90 tJPPEB TRANSITION OB MOUNTAIN LIMESTONE. 



Mountain limestone is one of the most important calcareous roeks 

 in England and Wales, both from its extent, the thickness and num- 

 ber of its beds, the quantity and variety of its organic remains, and 

 its richness in metallic ores, particularly of lead. In Derbyshire, 

 where the different beds of limestone have been pierced through by 

 the miners, the average thickness of the three uppermost is about 

 160 yards ; the beds are separated by beds of trap or basalt, resem- 

 bling ancient lavas. The lowest limestone has not been pierced 

 through. In the northern part of Yorkshire, and in Westmoreland 

 and Cumberland, the beds of mountain limestone alternate with beds 

 of greywacke-slate, and of coarse sandstone. In North Wales, and 

 in Somersetshire, mountain limestone forms entire mountain masses, 

 of vast thickness, distinctly stratified ; the strata often varying in col- 

 our, and sometimes in the nature of their organic remains. 



The beds of mountain limestone in England and Wales vary much 

 in colour and quality. The colour is most commonly light grey, but 

 it is sometimes black and sometimes a reddish brown, or is varie- 

 gated. The limestone is generally sufficiently hard to receive a high 

 polish, and forms what is denominated marble, of considerable beau- 

 ty. The texture is more or less crystalline. — The prevailing chaF- 

 acteristic organic fossils are encrinites and madrepores. The upper 

 beds of mountain limestone in Derbyshire appear to be composed al- 

 most entirely of encrinites. A bed of black limestone with madre- 

 pores occurs in Westmoreland ; it is more rare in Derbyshire, but is 

 found in the lower part of the mountain limestone in North Wales, 

 and Shropshire, and also in Devonshire. It takes a beautiful polish, 

 and is much used for chimney-pieces. The black colour appears to 

 be derived from bitumen, for it is injured by heat, and is entirely ex- 

 pelled by burning. Mountain limestone is generally a nearly pure 

 carbonate of lime ; but some beds, and even entire hills of this lime- 

 stone, contain a large portion of magnesia, like the dolomite of the 

 Alps. The mountain magnesian limestone of England is generally 

 harder than the common limestone, and has frequently a reddish brown 

 colour. Bredon Hill, in Leicestershire, and Cloud's Hill, in its vi- 

 cinity, are composed entirely of magnesian limestone ; there are 

 several beds of similar limestone which form low hills in the adjacent 

 country : they may all be regarded as an extension of the Derby- 

 shire mountain limestone, ranging southward towards Charnwood 

 Forest, and terminating at Grace Dieu, where the limestone is nearly 

 in contact with the granitic and porphyritic rocks. I say these may 

 be regarded as an extension of the Derbyshire mountain limestone, 

 though the continuity is partly concealed by a covering of the red 

 marie, and by coal measures : the limestone contains the same char- 

 acteristic fossils as the Derbyshire limestone, particularly encrinites 

 (screw stones), and the euomphalus ; but these are not abundant. 

 The strata of Bredon Hill and Cloud's Hill are much exposed, hav- 

 ing been extensively quarried for lime during a long period;; they 



