most abundant form is Lidymograptus 

 dejleceus, E. & Wl, but Didymograptus extensus, Hall, is not rare, 

 and Mr. Williams has also specimens of Loganograptas Logani, and 

 a Tetragraptus which is probably Amii. Tails of some large 

 Ogygia have also been found, and worm-tracks and castings (both 

 small and great) are extremely abundant. The markedly-flaggy 

 beds are from 150 to 200 feet thick. 



The best collecting-ground is afforded by the old quarries south 

 of Hafotty Filltirgerig, or by the stream-section which adjoins 

 them. The general character of the beds, however, is better 

 shown in the precipitous north-eastern face of Moel Llyfnant, or in 

 the more shelving slopes of Llechwedd Erwent. 



The Henllan or Calymene -Ashes [11]. 



Passing upwards, the well-bedded character of the Llyfnant LTags 

 gradually disappears, and, as the sediment does not become corre- 

 spondingly finer, the succeeding beds assume a notably ' blocky ' 

 character. With this they also take on a toughness which defies 

 description, and, although well-jointed on a small scale, appear to 

 retain neither bedding nor cleavage. Their colour is grey, and they 

 are granular and conchoidal in fracture, and in weathering become 

 covered with a characteristic, thick, cream- white crust. They are 

 more calcareous than the underlying flags, and contain much less 

 detrital quartz and practically no mica (4643). They appear to be 

 derived from some more or less distant volcanic source, which, 

 judging by the thickening and increase of ashy material southwards, 

 probably lay in that direction, and provided materials richer in 

 lime (and therefore more basic) than any of the products of the 

 later volcanoes of the district. 



