PART II. CHAPTER XXII. 



267 



Lower Silurian Rocks, and their Fossils. 



Wenlock formation. The well-known 

 rock of Dudley, so rich in organic re- 

 mains, belongs to this member of the 

 upper Silurian group, which consists 

 in its higher division of limestone more 

 or less crystalline, and highly charged 

 with corals and encrinites of species 

 distinct fromi those of the mountain 

 limestone. In its lower part it is prin- 

 cipally composed of argillaceous shale. 

 In the Wenlock limestone, the chain- 

 coral, called Catenipora escharoides, 

 abounds. Among the shells appear the 

 genera euomphalus, productus, atrypa, 

 and many others. 



LOWER SILURIAN ROCKS. 



Caradoc sandstone. — This formation, which is 2500 feet 

 thick, consists chiefly of sandstones of various colours, with 

 some subordinate beds of calcareous matter. Almost all the 

 more abundant fossils belong to the same genera as those of the 

 upper Silurian rocks, but the species are distinct. 



Llandeilo formation. — This division, forming the base of the 

 Silurian system, consists of hard dark-coloured flags, sometimes 

 slightly micaceous, frequently calcareous, and especially distin- 

 guished by containing the large trilobites 

 Asaphiis Buchii (Fig. 279.), and A. ty- 

 ranus.* There are also several genera 

 of mollusca in this deposit ; and it is an 

 interesting fact, that with many extinct 

 forms of testacea peculiar to the lower 

 Silurian rocks, such as orthoceras, pen- 

 temerus, spirifer, and productus, others 

 Asaphus Buchii, Brong. associated belonging to genera still 



existing, as nautilus, turbo, buccinum, turritella, terebratula, 

 and orbicwla.f 



No land plants seem yet to have been discovered in strata 

 which can be unequivocally demonstrated to belong to the Silu- 

 rian period. 



In Norway and Sweden, the Silurian strata extend over a 

 wide area, and so much resemble those of England in lithologi- 

 cal character and fossils, that they will probably be found to be 



Catenipora escharoides. 



* Murchison, Silurian System, p. 223. 



t Ibid. p. 351. 



