766 



THE WONDERS OF GEOLOGY. 



Lect. VIII. 



mena which were previously involved in doubt and ob- 

 scurity. 



The Silurian system is largely developed in the border 

 counties of England and Wales, and spreads over a con- 

 siderable area of South Wales, forming a link which 

 connects the Devonian series with the ancient slate rocks 

 of that country. The strata are named and characterized 

 by Sir R. I. Murchison according to the following table : — 



The Silurian System. 



{Commencing with the uppermost). 



Ludlow* rocks — slightly micaceous grey-coloured 

 sandstone. Blue and grey argillaceous limestone. 

 Dark-coloured shales and flag-stones, with con- 

 cretions of earthy limestone, containing ortho- 

 cerata, spirifers, and trilobites. Fishes. 



Wenlock* or Dudley limestone — sub-crystalline 

 blue and grey limestone — abounding in trilobites, 

 crinoidea, polyparia, spirifers, orthocerata, &c. 



Wenlock shale — dark-grey argillaceous shale, with 

 nodules of sandstone. 



Caradoc\ sandstone — shelly limestones, and finely 

 laminated, slightly micaceous greenish sand- 

 stones. Corals, mollusca, trilobites. Fishes. 



LlandeiloX flags and sandstones. Freestone, con- 

 glomeritic grits, sandstones, and limestones. 

 Dark-coloured flags. Beds of schist with abun- 

 dance of trilobites and mollusca. 

 Total thickness, nearly 8,000 feet. § 



Upper 



Silurian : 



thickness about 



4,000 feet. 



Lower 



Silurian : 



thickness about 



3,500 feet. 



/ 



* Towns in Shropshire, situated on the respective strata thus desig- 

 nated. 



f Caradoc, or Caractacus, from the Caradoc hills of Shropshire, so 

 named from the celebrated British chief. 



X Llandeilo; so named from a town in Caermarthenshire. 



§ The Silurian System, founded on geological researches in the 

 border counties of England and Wales, with descriptions of the coal 

 fields and overlying formation ; by Roderick Impey Murchison, F.E.S., 



