THE SILURIAN SYSTEM. 



669 



name, and to designate its subdivisions ; that nothing could be more natural than to 

 describe the system and its components, till then untraced in other quarters by the 

 geographical names of the localities where they were most conspicuously developed ; and 

 that so widely has the nomenclature been adopted by the geologists of Europe, as to 

 render it desirable that the term Silurian, which involves no theory, should be retained 

 as a generic appellative. 



In composition, the Silurian rocks are arenaceous, argillaceous, and calcareous, show 

 ing in all cases evidence of a sedimentary origin, and in general of a regular and tranquil 

 deposition, the original lamination of the beds being quite obvious. The system is divided 

 naturally into two principal groups, now called the Upper and Lower Silurians, each of 

 them supposed to be about 4000 feet in thickness. The upper group consists of soft, 

 greenish, or drab-coloured, shaly sandstone, locally called "mudstone," from having been 

 completely broken down and softened by exposure. The second division contains hard 

 slaty sandstones, with beds of limestone, having a concretionary sub-crystalline texture, 

 the lower members consisting of dark -coloured flags, resembling the Cambrian slates 



Ludlow Hocks. 



Wenlock Rocks. 



Caradoc Hocks. 



Llandeilo Formation. 



below and in many cases passing into them. The annexed is Sir E. Murchison's section of 

 the series, and his summary, with some additions from other sources : 



Upper 

 Silurian 

 rocks. 



Lower 



Silurian 



rocks. 



Ludlow 

 Group 



Thickness. 



2000 feet 



Wenlock | 

 Group J 



Caradoc 

 Group 



1 2500 feet 



Llandeilo ") 

 Group. ) 1200 feet 



Subdivisions. Lithological Characters. 



Upper Ludlow j" Sli g lul y micaceous thin-bedded, 

 i soft, greenish-grey sandstone, 



I- decomposing into soft mud. 



Aymestry lime- J Dark g rev > and blue or mottled 

 s t one argillaceous limestone ; sub- 



* crystalline. 



Sandy shale and flags; dark 

 grey and liver-coloured with 

 concretions of earthy lime 

 stone, very liable to decom 

 position into "mudstone." 



Wenlock lime- J Grev> and blue > concretionary 

 limestone, with much cal- 

 l- careous spar. 



{Argillaceous shale, liver-coloured 

 and dark grey, rarely mica 

 ceous, with nodules of earthy 

 limestone. 



Thin-bedded, impure, shelly 

 limestone ; and finely la 

 minated, slightly micaceous, 

 greenisli sandstone. 

 Thick-bedded, red, purple, green 

 and white freestone ; con- 

 glomeritic quartzose grits ; 

 L sandy and gritty limestones. 

 Dark-coloured grey flags, often 

 calcareous, with sandstone and 

 schist. 



Lower Ludlow 

 rock 



stone 



Wenlock shale 



Caradoc sand 

 stone 



Localities. 



Ludlow y vicinity 

 of Usk. 



Aymestry; Sedge- 

 ley near Dud- 

 ley. 



Ludlow ; Usk. 



Dudley ; Wen- 

 lock ; near Usk. 



Wigmore valley. 



Caer Caradoc ; 

 May Hill ; near 

 Llandeilo. 



j Llandeilo flags -j 



}Xeai 

 LI 

 bn 



Near Builth ; 

 andeilo; Pem 

 brokeshire. 



The whole of these groups are crowded with organic remains, and include some beds 

 almost entirely composed of them. 



