the Geological Structure of North Wales. 253 



brought into any close comparison with the well-defined Upper Si- 

 lurian groups of Mr. Murchison ; neither do they closely resemble 

 hi mineral structure the Upper Silurian groups of Westmoreland : 

 but he compares the Denbigh flagstone with the fossiliferous slates 

 and flagstones of Horton and Settle in Yorkshire. 



§ 3. Classification of the three preceding divisions of the Welsh slates. — 

 Organic remains, fyc. 



The group of chlorite, slate, &c. contains no organic remains, 

 and forms no passage into the rocks of the other division ; it there- 

 fore offers no sure means of classification ; but it seems to be infe- 

 rior to the other slate rocks in the southern promontory of Car- 

 narvonshire. 



The age of the middle division is decided by the organic remains. 

 None have yet been discovered in the low country east of the Menai, 

 but it is much concealed by alluvial drift. 



Commencing with the line of the Nant Francon and Llanberris 

 slate quarries, the author describes a series of regular ascending sec- 

 tions, continued through a horizontal distance of three miles, and 

 intersecting beds without a single flexure, inclined more than 50°. 

 In this great mass of strata are no described fossils. But at its top, 

 fossil bands appear containing Orthis flabellulum and canalis in abun- 

 dance ; together with corals (Turbinolopsis ?) and stems of Encrinites. 

 These bands are traced on the east side of the highest summits of 

 the chain from Moel Hebog to Carnedd Llewelyn. All the country 

 east of that range might be represented by a peculiar colour ; but 

 it is in physical structure identical with the eastern parts of the 

 chain ; and the author wishes not to separate it on supposed nega- 

 tive evidence, which may be upset by new observations. 



After two rapid undulations, there is again a regular ascending 

 section to Capel Curig ; and thence over the shoulder of Moel Shabod 

 to the bottom of the valley near Dolwyddelan. The ascending sec- 

 tion (interrupted only by one very short undulation), measured on a 

 horizontal base at right angles to the strike, is more than five miles 

 long, and is through highly inclined beds. The thickness of this 

 fossiliferous system must therefore be very great. But in the hills 

 east of Penmachno, and south of Bettws y Coed, are calcareous 

 beds (with more numerous fossils), which are placed in a still higher 

 part of the ascending section. These calcareous beds (sometimes 

 burnt for lime) the author places nearly on the parallel of the Bala 

 limestone ; though on general analogy, and not on any direct evi- 

 dence of sections. 



Again, from the great Merioneth anticlinal (above described) to 

 the Bala limestone, there is a great ascending section ; on two or 

 three parts of which are found organic remains, far below the parallel 

 of the limestone. And above the Bala limestone, to the crest of the 

 southern Berwyns, is a series of beds, some of which contain many 

 Lower Silurian fossils, and at least one more calcareous band. 



Lastly, the fossiliferous groups south of Llansaintfraid Glyn Cei- 

 riog, are (at least provisionally) brought into comparison with the 

 Bala limestone and other fossiliferous beds in the trough of the 

 southern Berwyns. 



