679 



SECTIONS OF NORTH WALES, &C. 



The author next discusses a series of sections illustrating the 

 structure of North Wales. One is drawn from the Menai Straits, 

 in a direction about E.S.E., so as to cross the Berwyn chain and end 

 in the carboniferous series near Osw^estry. The others are drawn 

 from the Berwyn chain to different parts of the carboniferous lime- 

 stone range on the north side of Denbighshire. The greater por- 

 tion of the first section crosses the older beds (the Cambrian Systeni) 

 which strike towards the N.E- The other sections intersect the 

 upper series (^Silurian System) which strike towards the N. W., 

 passing (in some places unconformably) round the beds of the older 

 system. From a consideration of the whole evidence the rocks are 

 grouped in the ascending order as follows. 



(1.) Chlorite slate, quartz rock, and mica slate of Anglesea and 

 Caernarvonshire. These are placed on the parallel of the first class ; 

 and nothing is discovered in the section that is perfectly analogous 

 with the Skiddaw slate, or first Cumbrian group, above described. 



(2,) The old slate series of Caernarvonshire and Merionethshire, 

 alternating indefinitely with bands of porphyry and felspar rock ; 

 many parts absolutely identical in structure with the second Cum- 

 brian group above- described. It is of enormous but unknown thick- 

 ness, and is bent into great undulations, the anticlinal and synclinal 

 lines of which are parallel to the strike of the chain. Through wide 

 tracts of country it is without fossils ; but at Snowdon and Glider 

 Fawr, encrinites, corals, and one or two species of bivalves have 

 been discovered in it. It ends with the calcareous beds which range 

 from Bala to the neighbourhood of Dinas Mowddy. This is called 

 the Lower Cambrian System. 



(3.) The next group (the Upper Cambrian System) commences with 

 the fossiliferous beds of Bala, includes all the higher portion of the 

 Berwyns, and all the slate rocks of South Wales which are below 

 the Silurian System. Its slate beds are less crystalline, and its 

 general structure is more mechanical, than the preceding group, and 

 it contains incomparably more fossils, which (though there are many 

 extensive portions of the group without fossils) are disseminated 

 through the more calcareous beds in great abundance. Many of the 

 fossils are identical in species with those of the lower division of the 

 Silurian System, nor have the true distinctive zoological characters 

 of the group been well ascertained. 



In many parts of South Wales it is separated from the Siluria?i 

 System by great faults and derangements of the strata, marked by a 

 broad band of rotten non-fossiliferous schist. At the north end of 

 the Berwyn chain it appears to pass by insensible gradations into the 

 lower division of the Upper System (the Caradoc Sandstone). 



(4.) The last natural group (the Silurian System). For all details 

 respecting this system the author refers to the abstracts of Mr. Mur- 

 chison's papers, and to his forthcoming work. He then describes the 

 sections : 



(1.) East of the Berwyns, in which the Caradoc Sandstone is finely 



