LINGULA FLAG SERIES. 6l 



thence circling round the Cambrian grits by Ffestiniog, they pass 

 out to sea on the south side of Traeth Bach. They may be studied 

 at St. Davids in South Wales, where they rest conformably upon 

 the Menevian beds, and attain a thickness estimated by Dr. Hicks 

 at 2000 feet. In Shropshire they are represented by the Shineton 

 Shales ; and in the Malvern area by the Hollybush Sandstone and 

 Malvern Shales. 



The gold lode of Dol-y-frwynog occurs in a talcose schist asso- 

 ciated with igneous rocks on the horizon of the Lingula Flags. 



In the maps of the Geological Survey, all the beds (excepting 

 the Igneous rocks) from the Lingula Flags to the Lower Llan- 

 dovery strata have received one colour ; for though an order of 

 succession can be made out by the help of fossils, yet practically 

 most of the formations pass so gradually into each other that it 

 was found impossible to define their limits on the map.^ 



In Wales the beds have been divided as follows : — 



T • 1 r ^. Dolgelly Beds. 



Lmsrula \ -^ -ur ^- ■ -d ^ 



2. tfestmiog Beds. 



I. Maentwrosf Beds. 



Flas: Series 



Maentwrog Beds. 



The name Maentwrog Beds was proposed by Mr. T. Belt in 

 1867, for the grey, yellow, and bluish-coloured slates and flags with 

 bands of sandstone, characterized by typical forms of Olenus, and 

 sometimes termed the Olenus Beds, which are exhibited in great 

 perfection at and around the village of Maentwrog, in Merioneth- 

 shire.'' The Maentwrog group is specially characterized by its dark 

 blue, jointed, ferruginous slates. It attains a thickness of about 

 2500 feet. 



Two divisions have been made : — 



^ 1' Dark dull blue slates, 



^ Upper. \ ractes. 1200 feet. 



M / ( Fine-grained grey flags, w 



Dark dull blue slates, with Agnostus pisifonnis, Olenus cata- 



ractes. 1200 feet. 

 Fine-grained grey flags, with A. pisiformis, O. tnmcatus. 600 feet. 

 Bluish-grey and black slates, alternating with grey and yellow flags : 

 J \ the slates contain Olenits gibbosiis, Agnostics nodosus, A. pisi- 



' ^ /orm/s, var. obestis. 300 feet. 



Fine-grained grey and yellow pyritic flags. 400 feet. 



On the whole, the fauna is very restricted. Phyllopoda make 

 their appearance. Mr. John Plant obtained many fossils from the 

 slates of Tyddyngwladis and Cwm Eisen.^ 



The Maentwrog beds extend from a little above Barmouth to 

 Llanelltyd. They are well developed in the Waterfall Valley and 

 in the valley running from Tafarn-helig to Caen-y-coed. 



^ A. C. Ramsay, Geol. N. Wales, edit. 2, p. 343. 



2 G. Mag. 1867, p. 538. 



3 Q. J. xxii. 505. 



