210 WENLOCK LIMESTONE (CONCRETIONARY STRUCTURE OF). 



hard, blue, subcrystalline, thin- bedded, limestone. Some of the flattened nodules are concreted 

 around nuclei of black chert. 



Near Easthope the uppermost strata put on a different lithological aspect, becoming a marble of 

 red and dull green colours, which has been a little worked for ornamental purposes. The red- 

 coloured portion is composed of broken stems of encrinites and small corals ; and the dull green 

 colour is due to an intermixture of very argillaceous limestone. Beds of this description are ex- 

 ceptions to the prevalent concretionary characters of the upper portion of the limestone which are 

 persistent over considerable areas. In those situations, however, where the formation is little de- 

 veloped, the thicker and central limestone having thinned out, it is only by such nodules and their 

 associated fossils, that the place of this formation can be traced. 



The central or chief calcareous masses are of very irregular thickness and dissimilar 

 appearance. In certain quarries they consist of regular beds of argillaceous limestone, 

 separated by way-boards of shale, which enters into all the crevices and depressions of 

 the limestone; and occasionally is even more abundant than the calcareous matter. 

 This earthy aspect, and the prevailing dingy ashen or greenish yellow to which the shale 

 weathers, is one of the most striking external characters of this rock. In other quarries 

 all traces of bedding are wanting, and the whole calcareous mass is made up of con- 

 cretions, sometimes of immense size, surrounded by beds of shale and impure limestone. 

 Examples, both of regular bedding and concretionary structure, are frequently exhibited 

 in the same locality. When the limestone lies in regular beds it is generally more or 

 less impure and mixed with argillaceous matter, but when arranged in concretions it is 

 purer and more crystalline. These large concretions are called " Ball-stones " by the 

 workmen, to distinguish them from the common beds, which they term "measures." 



25. 



This wood-cut represents the order of the beds to the north of Wenlock. a Upper beds of lenticular limestone with 

 nodules. b Chief mass of limestone with ball-stones. c Shale with small concretions. 



The colour of the rock is usually dull grey, but the crystalline varieties are sometimes 

 dark blue, and more rarely pink, the mass being freckled with veins and strings of white 

 crystallized carbonate of lime. The same number of calcareous beds is not continuous 

 over a large area. At Lincoln Hill, for example, where the limestone in the chief 

 quarries is inclined at about 55° and 60°, we have the upper and lower suite of small 

 concretionary beds, and the central system of purer thick-bedded limestone, having a 

 thickness of about forty or fifty feet, but the best beds not exceeding in amount twenty 

 to twenty -five feet. In these the workmen distinguish an upper and a lower limestone. 

 These masses present the edges of distinct beds at the escarpment, but on descending 

 into the subterranean quarries the lines of separation often disappear, and the whole of 

 the calcareous matter unites at such points into large concretionary lumps or ' ' Ball- 



