Vol. 62.] BOCKS OF WESTEKX CAEE3IAETHEXSHIEE. 



603 



Arexig. 



Table I. (continued). 



f Blue-black shales and mudstones con- 

 taining: Tetragraptus and trilobites. 



I Striped flaggy shales, with Ogygia mar- 

 gin a ta, etc, 



Tetragrapttis- 



Beds. 



Gritty beds, with Dictyograptus and 

 Dendrograptus. 

 j 0?'^'s-mudstones. 

 Grits and conglomerates, with Orthis and 

 trilobites. 



Black ashy shales, with local bands of blue 

 ash. 



I v Blue-black shales, with few fossils. 



Table II. — The Succession in the 

 Lower Llandovery 



f Slade Beds. 



I Eedhill Bed 

 1 

 Bal.v-Caradoc ■{ 



j Sholeshook Beds, -j 



Robeston-Wathen / 

 Limestone. \ 

 \ Dicra.nogra.pt us- 1 

 r Shales. 



Llandeilo -j Asaphus-tyrannus 



Llanvirx 



Arexig 



oplius-tyr annus f 

 Limestone. \ 



( D.-Murchisoni 



Beds. 1. 



I D.-bifidus Beds. \ 



I Tetragraptus- 1 



1 Beds. j 



Southern Limb of the Anticline. 



Grey sandstones and flags, with black 

 mudstone-partings — very few fossils. 



Grey grit and conglomerate. 



Blue-grey mudstones, with bands of 

 shelly limestones — usually rotten. 



Blue-grey mudstones, with trilobites 

 and brachiopods (Trinucleus seti- 

 cornis, etc.). 



Arenaceous limestones, with many 

 fossils. 



Black limestones, with calcareous shale- 

 partings. 



Black shales, with graptolites, etc. 



Black limestones, with Asaphus tyran- 



nus and Ogygia Buchii. 

 Black mudstones, with Didymograptus 



Murchisoni (?). 

 Black shales and mudstones. with bands 



of ash, and containing Didymograptus 



hifidus, etc. 



As in Table I. 



(0) Arenig Rocks. 



These are the oldest rocks exposed within the area, and occupy a 

 belt of ground extending from Banc-y-felin on the east, to Login on 

 the west. East of the Fenni Valley the ground is generally low — 

 being part of the peculiar hollow referred to on p. 602, but to 

 the west the ground is more hilly and broken. This variation is 

 accounted for, principally by a difference in the character of the 

 deposits. The beds in the lower ground consist almost entirely of 

 shales and mudstones, while in the high ground west of the Fenni 

 there is a considerable development of grits and conglomerates. 



The shales and mudstones are usually black and iron-stained. 

 The staining varies from deep chocolate to iridescent black. 

 Concretionary structure is very common ; and, on splitting, the 

 surfaces of slabs present numerous wavy concentric lines of light 

 and dark tints. This characteristic feature is a very useful means 



