the Geological Structure of North Wales. 247 



Cambridge Transactions. The carboniferous series of Denbighshire 

 and Flintshire is passed over with a slight notice, and without any- 

 detailed sections. His chief details are confined to the counties of 

 Denbigh, Carnarvon, Merioneth, and Montgomery ; and the southern 

 limit of his survey is denned by an irregular line drawn from the 

 Severn, near Welch Pool, to the coast near Aberystwith. The 

 southern boundary is purely arbitrary, marking only the limits of his 

 survey, and not any physical separation of the older rocks, which 

 are continued in great undulations through all the higher parts of 

 North and South Wales. He then describes the several connected 

 traverses by which he was led to the general views now laid before 

 the Society; and to the modifications his views have undergone 

 in consequence of traverses made by him in 1834 and 1842 along 

 the line of the Holyhead road, and from that line to the mountain 

 limestone ridges of Denbighshire and Flintshire. 



As a general conclusion from these details, he states that the older 

 stratified rocks (including all the formations of the region inferior 

 to the mountain limestone) may be separated into three great phy- 

 sical groups or primary divisions. 



(1.) Chlorite and mica slate, &c, occupying the south-west coast 

 of Carnarvonshire, and a considerable portion of the Isle of Anglesea. 



(2.) Greywacke, roofing-slate, &c. (alternating with masses and 

 beds of contemporaneous plutonic rocks), spread out from the Me- 

 nai to the edge of Shropshire, occupying all the high ridges of Car- 

 narvonshire and Merionethshire ; to the south blending themselves 

 with the system of South Wales ; and to the north nearly bounded 

 by the line of the great Holyhead road. 



(3.) A great overlying (and sometimes unconformable) deposit of 

 flagstone, slate, &c. (Upper Silurian), extending through the hills 

 north of the Holyhead road, and overlaid by the mountain limestone. 



These three primary divisions the author represents by three co- 

 lours on a geological map ; and the same system of colouring may 

 be extended through all the older formations of South Wales. The 

 middle group is however of enormous thickness, and may hereafter 

 be further subdivided. Its lower part contains no fossils, and in its 

 upper part they abound ; but between its upper and lower parts 

 there is no true physical separation, and the fossils seem gradually 

 to disappear in the descending sections. He could indeed represent 

 the fossiliferous and non-fossiliferous slates of Carnarvonshire by two 

 colours ; but in extending these colours through other counties of 

 North Wales, he would be compelled, in the present state of his in- 

 formation, to adopt arbitrary, and perhaps inconsistent, lines of de- 

 marcation. Hence he has been induced, for the present, to adopt 

 a more simple system of colouring than he at first attempted. 



§ 2. Physical structure of the country under notice. — Strike and undu- 

 lations of the strata. — Structure and relations of the three great divi- 

 sions, 8(C 

 South Carnarvonshire. — Carnarvonshire is divided into two physical 



regions, — one to the south-west and the other to the north-east of 



