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 Rocks. 



Wenlock Rocks. 



Caradoc Rocks. 



Llandeilo Formation. 



below and in many cases passing into them. The annexed is Mr. 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 1 

 Group J 



j goo f eet< 



Caradoc - 

 Group ] 2500 feet 



Llandeilo ~\ 

 Group. ] 1200 feet j Llandeilo flags <j 



Subdivisions. Lithological Characters. 



Upper Ludlow f Sli g htlv micaceous thin-bedded, 

 roc k soft, greenish-grey sandstone, 



L decomposing into soft mud. 

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

 stone 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." 

 {Grey, and blue, concretionary 

 limestone, with much cal- 

 careous spar. 



f Argillaceous shale, liver-coloured 

 J and dark grey, rarely mica- 

 ceous, with nodules of earthy 

 fc limestone. 



Thin-bedded, impure, shelly 

 limestone ; and finely la- 

 minated, slightly micaceous, 

 greenish 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 



Wenlock lime- 

 stone 



Wenlock shale 



Caradoc sand- 

 stone 



Localities. 



Ludlow ; vicinity 

 of Usk. 



Aymestry; Sedge- 

 ley near Dud- 

 ley. 



Ludlow ; Usk. 



Dudley ; Wen- 

 lock ; near Usk. 



Wigmore valley. 



Caer Caradoc ; 

 May Hill ; near 

 Llandeilo. 



Near Builth ; 

 Llandeilo; Pem- 

 brokeshire. 



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

 almost entirely composed of them. 



