102 PEOCEEDINGS OF THE GEOLOGICAL SOCIETY. 



out the two faunse in the Tremadoc area in 1853, to regard the 

 Arenig as the base of the Upper Cambrian of Sedgwick " *. Phillips 

 regarded the Arenig as the upper or terminal member of the Middle 

 Cambrian. Upon fossil evidence we may commence the Upper 

 Cambrian of Sedgwick with the Arenig series, which is represented 

 in the " Stiperstones " district. It was not until 1859 that Mur- 

 chison and Salter described the fossils of that area, although Sedg- 

 wick in 1843 established the Arenig group, and subsequently 

 obtained the same fossils from the Skiddaw Slates, which were 

 described by M'Coy previous to 1851. Salter, from the majority of 

 the fossils in the Tremadoc and Arenig groups, considered (and, I 

 believe, rightly) that the Tremadoc is the " natural termination " 

 of the Ffestiniog or Middle Cambrian series, and the Arenig the 

 base of Sedgwick's Upper Cambrian. Lyell adopted this view in his 

 manual; and Hicks and others have followed Salter in thus placing the 

 two groups. In the Cambridge Catalogue the Arenig is treated partly 

 as an intermediate group ; and although it is 4000 or 5000 feet in 

 thickness, yet the number of known species is few — about 60, all 

 named, in the Woodwardian Catalogue. 



That there is a perfect, continuous, and conformable succession 

 from the Tremadoc into the Arenig rocks, admits of no doubt. It 

 is true that this is seldom seen, even in the areas where they are 

 typically developed. In the Shelve and Stiperstones districts, where 

 36 species are known, and in the Skiddaw area, yielding 76, no 

 downward stratigraphical succession has been truly or definitely de- 

 termined. The Arenigs seem to stand alone, especially in Westmore- 

 land, where no older stratified rocks occur with which to compare 

 them. In South Wales, however, at St. David's, their base rests upon 

 the Tremadoc, and their summit is overlain by the Llandeilo beds of 

 Abereiddy Bay, both clearly defined. The three divisions here have 

 yielded no less than 96 species, 40 of which are Hydrozoa, and 31 

 Crustacea. Every Graptolite here makes its first appearance in 

 time ; none are known below the Arenig rocks. Six Crustacea out 

 of the 31 are derived from the Tremadoc, and only 3 pass to the 

 Llandeilo ; therefore 22 species are peculiar to the Arenig. Indeed, 

 out of the whole Areirig fauna, comprising 149 species, all but 38 

 are restricted to it. No more distinct group occurs in the British 

 Islands. 



Aeenig Rocks of Nokth Wales. 



Professor Sedgwick, in 1852, described the Arenig slates and 

 porphyries of North Wales as forming a distinct and well-marked 

 subgroup in his previously named " Ffestiniog group," and as rest- 

 ing, upon the underlying Tremadoc slate. It is also the "great 

 group of roofing-slate and contemporaneous porphyry " described as 

 occurring in the chains of Arenig, Arran Mowddwy, and Cader Idris, 

 and in the Ffestiniog mountains, or the western base of the Arenigs 

 and Arran, and probably the Stiperstone rocks. For many years 



* Catalogue of the Collection of Cambrian and Silurian Fossils in the Geolo- 

 gical Museum of the University of Cambridge, p. 18. 



