part 3] CHELLASTOX GYPSUM BRECCTA. 199 



(5) With falling temperature the deposition of anhydrite ceases at 36° C, 



and gypsum is again deposited in a solution with temporarily 

 increased concentration. Plates of gypsum ophitically enclosing 

 anhydrite begin to be formed. 



(6) At about 32° C. a little of the anhydrite at both the upper and the 



lower contact is converted into gypsum, the contact favouring the 

 change, which, however, for various reasons (as, for example, fall 

 in temperature or absence of much mother-liquor near the contact) 

 might not be extensive. 



Thus the microscopic evidence, so far as I have studied it, seems 

 to support the conclusion arrived at from a consideration of the 

 field relationship, that most of the gypsum-anhydrite 

 deposits of this country are original, and that conver- 

 sions from anhydrite into gypsum are mainly confined 

 to the zone of contact, and usually took place at the 

 time of deposition. 



In discussing the upper gypsum of Gypsum ville, Prof. E-. C. 

 Wallace mentions that there has unquestionably been a certain 

 -amount of transformation into gypsum of the top beds of the 

 underlying anhydrite since Tertiary times. Analyses of the upper 

 anhydrite-beds show a continuously increasing water-value from 

 the middle of each bed to the margin, which he takes as evidence 

 that a gradual change is taking place. He believes, however, that 

 only the lowest beds of the upper gypsum have thus originated, 

 the higher beds having been precipitated as such. 1 



Microscopic instead of chemical analysis might show an in- 

 creasing proportion of gypsum towards the upper and lower 

 surfaces of these upper anhydrite-beds, due to disturbance of the 

 ■chemical balance at the time of deposition, in the manner discussed 

 above. 



VIII. Fibrous Gypsum. 



A few words may, perhaps, be added with regard to the occur- 

 rence of fibrous gypsum. 



Most of the fibrous gypsum that I have seen appears to be 

 of secondary origin, formed by solution and redeposition of the 

 normally-deposited gypsum ; a little may have been formed in 

 the beginning. It has been forming from the time of deposition 

 of the seams of massive gypsum down to the present day. 



Whenever cracks appeared — in the freshly deposited and scarcely 

 hardened gypsuni ; in the marls themselves, or in the seams and 

 balls long after consolidation ; and at times when later earth- 

 movements bent, squeezed, and cracked the rocks — fibrous gj^psum 

 seems to have healed the scar. We find it as part of the trans- 

 lucent alabaster of the irregular deposits at Aston Glebe and 

 Chellaston, but it rarely occurs in the more regular seams at 

 Kirkby Thore. It forms part of the coarsestone, and riddles the 

 foulstone through and through in tiny veinlets. It occurs where 



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

 (1914) p. 275. 



