The Quarterly journal of the Geological Society of London . sharp line of demarcation fromthe gypsum suggest original deposition. In specimens in mypossession the change in the upward direction from anhydrite togypsum 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 varyingquantities, and is found sometimes in one and sometimes in anotherpart of the seam, although never immediately in contact with theroof or floor of the mine, and it appears and disappears withouta
The Quarterly journal of the Geological Society of London . sharp line of demarcation fromthe gypsum suggest original deposition. In specimens in mypossession the change in the upward direction from anhydrite togypsum 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 varyingquantities, and is found sometimes in one and sometimes in anotherpart of the seam, although never immediately in contact with theroof or floor of the mine, and it appears and disappears withoutany apparent reason. There seems to be no dividing line and noclear cleavage between gypsum and anhydrite, the latter beingoften intimately mixed with the best gypsum-stone. Near thesurface 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.). [Scalea—seam of normal thickness d=fine white gypsum 1 inch=32 feet.] b—thickening of seam ; c=anhydrite ; Where the seam is thickest (over 20 feet), however, a bed ofanhydrite is often found at points rising above the average heightof the gypsum, with a further thickness of best-quality gypsumabove it (fig. 7). Had the gypsum been derived from originalanhydrite one would have expected the unaltered cores to besituated lower down and nearer the centre of the seam, not inmasses rising above the average level. These thickenings at the top of the seam are said to be usuallyfrom 15 to 40 feet in diameter, more or less circular in form,1and the anhydrite in them varies from 2 inches to 3 feet in are, like the domes of Nottinghamshire and other districts(p. 187), welded on to the main seam ; and, since these masses ofanhydrite are apparently not relics of what was previously a thick 1 T. Trafford Wynne, Gypsum, &
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