part 3] chellaston gtpsum: BRECCIA. 191 



VII. ANHYDRITE IN ITS RELATION TO Gl'PSUM. 



(a) Field Evidence. 



In cases where anhydrite is associated with gypsum it is 

 interesting to determine, if possible, whether the present asso- 

 ciation is original, or whether conversions from the one to the other 

 have taken place since the time of deposition. The subject is 

 difficult and full of pitfalls ; but I venture to discuss it because 

 it seems to be inseparable from the subject of this paper, and also 

 in the hope that it may lead to further light being thrown upon 

 the subject. 



We may consider the following possible changes, which might 

 have taken place in different cases : — 



(1) The whole deposit was originally gypsum, which was first converted 

 > into anhydrite, and then partly reconverted into gypsum. 



(2) The whole deposit was originally anhydrite. 



(3) The deposit originally was partly anhydrite and partly gypsum. 



(1) An example under this head occurs at Luclwig Mine, Lyon 

 County (Nevada), where the change from gypsum to anhydrite is. 

 thought (by J. Claude Jones 1 ) to have been due to an igneous, 

 intrusion, and the subsequent reconversion to gypsum to vadose 

 waters operating near the surface of the ground. 



Another example is the gypsum-anhydrite association in New 

 Brunswick and Nova Scotia described by H. E. Kramm, 2 who 

 assumes that the minerals Avere deposited from sea- water as 

 gypsum, and converted into anhydrite by being warmed to 63| c C, 

 or more, under a thick cover of sediment. With removal of the 

 load rehydration began, and is still going oh. 



According to Prof. R. C. Wallace, who has recently discussed 

 the genetic relationship of gypsum to anhydrite in North America, 3 

 direct geological evidence has yet to be adduced before any theory 

 can be accepted that transformation of gypsum to anhydrite at 

 great depths below the surface takes place to such an extent as to 

 be of geological importance. 



No one, so far as I am aware, has suggested that the gypsum- 

 deposits of Britain have suffered so many vicissitudes, therefore 

 we may pass on to a consideration of the next possibility. 



(2) This is a simple case to state, but one quite as difficult to 

 prove. Burns puts it as follows 4 : — 



' The prevailing idea is that the whole bed was at one time anhydrous and 

 has gradually, through geological ages, been converted into gypsum, a process 

 which is still going on.' 



1 In a letter (' Economic Geology' vol. vii, 1912, pp. 400-402) discussing a 

 paper by Dr. A. F. Rogers (ibid. pp. 185-89), who considers that the 

 anhydrite is original. 



2 Summ. Rep. Geol. Surv. Canada fcr 1911 (1912) pp. 322-27. 



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

 (1914) pp. 271-76. 



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

 (1902-1903) pp. 417-18. 



Q2 



