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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 phjrsical 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 county under notice Strike and undu- 

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

 sions, 8fC. 



South Carnarvonshire. — Carnarvonshire is divided into two physical 

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

 the road from Carnarvon through Llanllyfni to Tremadoc. In the 

 south-western country, the coast, from Porthdinlleyn to the end 

 of the great promontory, and to Bardsea Island, is composed of chlo- 

 rite and mica slate. It forms a band on the average not two miles 

 in breadth, but it is evidently the prolongation of a formation which 

 is widely expanded in the Isle of Anglesea. At its north end it is 

 associated with a mass of brecciated serpentine (like that subordi- 

 nate to the same rocks in Anglesea), and it is here and there pene- 

 trated by veins of calcareous spar, sometimes so abundant as to re- 

 place the ordinary rock, which in such cases passes into great irre- 



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