THICKNESS OF COAL STRATA. 



103 



far as I have examined, the remains of vegetables exclusively, but 

 no beds of workable coal occur in it. Where the strata crop or 

 basset out, this rock forms abrupt and picturesque cliffs. Above the 

 grit, are laid the regular series of coal-measures or strata, comprising 

 sandstone of various qualities, indurated clay called clunch, ironstone,, 

 softer argillaceous beds called bind, and schistose argillaceous beds, 

 called shale. There are also two argillaceous strata containing nu- 

 merous shells allied to fresh-water muscles, and hence called Mus- 

 cle-bind. 



A gentleman extensively engaged in the working of coal mines in 

 this district, had an approximate measure taken of the thickness of 

 the different beds, which he sent me, and it was published in the first ' 

 edition of this work ; from which " it appears, that the total depth 

 taken on the level line of the measure of the whole Derbyshire strata, 

 including part of Nottinghamshire, is thirteen hundred and ten yards, 

 in which are thirty different beds of coal, varying in thickness from 

 six inches to eleven feet, making the total thickness of coal twenty 

 six yards : of course the above estimate can be regarded only as an 

 approximation to truth, since the thickness of the strata was taken 

 upon a level line, and not perpendicular to the line of their inclina- 

 tion or dip." Making an allowance for excess in the above measure- * 

 m,ent, the true thickness of the strata may fairly be estimated at about 

 two thousand five hundred feet. 



What is particularly deserving of notice in the bed of limestone- 

 shale before mentioned, below the coal-measures, and above the 

 mountain limestone, is, that this bed presents a transition from marine 

 calcareous strata with animal remains, to fresh-water strata with ter- 

 restrial vegetables : as both occur in different parts of the bed, it 

 would imply, that the subjacent limestone had been gradually but 

 unequally raised above the sea, and during its elevation some parts 

 remained immersed in the ocean, while other parts were covered 

 with vegetable depositions. In the western side of Durham and 

 Northumberland, the alternations of coal of inferior quality, with beds 

 of mountain Hmestone, are more distinct, and the transition from ma- 

 rine to fresh-water formations on a larger scale : both prove that the 

 elevation of the beds above the sea was effected by the operation of 

 an elevating force acting slowly, or at distant intervals, — a subject 

 which it is proposed to advert to in another part of the volume. 



Coal-fields, as before stated, are of limited extent, and the strata 

 frequently dip to a common centre, being often arranged in basin- 

 shaped concavities, which appear to have been originally detached 

 lakes, that were gradually filled by repeated depositions of carbon- 

 aceous and mineral matter. In some of the larger coal-fields, the 

 original form of the lake cannot be traced, but in the smaller ones 

 it is distinctly preserved. 



The different strata under a bed of coal are frequently similar to 

 the strata over it ; and the same series is again repeated, in some 



