196 MR. B. SMITH ON THE [vol. lxxiv,. 



The plates of gypsum evidently crystallized later than the 

 anhydrite, and these have obviously not been derived from it by 

 hydration, but crystallized out from the mother liquor. The 

 dolomite probably came out simultaneously with the anhydrite. 



(ii) This association of dolomite, anhydrite, and gypsum occurs, 

 frequently in the Permian rocks east of the Pennines. Thin slices 

 of Lower Magnesia n Limestone from a deep boring in Lincolnshire 

 show granular dolomite with plates and patches of anhydrite, in a 

 ground-mass of gypsum, and with occasional parallel intergrowths 

 of anhydrite and gypsum. The whole is suggestive of chemical 

 precipitation and assemblage of the dolomite rhombs as a sediment, 

 with nearly contemporaneous crystallization of the anhydrite and 

 gypsum. 



With regard to this dolomite-anhydrite, or dolomite -anhydrite - 

 gypsum phase, we may refer to Dr. A. Wade's conclusions l as to 

 the conditions of deposition of a similar series in the Eastern Desert 

 of Egypt, namely, that it is an original deposit, occurring on the- 

 edge of an enclosed basin, in the centre of which rock-salt is the 

 most important deposit. 



(iii) A specimen of the contact collected by Dr. P. L. Sherlock,, 

 from a loose block at G-otham, consisted of bundles of crystals of 

 anhydrite set in a matrix of gypsum, but passed rapidly into a 

 rock in which anhydrite was predominant, the anhydrite being- 

 merely veined by gypsum. It is evident that the gypsum is of 

 later formation than the anhydrite, while the latter is bounded 

 by sharp outlines determined by cleavage-planes, and furnishes 

 little evidence of alteration, although gypsum forms an infilling. 



(iv) A slice of the anhydrite from Fauld, proved to be similar- 

 to that from Gotham, and the cracks formed in course of pre- 

 paration of the slide are similar, as regards their boundaries, to the 

 gypsum veins in the Gotham anhydrite. 



(v) Examination of the gypsum-anhydrite contact at Cocklakes r 

 however, discloses clear evidence of a conversion having occurred. 

 The anhydrite occurs here between two beds of gypsum. 



A. At the contact between the top of the anhydrite and the 

 overlying seam of gypsum a change is seen, as at Gotham (iii),. 

 from gypsum at one extremity to anhydrite at the other, with an 

 intermediate zone consisting partly of the one mineral and partly 

 of the other (PI. XVIII, fig. 1). The anhydrite contains gypsum- 

 filled veins, which are arranged more or less parallel to the upper- 

 surface of the bed, become gradually smaller as they occur deeper 

 in the body of the anhydrite, and finally taper off in thin threads. 

 On the other hand, the isolated fragments of anhydrite, set in 

 the gypsum mosaic, become progressively smaller as their distance 

 from the main body of anhydrite increases. 



The bulk of the gypsum is a mosaic of crystals averaging 1 1 mm. 



1 ' Some Observations on the Eastern Desert of Egypt ' Q. J. G. S. vol. lxvii 

 (1911) p. 257. See also Sir Archibald Geikie, ' Text-Book of Geology ' 4th ed. 

 vol. i (1903) p. 530. 



