768 



THE WONDERS OF GEOLOGY. Lect VIII. 



other deposits. The lowermost strata, to a thickness of 

 thirty feet, are literally made up of brachiopodous shells 

 {Terebratula navicula). 



(b.) Aymestry limestone. Principally argillaceous and 

 subcrystalline limestones ; characterized by the prevalence 

 of the remarkable brachiopodous shell, the Pentamerus* 

 a genus only found in the Silurian deposits. 



(c.) Lower Ludlow rocks. A series of shales and con- 

 cretionary limestones, distinguished by the presence of 

 some peculiar chambered shells (Litnites, Phragmoceras), 

 Trilobites (Homalonotus), and zoophytes allied to the 

 Sea-pens (Graptolites).^ 



The inferior series of the Upper Silurian consists of the 

 Wenlock shales and limestones ; but the latter are better 

 known as the Dudley limestones, from the interest and 

 variety of the organic remains with which the strata in 

 the vicinity of that town abound. 



From the profusion of Trilobites, shells, and corals, dis- 

 played in relief on the surface of the slabs of the Dudley 

 limestone, many of the specimens are of surpassing interest ; 

 they are, indeed, tablets of stone, inscribed with the typical 

 hieroglyphics of the palaeozoic ages. 



The Lower Silurian formation is divided into two 

 groups ; — 



(a.) The Caradoc sandstones, consisting of shelly limestones 

 and sandstones, which contain trilobites, and cephalopodous 

 and brachiopodous shells, unknown in the upper strata (as 

 Pentamerus Icevls, Or this grand* s, &c.) ; and, (b.) the Llan- 

 deilo flags, which are dark-coloured micaceous schists and 

 flagstones, that form the base of the Silurian system. In 

 these lowest deposits some large Trilobites {Asapkus liuchii), 

 and certain species of Graptolites occur, and are regarded as 

 characteristic. 



12. Silurian System of England. — Both the litho- 

 * Medals of Creation, p. d. p. '290. 



