02 



MOUNTAIN LIMESTONE. 



tion. If the unstratified masses at Cloud's Hill owe their form to the 

 action of heat, it is difficult, if not impossible, to conceive how this 

 heat could have changed internal portions of the limestone, without 

 affecting the surrounding strata. In Devonshire, and elsewhere, hills 

 of mountain limestone may be seen in which the stratification of the 

 entire mass is obliterated or nearly so ; but there can be no difficulty 

 in this case, — indeed, it may be said that we do not know that the 

 limestone was ever stratified, though the probabilities are greatly in 

 favour of its having been so. 



Remarkable sudden changes may be sometimes observed in the 

 quality of the same beds of mountain limestone. At Llanymynah^ 

 in Shropshire, (a hill composed of this limestone,) the quality of the 

 limestone, on one side of the hill, is considered by the lime-burners 

 of the very best kind ; while, at a little distance, the same strata are 

 so impure, from an intermixture with sand and clay, that they cannot 

 be used with advantage : but what is more remarkable, I have seen, 

 in this hill, a stratum of the best limestone, lying regularly between 

 other strata, suddenly terminate, and a whitish calcareous marie oc- 

 cupy hs place, preserving the same degree of thickness, and the 

 sanie direction. As these strata contain marine organic remains, 

 and were deposited at the bottom of the ocean, we may suppose that 

 a submarine current had prevented the limestone from extending 

 further, and supplied its place by a deposition of clay, before the stra- 

 tum above was deposited. In the former case, where the strata of 

 good limestone become, in some parts, calcareous and impure, we 

 may suppose that submarine currents, carrying away particles of sand, 

 had intermixed them with the calcareous depositions in one part, but 

 not in another. Indeed, this sudden change in the quality of the 

 limestone is so common in that part of Wales, that the workmen 

 have given to it the expressive name of Balkslone. When I was 

 first informed of the balkstone, and saw that it impeded the opera- 

 tions of the quarrymen, I expected to have found a dyke of basalt, 

 and was surprised to observe a mass of stratified limestone, of an 

 impure quality, cutting througli the best limestone like a thick wall 

 and left standing, the good limestone being worked away on each side 

 of it. This wall of limestone is of a darker colour than the rest ; it 

 contained the remains of the encrinites. It is owing, I conceive, to 

 the irregularities in the deposition of the strata, from causes attend- 

 ing their original formation, that soft and irregular beds or masses of 

 clay occur in mountain limestone, which have subsequently been 

 washed out by subterranean currents of water, and formed excava- 

 tions and caverns of considerable magnitude. Many instances might 

 be cited of large streams, and even rivers, engulfed in mountain 

 hmestone, and rising again at the distance of several miles. In the 

 northern counties these openings are called Swallow Holes. Mr. 

 Farey has enumerated twenty-eight swallow holes in the mountain 

 limestone of Derbyshire. 



