part 3] CHELLA.STON" Oi'PSUM BRECCIA. 193 



pure gypsum are most easily attacked by water and denuded, so 

 that the gypsum surrounding the anhydrite areas is on the edges 

 of the areas of pure gypsum that have been removed. He also 

 points out that it is difficult to see how a ball of anhydrite, in the 

 midst of gypsum, could be converted into gypsum without bursting 

 the surrounding rock. He finally concludes that the central parts, 

 where they are now anhydrite, have always been anhydrite. 



On the north-western boundary of Cocklakes Mine the seam is 

 faulted parallel to a line along which the anhydrite thins out, and 

 the upper and lower beds of gypsum coalesce. The writer inter- 

 preted this as implying that the fault occurred along a line of 

 weakness where the strengthening rib of anhydrite was absent. 



With regard to the action of subsoil water, we may observe 

 that at Acorn Bank, where there is no underlying gypsum, the 

 anhydrite has been as freely exposed at the bottom as at the top 

 to the action of percolating water for countless years. Again, 

 at Cocklakes the open joints, or ' backs," in the anhydrite are 

 merely faced by 1 or 2 inches of gvpsum, which occurs in thin 

 layers like stalagmite, the anhydrite being apparently unaffected. 

 The conversion of the upper and lower layers of a bed of anhydrite 

 into gypsum would give rise to the shattering of the remainder of 

 the anhydrite, but this has not happened. 



At Faukl, also, the anhydrite is apparently unaffected, and it is 

 found where quantities of water are met with. 



Lumps of anhydrite left in the open doubtless become covered 

 with a film of gypsum in a few years, but this is probably due to 

 actual solution and redeposition rather than replacement. 



Although we have no records of the conversion having taken 

 place in this country, cases have been met with and reported on 

 elsewhere. 1 At least one case is known in Europe, in which a 

 tunnel was driven through a deposit of anhydrite and thrown out 

 of alignment by the swelling of the material when changed into 

 gypsum, the alteration being brought about by trickling water. 2 



(3) Prof. Wallace concludes that, while secondary transforma- 

 tions are possible at considerable depths, and also at the surface, 

 the character of the gypsum-anhydrite deposits of North America 

 may be accounted for most directly, and with least difficulty, as 

 due to original deposition. 3 British occurrences seem to call for a 

 similar generalization. 



Since, in the first place, there are no indications that the gypsum 

 of the Chellaston pots, or the gypsum of the Kirkby-Thore quarry, 

 was ever anhydrite, we might argue that there is no reason why 

 the gypsum associated with anhydrite in the equivalent seams 

 should ever have been in the anhydrous state. 



1 Sir Archibald Geikie, ' Text-Book of Geology ' 4th ed. vol. i (1903) pp. 400 

 & 453. 



2 H. Ries & T. L. Watson, 'Engineering Geology ' 2nd ed. (1916) p. 114. 



3 ' Gypsum & Anhydrite in Genetic Relationship ' Geol. Mag. dec. 6, vol. i 

 1914) p. 276. 



