SEDGWICK ON NORTH WALES. 19 



lower flags (b b'\ of the upper Silurian series. Towards the 

 northern extremity of the trough, a mass of the older rocks (c), 

 abounding, like the similar mass of Cyrn-y -brain, in fossils of the 

 Caradoc sandstone, breaks out at Cricor Mawr. This mass is 

 covered, on its north-western flank, by upper Silurian rocks ; and 

 these are overlaid by mountain limestone (d\ a range of which 

 bounding the vale of Clwydd on the S. E. runs from the point 

 represented in the Section, beyond Abergele. The mountain lime- 

 stone is followed by new red sandstone (/). 



Section IX. 



From Garn Brys, S. W. of Cernioge, to Abergele. 



Horizontal base 20 miles. 



S. 10° W. N. 10° E. 



ecj 

 >> 



Mwdwl j* 



Gam Brys. 3£g Eithin. tq Abergele. 



/ 8 



In this Section, we have, near Cernioge, first, the rocks of the 

 older series (a), abounding with fossils of the Caradoc sandstone ; 

 and secondly, lying unconformably on the preceding, are the con- 

 glomerates and sandstones (&) which there constitute the base of 

 the upper Silurian flagstones. These conglomerates, &c, the author 

 compares to the coarse greywacke and flagstone which constitute 

 the unconformable base of the upper series at the south-eastern 

 extremity of the Berwyns. These coarse mechanical rocks do not 

 appear in any distinct form in the country traversed by the two 

 former lines of section. The conglomerates pass into sandstones of 

 a finer structure, which alternate with bands of dark coarse slate 

 having occasionally true slaty cleavage. 



In this part of the Section, the author interpolates a fault, by 

 overlooking which, he was led, when he first exhibited this Sec- 

 tion, to estimate the conglomerates and sandstones at too great a 

 thickness. To the north of this fault, the finer sandstones (c') 

 are repeated. 



The lower sandstones have been already mentioned. The middle 

 flags, the author formerly described as consisting of beds resem- 

 bling those of the lower flags ; but these beds are more indurated, 

 and contain, here and there, many fossils. In this middle division 

 he now proposes to include the coarse greywacke and slates of 

 Bronhaulog (e) ; whereupon he takes occasion to remark that, in 

 North "Wales, slates arising from transverse cleavage extend to 

 a higher geological level than they do in Westmoreland, and to a 

 still higher level in Devonshire than they do in North Wales ; 

 and, consequently, that such cleavage does not define the age of 

 any rock, but serves only, like other peculiarities of structure, to 

 mark the existence of certain physical conditions. 



c 2 



