562 LOWER SILURIAN ROCKS. [Ch. XXVII. 



South. Wales, where they are well developed, and where they are over- 

 laid unconforniably by the equivalent of the May-Hill sandstone. 

 They consist chiefly of hard slaty rocks, with bands of sandstone and 

 beds of conglomerate from 600 to 1000 feet in thickness. The fossils, 

 which are somewhat rare, consist of twenty-eight known species, some 

 few peculiar, a part agreeing with the May-Hill beds, and the rest, six- 

 teen in number, belonging to Lower Silurian species ; besides these, 

 no less than fifty-four species of fossils are given by Sir R. Murchison 

 as common to the Lower Silurian (Caradoc) and the Wenlock forma- 

 tions, and we cannot doubt that all these existed in the intermediate 

 Llandovery and May-Hill periods. 



The whole May-Hill and Llandovery series has been considered by 

 some geologists as constituting beds of passage between the Lower 

 and Upper Silurian, while others have assigned to it the rank of a 

 Middle Silurian group. It may, with some reason, be objected that 

 the number of peculiar fossils are not sufficient to entitle it to so im- 

 portant a distinction ; but there is no small difficulty at present in 

 adopting any other classification. The two formations, the May-Hill 

 and the Llandovery, are intimately connected by their fossils, the 

 Lower having about two-thirds of its species common to the Upper 

 zone. Again, half the species of the Llandovery pass down into the 

 Lower Silurian, just as half the May-Hill species pass up into the 

 Wenlock. In England we might draw the line, as Sir E. Murchison 

 inclines to do, between Upper and Lower Silurian, by classing the 

 May-Hill with the higher division, and the Llandovery with the 

 lower ; but in countries where there is no unconformability of strata 

 between the two zones, such a line of demarcation between the middle 

 of the Pentamerus beds would be impracticable. It has been some- 

 times suggested that Ave might form a better tripartite division of the 

 Silurian rocks by including the Wenlock with the May-Hill and Llan- 

 dovery beds as a middle group, classing the two Ludlow formations as 

 Upper, and the Caradoc and Llandeilo formations as Lower Silurian ; * 

 but I am not prepared to adopt so great a change in the generally- 

 received classification. 



LOWER SILURIAN ROCKS. 



Caradoc and Bala Beds. — The Lower Silurian has been divided 

 into — 1st, Caradoc Sandstone and Bala Beds; 2dly, the Llandeilo 

 Flags ; and 3dly, the Lower Llandeilo or Arenig formation. The 

 Caradoc sandstone was originally so named by Sir R. I. Murchison 

 from the Mountain called Caer Caradoc in Shropshire ; it consists of 

 shelly sandstones of great thickness, and sometimes containing much 

 calcareous matter. The rock is frequently laden with the beautiful 

 trilobite called by Murchison Trinucleus Caractaci (see fig. 647), 



* See Report of Canada Survey Table of Equivalents, p. 932, 1863 



