664 THE WONDERS OF GEOLOGY. Lect. VIJ. 



are seen above in an unconformable position. The Mountain 

 limestone and Millstone grit (1, 2) also appear on the opposite 

 flank of the elevated ridge of the Mendips. It will be in- 

 structive to enumerate the deposits exhibited in this section, 

 in chronological order; that is, in their original position 

 before they had suffered displacement. Commencing with the 

 lowermost or most ancient, the Devonian strata of the Mendip 

 Hills, we have — 1. Mountain limestone. — 2. Millstone grit ; 

 upon this are alternations of coal and shale, with Pennant- 

 grit. — 3. New Red-sandstone. — 4. Lias. — 5. Inferior Oolite. 

 — 6. Great Oolite. — 7. Oxford Clay, south of Malmsbury. 



The term basin, applied to these accumulations of car- 

 boniferous strata, must be taken in a general sense ; for 

 though some of these groups of deposits may have been 

 formed in circumscribed depressions, it is evident that, in 

 general, the beds have extended over large areas, and that 

 their present isolated and confined limits are attributable 

 to subsequent elevations and depressions of the rocks on 

 which they repose, and by which the faults and dislocations 

 of the coal and associated strata have been produced. 



4. Coal-field of Derbyshire. — The Derbyshire Coal- 

 field will serve as a type of the English series. 



The strata of carboniferous limestone which form the 

 grand mountain-chains of Derbyshire, decline towards the 

 eastern side of the county, and sink beneath the coah mea- 

 sures. Immediately upon the limestone is placed a bed of 

 calcareous slate or shale, varying in thickness from three 

 to six hundred feet. The compact strata are separated by 

 coarse layers, which readily disintegrate, and these form 

 the exposed face of Mam Tor, or the " shivering mountain" 

 near Castleton. They are succeeded by a mass of grit, or 

 conglomerate, with vegetable remains, which is worked for 

 mill-stones. Above the millstone-grit are the regular coal 

 strata, comprising sandstones of various qualities, and often 

 in exceedingly thin Laminae ; indurated clays : iron-stones. 



