WENLOCK LIMESTONE. 



209 



This formation differs from the rocks above and below it, in containing thick masses 

 of a crystalline and subcrystalline limestone, highly charged with corals and encrinites. 

 In parts, indeed, the latter are so abundant, that the rock on a superficial glance might 

 be mistaken for the mountain or carboniferous limestone; an examination, however, 

 shows that the crinoidal remains, and the other fossils contained in the Wenlock lime- 

 stone, are distinct from those of the Carboniferous System. Further, the strata are 

 constantly arranged in concretionary masses, which are separated from each other by a 

 vast predominance of argillaceous matter, an arrangement rarely, if ever, perceptible 

 in the limestones of the carboniferous series. 



The formation is naturally separable into two divisions, the upper calcareous, the lower 

 highly argillaceous, with a few calcareous concretions. The first of these occupies the 

 summit and slopes of the Wenlock Edge, the second appears in the escarpment and is 

 spread out on the north-west in a longitudinal valley running parallel to the Edge. Let 

 us first examine the upper or calcareous zone in the neighbourhood of Wenlock, where 

 it is most expanded, and then follow it in its prolongation to the north-east and south- 

 west to the banks of the Lugg. 



Wenlock Limestone. (<?. of wood-cut p. 196. and PI. 31. figs. 2, 3, 4, and 5.) 



The chief calcareous strata of this formation are overlaid and underlaid by a number 

 of small concretionary nodules of grey argillaceous limestone, running in layers and 

 held together by a matrix of shale, which weathers to an ashen or yellowish green co- 

 lour, whilst in some places the nodules unite and form irregularly thin-bedded, lenticular 

 limestones. Both these varieties are exposed in beds above and below the principal 

 masses of limestone in many quarries on the summit and south-eastern slopes of Wen- 

 lock Edge, and by the sides of the roads which traverse that ridge. In the Gleedon 

 Hills, north of Wenlock, where the calcareous zone is most expanded, the strata undu- 

 late, and thick beds of impure concretionary limestone are exposed in the central and 

 more solid masses of the rock. 



To study the connexion of this upper subdivision of the Wenlock limestone with the lower 

 Ludlow rock, we must follow the south-eastern face of the Wenlock Edge till it is flanked by the 

 parallel chain of hills of the Ludlow rocks, as expressed in the opposite wood-cut, and we then per- 

 ceive, that for many miles, the highest beds of the limestone rise up conformably, and for the most 

 part at small angles of inclination, from beneath the lower shale of the overlying formation. At 

 Presthope, courses of lenticular limestone, made up of irregularly flattened concretions and alter- 

 nating with light- coloured shale, rise from beneath the lower Ludlow rocks, and pass down into a 



the Wenlock limestone to be at least 1500 feet below the base of the Old Red Sandstone. (See section, PI. 31. 

 fig. 2, 3, and 4.) Whilst, therefore, the type of the formation, like that of those above and below it, is derived 

 from examples in Shropshire, any peculiarities in these rocks at Dudley and other places will be mentioned in 

 separate and subsequent descriptions. 



