540 Belt— On the ** Lingula Flags." 



flags. The Lower beds are bluish grey and only slightly arenaceous 

 and micaceous, and contain Lingidella Davisii, McCoy, in abundance, 

 and also mimerous worm tracks. Thin, hard felspathic layers 

 alternate with thicker and more shaly ones. These are succeeded 

 by thick beds of yellow and yellowish grey arenaceous flags, con- 

 taining also hard felspathic layers. The arenaceous beds are often 

 coarsely and strongly cleaved, and the cleavage planes filled with 

 iron rust, so that, but for the uncleaved interbedded felspathic layers, 

 it would be most difficult to determine the planes of bedding, and 

 those of cleavage might easily be mistaken for them. Lingidella 

 Davisii occurs only sparingly in this part of the series. The thick- 

 bedded arenaceous flags are followed by thinner-bedded grey and 

 yellowish grey flags, much flner grained than those lying below 

 them, and crowded with the shells of Lingulella Davisii. In these 

 beds in 1865 I found, near Penmaen-pool, the only specimen of a 

 true fucus recorded from British Lower Silurian or Cambrian rocks. 

 It branched dichotomously over the face of a slab about four feet 

 long and three feet broad. It belongs to the genius Buthotrephis, but 

 the species has not yet been described. 



The highest of the Lower Festiniog beds are bluish and brownish 

 grey fine-grained flags. They too are crowded with Lingulella 

 Davisii, and contain also Hymenocaris vermicauda, Salter, which has 

 been found near Penmaen-pool, on Mynydd-gader and on Moel 

 Hafod-Owen, but only sparingly. The Lower Festiniog beds are 

 about 2000 feet thick. 



Upper Festiniog Beds (No, 6 in Section). — Lying on the last- 

 named beds is a band of tough blue grey flags, not more than fifty 

 feet thick, but containing an assemblage of fossil remains, nearly 

 distinct from those in the beds below, and quite so from those above. 

 A species of Lingulella, probably a variety of L. Davisii, but only one- 

 third the size of that species, still occurs, and is accompanied by 

 Hymenocaris vermicauda. Along with these occur, for the first time, 

 Gonocory-phe micruua, Salter, and Bellerophon Cambriensis, Sp. n. I 

 have found this band, with its characteristic fossils, at Gwern-y- 

 barcud ; in the Mawddach near Craig-y-dinas, and on Mynydd-gader. 



The Festiniog Group, as above defined, comprises the Upper and 

 Lower Festiniog beds, and is a little more than 2000 feet thick. 

 The river Mawddach cuts through the whole of the beds between 

 Ehiufelyn and Hafod-fraith. From thence they range across the 

 east-end of Moel Hafod-Owen and by Pen-y-bryn, skirting the 

 igneous rocks of Ehobell-fawr, where, however, only the lower beds 

 are seen, as the upper ones have been thrust a mile and a half 

 over to the eastward, by the intrusion of the diabase, and are seen 

 on the east flank of Moel Cors-y-gamedd completely inverted, so 

 that they overlie the newer Dolgelly and Tremadoc beds, as shown in 

 Section, page 537. From Pen-y-bryn the lower beds, much disturbed 

 by intrusive rocks, run south-westerly past Glasdir-isaf and Llyn 

 Cynwch to Tyddyn-bach, but are not well seen, excepting on the 

 west bank of the lake, where good specimens of Lingulella Davisii 

 abound. The whole of the beds, having escaped from the disturb- 



