BRITISH SILURIAN ERACHIOPODA. 69 



are depauperated and smaller than in the higher and lower zones, where they are more 

 abundant, indicating a condition (perhaps increased depth of water), prejudicial to life and 

 multiplication, excepting only that at one third from their base a remarkably rich zone 

 occurs, the horizon of which seems to correspond closely with that of the Woolhope 

 Limestone of Herefordshire, and possibly of the Barr Limestone of Staffordshire, though 

 in Shropshire the calcareous element is wanting. It is exposed on the east bank of the 

 River Severn, a short distance above Buildwas Bridge, in a section including from 70 to 

 80 feet of shale beds, which we propose to call ' The Buildwas Beds.' They are also 

 exposed further to the west by the side of the brook south of Harley. Just above the 

 fossiliferous zone of the Buildwas Beds, the monotonous ' Mudstone ' character of the 

 Wenlock Shale is broken by the occurrence of a few thin bands of a remarkable cream- 

 coloured clay, resembling steatite in texture. The late Mr. David Forbes made for me 

 an analysis of these bands, which were found to consist of — 



= 11 10 of Carbonate of Lime, 



Water 13-88 



Carbonic Acid 488 



Lime 622 



Silica 45-48 



Alumina 23-52 



Protoxide of Iron 1-76 



Protoxide of Manganese , , 0'07 



Magnesia 1*44 



Potash 2-15 



Soda 0-54 



99-94 



and remarked on the smallness of the percentage of magnesia in the mineral, which so 

 closely resembles compounds which, from their unctuous feel and external characters, are 

 usually considered to be highly magnesian. 



" The pale colour of these bands is evidently due to the occurrence of the iron in a 

 state of protoxide, which may perhaps have resulted from the presence of the deoxidising 

 agency of organic matter. 



" If we place the Tickwood Beds as forming a connecting hnk between the Wenlock 

 Limestone and the Wenlock Shale, the remainder of the shale may be subdivided as 

 follows : 



Barren shales of Coalbrook Dale and Ape Dale, or " Coalbrook Dale 



Beds" 1100 to 1200 feet. 



Yossiliterous zone q{ Buildva-s, or " Buildwas Beds" 80 to 100 „ 



Barren shales of Buildwas Park, or " ^asewze?!^ J8ec?5 " 500 to 600 „ 



" The barren ' Basement Shales ' in the railway-cutting below Buildwas Park are red 



