T. Davidson & G. Maw — U. Silurian Rocks of Shropshire. 103 



zone " The Tickwood Beds," which may be roughly estimated to 

 include a thickness of from 300 to 500 feet of strata. 



They are exposed in the deep road-cutting near the railway bridge 

 between Tickwood and Farley Dingle. There is also a fine natural 

 exposure 2^ miles to the east, by the side of a small stream flowing 

 down the east end of Benthall Edge, opposite Ironbridge ; and most 

 of the adjacent cutting on the Severn Valley Railway passes through 

 the base of these nodular limestones and shales. The Tickwood 

 Beds are highly fossiliferous. They contain all the five species of 

 Spirifer found in the Upper Silurians of Shropshire, with a larger 

 proportion of individuals than in any other zone. The Tickwood 

 Beds are also the highest horizon in which the new genus Glassia 

 occurs, and Orthis biloba here attains its highest limit, with the 

 exception that a few individuals occur rarely in the Wenlock Lime- 

 stone and Lower Ludlow. 



Below the fossiliferous Tickwood Beds, from 1800 to 1900 feet of 

 soft shales occur, which are comparatively barren in organic remains, 

 excepting only that at one-third from their base a remarkably rich 

 zone occurs, the horizon of which seems to correspond closely with 

 tliat 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 Eiver 

 Severn, a short distance above Buildwas Bridge, in a section in- 

 cluding 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 Uarley. 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 one of us an analysis 

 of these bands, which were found to consist of 



> = 11 '10 of Carbonate of Lime. 



"Water 13-88 



Carbonic Acid 4-88 



Lime 6-22 



Silica 45-48 



Alumina 23-52 



Protoxide of Iron 1-76 



Protoxide of Manganese 07 



Magnesia 1-44 



Potash 2-15 



Soda 0-54 



99-94 



and remarked on the smallness of the per-centage 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 link 



