190 ME. B. SMITH ON THE [vol. lxxiv,, 



VI. ' Volcanic ' Theojry. 



Another theory has been advanced by various writers, namely,, 

 that the beds were originally limestones which have been converted 

 into gypsum by sulphureous waters, either directly on the sea-bed 

 or under a certain amount of cover. This explanation was offered 

 by Burns to account for the deposits of the Eden Valley in 

 particular, 1 assuming that the beds were covered by little or no 

 sediment at the time of alteration. 



The sulphureous waters are usually assumed to be due to volcanic 

 (solfataric) action ; yet among the British Mesozoic rocks there 

 is no evidence of such action, and there is very little in the rocks- 

 associated with the gypsiferous deposits of the Permian Era. 



If altered, the limestones must have been fairly pure to begin 

 with, but few of the known Permian and Triassic limestones ever 

 are pure, while those associated with the Sussex gypsum are impure 

 In the latter case it is hard to imagine, from the character of the 

 seam, how a pure limestone could be converted into gypsum in 

 so irregular a manner. At Kirkby Thore the structure of the 

 gypsum precludes this suggested mode of origin. 



Gypsum and anhydrite certainly occur in association with the 

 Magnesian Limestone ; but sliced specimens (p. 196) seem to show 

 that the whole deposit was original. 



If any limestones had been altered under cover, at a date long- 

 subsequent to their deposition, there would surely have been found 

 intermediate stages of the process. 



Dr. A. Wade describes how shells, or raised coral-reefs, become 

 converted into gypsum in the Eastern Desert of Egypt, 3 but con- 

 siders it due partly to capillary action, and partly to a metasomatic 

 replacement of the calcium carbonate, in which the shells are 

 destroyed. In such cases intermediate stages are found, and 

 volcanic action is not suggested. 



It would appear that, if I have rightly interpreted the origin 

 of the Chellaston Breccia, most of the important deposits of 

 calcium sulphate in this country must be regarded as original and 

 formed contemporaneously with the sediments in which they now 

 occur. An explanation is still required for the irregularity of some 

 of the seams and the segregation of the purer masses in pots, balls, 

 or domes. Yet, whatever the cause, which is a problem that 

 more immediately concerns the chemist, the facts of deposition 

 hold good. There seems to be a general tendency for 

 mineral salts to collect in lenticular masses, either bi- 

 convex or plano-convex, when the convexity may be above or 

 below. 



1 ' The Gypsum of the Eden Valley ' Trans. Inst. Min. Eng. vol. xxv 

 (1902-1903) pp. 410-34. 



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

 (1911) p. 253. 



