20 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



The upper flags have been described by the author in his former 

 paper as composed of softer beds than the lowest and middle sub- 

 divisions ; those beds being more or less slaty, and containing few 

 fossils. To this subdivision the author refers a bed, near the northern 

 extremity of this Section, which contains Graptolites ludensis. 

 Also 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 the lower, middle, and upper flags, 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. 



