438 



CARADOC SANDSTONE. 



[Ch. XXVIL 



Eamsay and Mr. Aveline, who observed that in the Longmynd Hills the 

 Caradoc sandstone rested unconformably on the Lower Silurian, and that 

 the latter or " Llandeilo flags," together with some still older rocks, must 

 have constituted an island in the Caradoc sea. Professor E. Forbes at the 

 same time observed that the island was probably high and steep land 

 rising from a deep sea, and that the Caradoc fossils, some of them of lit- 

 toral aspect, as Littorina and Turritella, were deposited round the mar- 

 gin of that ancient land. It was also remarked that while the sandstone 

 and conglomerate of this upper Caradoc"^ reposed unconformably on the 

 Llandeilo beds, it at the same time graduated upwards, as Sir R. Murchi- 

 son had stated, into the Wenlock Shale. 



Subsequently Professor Sedgwick and Mr. M'Coy, pursuing their inves- 

 tigations independently of the Survey in North Wales, became convincedf 

 that the Caradoc beds of May Hill and the Malverns, constituting the 

 Upper Caradoc, already mentioned, were full of Upper Silur. an fossils ; 

 and that the strata of Caradoc sandstone at Horderly and other places 

 east of Caer Caradoc belonged to the Bala group (or equivalent of the 

 Llandeilo), being distinguished by Lower Silurian species. This opinion 

 was finally substantiated by Mr. Salter and Mr. Aveline, in 1853, by an 

 appeal to parts of Shropshire where " the Caradoc" had been originally 

 studied by Sir R. Murchison, and where they found the Upper Caradoc 

 unconformable on the lower, and filled with a series of very distinct 

 fossils.J 



In the restricted sense, therefore, in which it is now understood, the 

 Caradoc Sandstone comprises a series of beds of passage from the Lower 

 to the Upper Silurian group. It is everywhere characterized by species 

 of Penfamenis and Atrypa unknown in the overlying Wenlock or Lud- 

 low beds, but which descend into the strata of the Llandeilo group. Fen- 

 tamerus loevis (fig. 589), and P. ohlongus may be particularly mentioned 



Fig. 589. 



Pentamerus lawis, Sow. Caratk-^ ^.u\:..^i^no. 

 Terhaps the young of Pentamerus oblongus. 

 «, T). Views of the shell itself, from figures in Murchison's Si!. Syst. 



c. Cast with portion of shell remaining, and with the hollow of the central septum filled with spar. 



d. Internal cast of a valve, the space once occu[)ied by the septum being represented by a hol- 



low in which is seen a cast of the chamber within the septum. 



* Quart Geol. Joiirn. vol. iv. p. 297. f Geol. Quart. Journ. 1S52, 



± Geol. Quart. Journ. vol. x. p. 62. 



