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ME. W. G. FEAEN SIDES ON THE 



[May 19 1 o, 



Tremadoc 



Series. 



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A 



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Garth Hill Beds: (Incomplete in the Ynyscynhaiarn area.) 

 Grey-blue slates with Angelina. 



P e n m o r f a Beds: 



Flaggy raudstones and thinly-bedded slates 

 with Shumardia and the Shineton fauna. 



Portmadoc Beds: 



Thickly-bedded felspathic slates, with occa- 

 sional Asaphellus. 



Moely gest Beds : 



Banded grey slates and mudstones; few 

 fossils, Acrotrcta and Bellerophon. 



The Bictyonema Band: 



A constant and characteristic band of 

 bright-rusting blue-grey mudstones, with 

 abundant Bictyonema sociale. 



Tynllan Beds: 



Thinly-bedded rusty shales, with some hard 

 grey mudstone-bands, containing Niobe 

 and Psilocejihalus (Symphysurus). 



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dolqelly 



Sertes. 



Pfestiniog 

 Series. 



Maentwrog 

 Series. 



" Sooty-black mudstones, with Pelturascarabcsoides. 

 Blue-black mudstone, with Agnostus trisectus. 

 Black slates, with calcareous bands often crowded 



with Orthis lenticularis. 

 Dark flaggy slates, with Barabolina spinulosa. 

 Grey-blue slates and flags, crowded with Lingu- 



lella davisii. 

 Grey flags and grauwacke, with some coarser 



bands (1800 feet thick). 

 Busty grey and blue slates, with thin bands 



felspathic grauwacke. 



er 



I shall now briefly describe the rock series in the order of their 

 formation. 



III. The Maentweog ob Busty Flag Seeies. 



The oldest beds in the Ynyscynhaiarn district are exposed in the 

 cliff-sections between Mount Pleasant and Cefn, and form the low 

 promontory to the south and west of Treflys Church, as far as Careg- 

 yr-Eryr and the Black Eock. They are a series of ill-cleaved, banded, 

 dark grey, black, or green slates, silky in fracture. Their exposed 

 surfaces are soon filmed over with rust, and in weathering they 

 break up into bladed fragments about the size and shape of a man's 

 hand. Interbedded with the slates, in all except the lowest bands, are 

 numerous coarser felspathic or gritty ' ringers,' which are massive 

 in their bedding and well defined from their surroundings. These 

 ringers range from something less than an inch up to a foot in 

 thickness, and, though irregular in their distribution, seem to become 

 more abundant as we pass up the series. 



The measured succession below the columnar greenstone of the 

 shore east of Craig- ddu (Black Rock) is : — 



