194 



ME. B. SMITH ON THE 



[vol. lxxiv, 



The regularity of the beds of anhydrite at Cocklakes, Cotehill, 

 and Acorn Bank, and their fairly sharp line of demarcation from 

 the gypsum suggest original deposition. In specimens in my 

 possession the change in the upward direction from anhydrite to 

 gypsum occurs in f inch, and in the downward direction in 1^ inches. 



At Fauld the conditions are somewhat different. The anhydrite, 

 or, as it is locally termed, ' hard stone,' is met with in varying 

 quantities, and is found sometimes in one and sometimes in another 

 part of the seam, although never immediately in contact with the 

 roof or floor of the mine, and it appears and disappears without 

 any apparent reason. There seems to be no dividing line and no 

 clear cleavage between gypsum and anhydrite, the latter being 

 often intimately mixed with the best gypsum-stone. Near the 

 surface it is rare, and it appears to increase with the thickness of 

 •cover — all of which might be taken as arguments in favour of the 

 •conversion of anhydrite into gypsum. 



Fig. 7. — Section showing the occurrence of hard stone' 

 (anhydrite) at Fauld. (After T. Trafford Wynne.) 



[Scale 

 -a = seam of normal thickness 



d=fine white gypsum 



1 inch =32 feet.] 



6=thickening of seam ; c= anhydrite ; 



Where the seam is thickest (over 20 feet), however, a bed of 

 anhydrite is often found at points rising above the average height 

 of the gypsum, with a further thickness of best-quality gypsum 

 above it (fig. 7). Had the gypsum been derived from original 

 anhydrite one would have expected the unaltered cores to be 

 situated lower down and nearer the centre of the seam, not in 

 masses rising above the average level. 



These thickenings at the top of the seam are said to be usually 

 from 15 to 40 feet in diameter, more or less circular in form, 1 

 and the anhydrite in them varies from 2 inches to 3 feet in thickness. 

 They are, like the 'domes' of Nottinghamshire and other districts 

 (p. 187), welded on to the main seam ; and, since these masses of 

 anhydrite are apparently not relics of what was previously a thick 



1 T. Trafford Wynne, ' Gypsum, & its Occurrence in the Dove Valley ' 

 'Trans. Inst. Min. Eng. vol. xxxii (1906-1907) p. 180. 



