550 



SUBDIVISIONS OF SILURIAN ROCKS. 



[Oh. XXVII, 



gether by argillaceous matter. Far too much importance has been 

 attached to this kind of rock, as if it belonged to a certain epoch in 

 the earth's history, whereas a similar sandstone or grit is found in 

 the Old Red, and in the Millstone Grit of the Coal, and sometimes 

 in certain Cretaceous and even Eocene formations in the Alps. 



The annexed table will explain to the reader the successive forma.- 

 tions into which the strata called Silurian by Sir Roderick Murchison 

 may be subdivided : 



UPPER SILURIAN ROCKS. 



1. Ludlow Formation. 



Upper 

 Ludlow. 



Lower 

 Ludlow. 



Prevailing Lithological Characters. 



a. Dovmton Sandstone. — Fine- 

 grained yellowish sandstones 

 and hard reddish grits ; at 

 the base a "bone-bed" with 

 fish remains. 



I b. Micaceous gray sandstone and 

 [_ mudstone. 



' a. Aymestry Limestone. — Argil- 

 laceous limestone. 



b. Lower Ludlow Shale. — Shale 

 with calcareous concretions, 

 often of large size. 



Thickness 

 in feet. 



80 



700 



50 



1000 



Organic Eemains. 



Marine Mollusca of almost 

 every order, the Brachio- 

 poda most abujidant ; 

 Annelides, Crino.des,and 

 corals; Placoid and Ga- 

 noid fish (oldest remains 

 of fish yet known); a few 

 Graptolites ; Crustacea 

 of the Eurypterid or^er ; 

 Seaweeds. 



2. Wenlock Formation. 



Upper 

 Wenlock. 



Lower 

 Wenlock. 



Wenlock Limestone. — Concre- 

 tionary and thick-bedded lime- 

 stone. 



a. Wenlock Shale.— Argillaceous 

 shale, frequently flagstone. 



b. Woolhope Limestone and Den- 

 bighshire Grit. — Argillaceous 

 limestone and shale, some- 

 times replaced by felspathic 

 sandstones and grit. 



Above J 

 h 3000 S 



Marine Mollusca and Ba- 

 diata ; Crustaceans of the 

 Triiobite and Eurypte- 

 rid orders; Graptolites 

 abundant. 



MIDDLE SILURIAN ROCKS. 



Llandovery Formation. 



Upper 

 Llandovery. 



Lower 

 Llandovery. 



a. Tarannon Shale. — Purple or 

 pale-colored shales. 



b. May-Hill Sandstone and Pen- 

 tamerus Limestone. — Nodular 

 limestone and dark shale ; cal- . 

 careous sandstone, with un- j 

 derlying coarse grits, often 

 red-colored. J 



Llandovery Slates. — Hard, sand- ) 

 stone and slate, frequently V 

 with conglomerate beds. ) 



1000 



800 



600 



to 



1000 



Crinoidea and corals very 

 abundant ; "Cystideae ; 

 Mollusca, chiefly Bra- 

 chiopoda ; Pentam.eris 

 Icevis being characteris- 

 tic of the limestones. 



