266 [Nov. 29, 



slate, having occasionally true slaty cleavage ; and these slates the 

 author compares with the dark roofing slates which form the base 

 of the upper series in the Llangollen district. 



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. 



To the sandstones succeeds a thick mass of Denbighshire flag- 

 stone (d), generally agreeing with that of the Llangollen district ; 

 and this is followed by the coarse greywacke and slate of Bronhaulog 

 (e) ; whereupon the author 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. 



At the end of the Section occurs a thick mass (/) in which 

 are a number of beds like those of the lower groups, but often pass- 

 ing into rotten slate or mudstone. The last bed (g) in this Section 

 is mountain limestone. 



On the fossils of these Upper Silurian rocks, as a whole, 

 it may be remarked, that they agree very nearly with those from 

 the upper Silurian rocks of Mr. Murchison ; but that the distribu- 

 tion of species is somewhat different. Thus, in the list of fossils from 

 the Lower Flags (vide list of fossils from Plas Madoc, Proc. Geol. 

 Soc, vol. iv. p. 221.), species are found which were once supposed 

 to be characteristic of the tilestone of Shropshire, a bed above the 

 upper Ludlow mudstone. This may be accounted for by the cir- 

 cumstance, that both the tilestones and the Plas Madoc beds belong 

 to an arenaceous deposit ; and hence, though widely separated by 

 intervening slates and flagstones, they have in common some 

 species not found in the intermediate beds, 



