Ch. XXVII.] MIDDLE SILURIAN ROCKS. 437 



mirus (fig. 586) is almost a globe when rolled up, the forehead of this 

 species being extremely inflated. The Homalonotus, a form of Trilobite 

 in which the tripartite division of the dorsal crust is Fj g . 537. 



almost lost (see fig. 58V), is very characteristic of 

 this division of the Silurian series. 



2. The Wenlock Shale. — This, observes Sir R. 

 Murchison,* is infinitely the largest and most per- 

 sistent member of the Wenlock formation, for the 

 limestone often thins out and disappears. The shale, 

 like the Lower Ludlow, often contains elliptical con- 

 cretions of impure earthy limestone. In the Malvern 

 district it is a mass of finely levigated argillaceous 

 matter, attaining, according to Prof. Phillips, a thick- 

 ness of 640 feet, but it is sometimes more than 1000 

 feet thick in "Wales. The prevailing fossils, besides 

 corals and trilobites, and some crinoids, are several 

 small species of Or this, with other brachiopods and 

 certain thin-shelled species of Orthoceratites. One ce^ahSoJXZs 

 species of Graptolite, a group of zoophytes before Castle; jnai size, 

 alluded to as being confined to Silurian Fig. 588. 



rocks, is very abundant in this shale, and '^^^^^^^^^^^ 

 occurs more sparingly in " the Ludlow." GraptoUthm Ludensis, Murchison. 



Of these fossils, which are more charac- Ludlow and Wenlock Shales. 



teristic of the Lower Silurian, I shall again speak in the sequel (p. 442). 



MIDDLE SILURIAN ROCKS. 



Caradoc Sandstone. — This sandstone, so named from a mountain called 

 Caer Caradoc, in Shropshire, was originally considered by Sir Roderick 

 Murchison as the sandy and upper portion of the Lower Silurian strata. 

 Subsequent investigations have led to the conclusion that the original or 

 typical Caradoc is divisible into two formations, — the lower, an arenaceous 

 form of Llandeilo flags, and containing identical species of fossils ; the 

 other or superior sandstone, a series of strata resting unconformably on the 

 Llandeilo beds, and chiefly characterized by Upper Silurian fossils, yet 

 having some intermixture of species common to the " Lower Silurian." 

 Hence the Caradoc, as distinct from the Llandeilo, must either be classed 

 as the base of the Wenlock Shale, an opinion to which some authorities 

 incline, — or it may be regarded as a Middle Silurian group, an alternative 

 which I have embraced provisionally in common with many officers of 

 our Government Survey. The larger part, therefore, of what was once 

 termed " the Caradoc" has merged into the Llandeilo, and is the equiva- 

 lent of the upper and middle portions of that division. 



The first step towards placing in a clearer light the relations of " the 

 Caradoc" to the strata above and below it, was made in 1848 by Professor 



* Siluria, p. 111. 



