THE SILURIAN SYSTEM. 



675 



locality it appears continuously in the longitudinal valley reaching from the Severn to 

 the Onny, running in a line parallel to the ridge called Wenlock Edge, and intermediate 

 between it and the Caradoc hills. The lower part of the shale contains concretions, 

 which exhibit, upon being broken, an internal structure similar to the " cone-in-cone," 

 common to the shale of the lias, the cones consisting of dark-coloured crystalline car- 

 bonate of lime in an argillaceous paste. The upper part, where exposed to the action 

 of the atmosphere, has been deeply denuded. The Wenlock limestone is seen con- 

 spicuously in the Edge, which extends a course of about twenty miles, and forms one 

 of the most remarkable features of the physical geography of Shropshire. The lime- 



Old Lincoln Quarry, near Iron Bridge. 



stone is disposed in two forms. It occurs in regular beds, termed "measures," of 

 varying thickness, dull grey colour, more or less impure through the presence of argil- 

 laceous matter, the strata being often wavy and contorted. It occurs, also, in large 



concretional masses, where 

 it is purer and more crys- 

 talline, (locally, they are 

 called ball-stones,) which 

 are sometimes dark blue, 

 occasionally pink, freckled 

 with veins and strings of 

 white crystalline carbonate 

 of lime. The ball-stones 

 are frequently of immense 

 size, having a diameter of 

 thirty feet, and, in one 

 case, a single mass has 

 been quarried to the depth 

 of eighty feet without its 

 dimensions being ascertained. The annexed view of one of the quarries near Iron 

 Bridge, shows the undulated and contorted beds of impure limestone, and the places 

 from whence the crystalline variety, the concretions or ball-stones, have been ex- 

 tracted. The Wenlock shale, as a bed of passage, between the Upper and Lower 

 Silurians, contains shells, corals, and trilobites, common to both ; but the limestone is 



XX 2 



1. Euomphalus sculptus. 



2. Euomphalus t'unatus. 



2. Euomphalus rugosus. 

 4. Euomphalus discors. 



