448 



LINGULA FLAGS OF NOKTH WALES. [Cii. XXVII. 



tion, and -would create much confusion, by disturbing a nomenclature 

 long received and originally established on well-defined paleontological 

 data. 



In Shropshire, the classical region, where the type of the Silurian 

 group \vas first made out by Murchison, the formations subjacent to 

 the Llandeilo consisted of quartzose rocks, sterile of fossils, or yielding 

 little more than some obscure fucoids. In JSTorth Wales, Professor Sedg- 

 wick found below the Bala Limestone, long since recognized ' as the 

 equivalent of the Llandeilo flags, a vast thickness of sedimentary and 

 volcanic rocks, the lithological characters and physical features of which 

 he studied assiduously for years, dividing them into well-marked forma- 

 tions, to which he affixed names. Collectively they constituted the chief 

 part of the rocks called by him " Cambrian." They were devoid of lime- 

 stone ; but in a group of micaceous sandstones Mr. E. Davis discovered 

 in 1846 the Lingula named after him, and from which the name of 

 " Lingula flags" has since been derived. In these flags, about 1500 or 

 2000 feet in thickness, several other fossils were afterwards found, of dif- 

 ferent species from those in the Llandeilo beds. Amongst them, trilo- 

 bites. Agnostics and Conoceplialus (for genus, see fig. 614), and some rare 

 Brachiopoda and Bryozoa, still unpublished by our Government survey- 

 ors, have been detected, and in the inferior black slates of ISTorth Wales a 

 tiilobite called Poradoxides (for genus, see fig. 613), a form still more 

 characteristic of this era, together with another of the genus Oleniis (fig. 

 610), and a phyllopod crustacean (fig. 608). 



Fossils of tin 

 Fi^. 608. 



Lingula Flags,'''' or lotcest Fossiliferoiis Rocks of Britain. 



Fig. 609. Fig. 610. 



Hymenocaris vermicauda, 



Salter. 



A Phyllopod Crustacean. 



I uat. size. 



"Lingula Flags" of Dolgelly, and Ffestiniog; N. Wales, 



Lingula Davisii, M'Coy. 



a. k natural size, 



&. Distorted by cleavage. 



Olemis micrurus, 



Salter. 



J nat. size. 



I have before observed, that between the Bala Limestone and the 

 Lingula Flags there is a thickness of 11,000 feet of strata, in which 

 Grapiolites and certain species of Asaphus, Calymeyie, and Ogygia 

 occur. These may be referred at present to the Silurian series, but 

 the exact limits between them and the Lingula Flags cannot yet be 

 assigned. 



We might have anticipated, as already remarked, p. 442, that, when- 

 ever a fossil Fauna was discovered in the Cambrian strata, it would be 

 found to consist of distinct species, and even, to a large extent, of distinct 

 genera ; for, although geological periods are of very unequal value in 

 regard to the lapse of time (see p. 103), and our lines of separation may 



