GEOLOGY OF DERBYSHIRE. 231 



chert, six or seven feet thick, is quarried to be sent to the china 

 manufactories in Staffordshire. Chert, however, most usually 

 occurs in nodules and irregular masses distributed in the beds of 

 limestones or in thin plates in the interstices between them, and in 

 these conditions precisely resembles the flints in the chalk forma- 

 tion, of which the geologist is instantly and most strikingly remind- 

 ed by many small cliffs in the upper part of the mountain limestone. 

 If the chalk were indurated to the texture of the mountain lime- 

 stone, no one could perceive the difference, except from the organic 

 remains.* The thickness of the mountain limestone is certainly 

 very great, but as the base of the formation is nowhere exposed in 

 Derbyshire, notwithstanding the deep valleys and dales which have 

 been scooped out of it, its entire thickness is unknown. Neither is 

 it at present ascertained what is the thickness of those parts which 

 are exposed; and this point is one beset with many difficulties. It 

 is obvious that where we get a thick formation composed of many 

 beds of the same material, and there is no section or pit deep 

 enough to ascertain its thickness at any one point, but we are 

 obliged to measure the different beds at many different places 

 wherever we can find them exposed, the operation is one not only 

 of great labour and difficulty, but liable to many errors. If all the 

 beds were limestone, with no other material between them, it would 

 be next to impossible to ascertain the whole thickness correctly. 

 This, however, is luckily not the case, since thin partings of clay 

 are sometimes found, seldom more than a few inches in thickness, 

 and sometimes only half an inch ; but occurring apparently be- 

 tween the same beds with considerable regularity over tolerably 

 wide areas. These partings will probably be of great use eventu- 

 ally in working out the geology of the limestone. But the most 

 important division of the mountain limestone is produced by the 

 presence in it of a rock entirely different, not only from the lime- 

 stone itself, but from all the others which we have hitherto met 

 with. This is, 



" This fact, and that of a tertiary limestone in the south of France con- 

 taining nodules of flint similar to the chalk, (as described by Mr. Lyell), are 

 remarkable, as showing us how accurately the same effects have been pro- 

 duced by Nature, when working under similar conditions, in aeras so remote 

 from each other as those of the production of the mountain limestone, the 

 chalk, and the tertiary strata. From the similarity of the effects we may 

 argue directly to the identity of the cause. 



