CHAPTEE XX. 



THE SILURIAN GROUP. 



Silurian System of Sir R. Murchison and English Authors; Systemes Quartzo- 

 Schisteux superieurs et ' inferieurs of M. Dumont ; Palaeozoic Rocks of 

 Authors. 



MUSEUMS : Geological Society ; Museums of Dudley, Ludlow, &c. ; Collec- 

 tion of Sir R. Murchison, Mr. Grey, and of other Local Collectors. 



AUTHORS : Murchison, De Verneuil, Dumont, Barrande. 



CHARACTERISTICS : A Marine deposit of vast extent containing a number and 

 variety of Marine organic remains. 



THIS group, which was formerly imperfectly known, and classed 

 under the general denomination of grey-wacke or transition 

 rocks, has been most ably investigated by Sir R. Murchison, 

 to whose great work on the subject we must refer for com- 

 plete details. The name of Silurian has been bestowed on 

 this group in consequence of the districts which it occupies 

 in this country being those formerly inhabited by that tribe 

 of the ancient Britons named the Silures. 



GEOGRAPHICAL DISTRIBUTION. The Silurian strata ex- 

 tend from the heart of South Wales to that of England, 

 over the counties of Eadnor, Montgomery, Caermarthen, 

 Brecon, Pembroke, Monmouth, Gloucester, Worcester, Here- 

 ford, and Shropshire. They are classified into two divisions, 

 the Upper and Lower Silurian, which are again subdivided 

 into two upper and two lower stages. 



UPPER SILURIAN ROCKS. 

 1. Upper Ludlow. Micaceous grey sandstone. 



Argillaceous Ihneston, 



q r nwm- T ndlnw I Strata with concretionary 

 i limestone. 



Containing 'shells of 

 every class, brachi- 

 opoda most abun- 

 dant, corals and sau- 

 roidiSsh. Thickness 



2000 feet, 

 hells as be 



formation. (2. Wenlock shale. ' Argillaceous shale. ( ness 1800 feet. 



