1846.] SEDGWICK ON THE FOSSIL SLATES OF N. WALES, ETC. 157 



Taking for granted all the previous details, it follows that the 

 great series of Cambrian slates (onaitting in this enumeration the 

 hypozoic rocks of Anglesea and Carnarvonshire) may be subdivided 

 into four or more great groups ; and when considered on a wide scale, 

 these groups are as well defined as the groups in the Upper Silurian 

 rocks of Denbighshire and Westmoreland. In the typical country of 

 Siluria, where the physical groups are better defined, the corre- 

 sponding groups of fossils are also better defined ; but this definition 

 is only local. 



1. The lowest of these groups may be called the Festiniog or the 

 Tremadoc group. Its lowest portions have no fossils — at least none 

 have been found in them ; but in its upper portion we have Fucoids, 

 Lingulae, and Trilobites. This group is seen on the shores of the 

 Menai, and in the low country to the east of it — in the south promon- 

 tory of Carnarvonshire (?) —in the Tremadoc promontory — in the 

 hills south of Festiniog— on the west side of the great Merioneth 

 anticlinal, in beds extending from the neighbourhood of Llanelltyd 

 to the ridges west of Great Arrenig — and lastly near the base of the 

 chain of Cader Idris. 



2. A great group of roofing-slate and contemporaneous porphyry 

 superimposed, bed upon bed, with greater or less regularity ; the 

 former passing into grits, flags, and coarse greywacke in many in- 

 definite alternations — the latter exhibiting indefinite alternations of 

 trappean breccias, trappean shales, and other forms of recomposed 

 plutonic rocks, mixed with those which have been erupted. In some 

 places fossils are abundant, in others they entirely disappear; and 

 they are seen both among the coarser slates and also (though more 

 rarely) among the recomposed plutonic rocks*. This group is seen 

 in the crests of the Carnarvon chain, in the chains of Arrenig, Arran 

 Mowddwy, and Cader Idris, &c. &c. The fossiliferous bands of Snow- 

 don, Moel Hebog, and Llyn Ogwen, &c., seem to be represented in the 

 chain of Arrenig and Cader Idris by irregular masses of crystalline 

 limestone, in which organic remains (if indeed they have ever existed) 

 have been obliterated by the action of the great ribs of porphyry 

 among which they are interlaced. As the genus Orthis first appears 

 in this group, it might be called the Orthidian group ; or, geographi- 

 cally, the Snowdonian group. 



3. A verj^ great group, in which porphyries are much more rare, 

 and in which (between the eastern flanks of Arrenig and the top of 

 the Berwyns) we have three, or more, calcareous bands, and nume- 

 rous fossils. In this group are included a part of the slate series 

 on the south flank of Cader Idris, descending towards the country 

 near Machynlledd, and all the lower groups of the South Cambrian 

 sections above described. This part of the comparison is however 

 difficult, from the comparative rarity of fossils in the parts of South 

 Wales alluded to — probably indicating deposits formed at a marine 

 depth beyond the ordinary limits of animal life. At the same time, 

 from the absence of contemporaneous porphyries in South V\'ales, 



* See the fossil list, supra, p. 149, where the letter (tt) indicates a porphyritic 

 rock associated with trappean shale. 



