192 MR. li. SMITH ON THE [vol. lxxiv, 



Cases in which transformation processes may have taken place, 

 and undoubtedly did take place, at or near the surface, 

 contemporaneously with the deposition of the beds and 

 while they were in a soft and wet condition, must be ruled out. 

 In these circumstances expansion due to hydration would have 

 no effect upon the underlying sediments, or upon those subse- 

 quently deposited. 



Prof. Wallace states 1 that the conditions under which natural 

 anh} r drite could be transformed into gypsum are as follows : — 



(a) at 30° C. in presence of a saturated solution of sodium chloride. 



(b) at 66° C. in presence of water. 



The figures given by Van't Hoff" are 32° C. and 63|° C. 

 respectively. 



With regard to conversion during past ages: — 



(a) In the Keuper of Cumberland, Nottinghamshire, Stafford- 

 shire, and Durham there is no convincing field evidence 3 to 

 support the theory that conversion has taken place under a thick 

 blanket of sediment. Even if the temperature were suitable and 

 a saturated solution of common salt available, it might be expected 

 to operate on beds of anhydrite in the same area in the same way ; 

 we should not expect to find, for example, one seam of anhydrite 

 converted into gypsum at both top and bottom, as at Cocklakes, 

 and another only altered at the top. as at Acorn Bank. In the 

 first case the expansion on conversion would be liable to shatter 

 the enclosed rib of anhydrite in all directions and separate it into 

 isolated fragments, instead of which we find a practically solid bed. 

 Again, the more irregular seams and isolated occurrences, where 

 the anhydrite occurs sporadically, would be easily penetrated in 

 all directions by invading solutions. In such cases we should 

 expect practically no anhydrite to be left unchanged, if any of the 

 gypsum was formed by this process. 



(/?) These arguments would apply also in the case of water at 

 the higher temperature. 



The conclusion that hydration is still going on (in the 

 Eden Yalle} r ) is founded on the following observations 3 : — 



' (i) Near the outcrop, where the circulation of water has been freer, the 

 anhydrite entirely disappears, as it does also at the edges of the bed where it 

 comes up against the clay ; (ii) following a deposit into the hill ... as the cover 

 increases, so usually does the proportion of anhydrite ; and (iii) wherever a 

 feeder of water has been found to traverse the bed, the rock has been found 

 hydrated in that neighbourhood.' 



From these observations Burns draws other conclusions, which, 

 hi my opinion, are nearer the truth, namely, that the masses of 



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

 (1914) p. 272. 



3 For certain details of these occurrences see Special Reports, vol. iii, 

 < Gypsum & Anhydrite ' Mem. Geol. Surv. 1915 ; 2nd ed. 1918. 



3 D. Burns, ' The Gypsum of the Eden Valley ' Trans. Inst. Min. Eng. 

 vol. xxv (1902-1903) p. 418. 



