104 Rev. A. Sedgwick on the Geological Relations and 



The mineralo"-ical character of these beds is much more constant than in 

 the lower parts of the formation : there are, however, many variations of 

 structure, some of which must be enumerated. In some quarries (for example, 

 at Carlton near Worksop) the separate beds cannot be distinguished by 

 colour or composition from the inferior deposit of yellow limestone. They 

 are not unfrequently cellular ; and we may, here and there, find traces of a 

 concretionary and globular structure. Such forms are, however, imperfectly 

 developed; partly perhaps in consequence of the separating marls, which keep 

 the thin beds distinct from each other, and prevent any modification of struc- 

 ture from operating on the whole mass. 



The prevailing colour of the series is grey, varying from ash-grey to dark 

 smoke-grey ; and the several shades are not unusually exhibited in stripes or 

 cloudy spots ; and among beds possessing such colours we may find portions 

 of a red, brown, or bluish tinge. It also deserves remark, that the colours of 

 the rock are much affected by the thin bands of marl, which not merely coat 

 over the surface, but partially penetrate the component strata. In quarries, 

 however, exhibiting the most ordinary structure and colour, we may occa- 

 sionally find beds of hard, bright yellow, subcrystalline dolomite. 



Some of the thin beds have a dull, earthy fracture, and must be regarded as 

 varieties of marl-slate. In general they are considerably indurated, and gra- 

 dually pass into hard and nearly compact limestone. Of the harder varieties 

 which have a dull surface, some are porous and almost granular ; some nearly 

 compact, with a rather uneven fracture ; others perfectly compact, with a 

 beautifully smooth conchoidal fracture ; the several modifications being mixed 

 together, replacing each other, or alternating in thin stripes. 



Associated with the preceding, and without any visible order, are other 

 beds which are nearly compact without the smooth conchoidal fracture, and 

 which, when examined with a lens, show a glimmering lustre. These pass 

 through many shades into a hard, small-grained, crystalline rock. All the spe- 

 cimens, which are both granular and crystalline, are, I believe, dolomites : but 

 of the other varieties, some do, and some do not, contain magnesia, the com- 

 position in this respect being apparently dependent upon circumstances 

 merely accidental or local. 



As some of the harder beds in this deposit form a good material for the 

 road, and as those parts of it which contain little magnesia are burnt and ex- 

 tensively used in agriculture, large quarries have been opened in it (nearly 

 along the line of the great North-road) which exhibit in great perfection all 

 its modifications. One or two of these localities may be briefly noticed. 



