TREMADOC SLATE SERIES. 63 



Dolgelly Beds. 



The term Dolgelly group was proposed by Mr. Belt (1867) for 

 the soft blue and black slates of the neighbourhood of Dolgelly. 

 Their thickness is estimated at 600 feet. 



The beds have been divided as follows :^ — 



{Soft black slates (Moel Griin Slates), with Conocorypke abdita, 

 Agnostiis triscctiis, Olenus [Peltura) scaraluvoides, Cte^iopyge 

 pecten, Spluvrophthabmis liisitlcatus, Orthis leniiciilaris. 300 feet. 

 ^^ \ ( Hard blue slates (Rhiwfelyn Slates) with Olemis (Parabolina) 



Q ^ Lffiver. \ spiimlosiis, Ortliis lenticularis, Agnostus, Liiiguldla, and 

 ( Protospongia. 300 feet (?). 



Mr. Belt states that the slates in the Upper Dolgelly beds give a black streak, 

 and that with the exception of a thin layer of black slate in the Ffestiniog group, 

 there are no other beds in the Dolgelly district so characterized. These beds 

 generally contain numerous grains of pisolitic iron-ore. 



A very fine section of the beds is exposed along a brook falling 

 into the Mawddach at Rhiwfelyn. The beds are developed at 

 Dolgelly and near Tremadoc. In South Wales the series com- 

 prises bluish and grey flags ; and the black iron-stained slates of 

 Leweston, Haverfordwest, with Olenus spinulosus, may belong to this 

 series. The Black shales at Malvern represent the Upper Dolgelly 

 Beds. 



TREMADOC SLATE SERIES. 



This formation, so termed by Sedgwick in 1 846, from the town 

 of Tremadoc in Caernarvonshire, consists of blue and grey earthy 

 slates, flags, and sandstones, having a thickness of about 1000 feet. 

 They rest conformably upon the Lingula Flags, and are exhibited 

 in many places in Caernarvonshire and Merionethshire. In 1866 

 Messrs. Salter and Hicks indicated their presence near St. Davids. 



Two divisions are made in this group :^ — 



_o f2. Upper. — With Asaphus (Asap/ulliis) Homfrayi, Conocorypke depressa. 

 'S ^' \ Tremadoc, Shineton ? 



p^ i I. LoTjer. — Dictyonema Beds, with Dictyoncma socialis. Tremadoc, 

 ^ V Shineton, Malvern. 



Other fossils include Ogygia scutatrix, Asaphus affinis, Angelina 

 Sedgivickii, Conocoiyphe depressa, Niobe, and Olenus. In this forma- 

 tion Cephalopoda first make their appearance, such as Orlhoceras 

 and Cyrtoceras ; Graptolites, Crinoids, Asteroids, and Lamelli- 

 branchs appear. Lingulella Davisii is met with, together with 

 Obolella and Orlhis, also Conularia. The oldest British Starfish, 



1 G. Mag. 1867, p. 541 ; see also C. Lapworth, G. Mag. 1881, p. 320 ; 

 John Plant, Q. J. xxii. 506. 



'^ C. Lapworth, G. Mag. 1881, p. 320, 



