part 3] CHELLA.8T0N GYPStfM BRECCIA. 195 



bed of anhydrous calcium sulphate, it is reasonable to suppose that 

 the gypsum surrounding the masses is original, unless indeed some 

 -of it suffered conversion at the time of deposition of the remainder. 



Since anhydrite, according to Van't Hoff, can be precipitated in 

 the presence of a strong solution of common salt at 36° C, but may 

 be converted into gypsum at 32° C. under the same conditions, and 

 since anhydrite is often found inside gypsum masses, and unmixed 

 with ordinary sediment (dolomite when precipitated chemically is 

 .an exception), there is a reasonable probability that many of the 

 anhydrite masses were formed during periods when the waters were 

 hotter and the land suffering under more intense conditions of 

 •drought than when the gypsum was deposited. With fall of 

 temperature — sometimes due to influx of land water -and change 

 in strength of the solutions, gypsum would be deposited again, and 

 part of the anhydrite might, or might not, be hydrated before 

 being buried too deeply to be affected. When drought was 

 greatest, ordinary sedimentation was least, hence large beds of 

 anhydrite are more frequently found in contact with gypsum rather 

 than with sediment. 



In those cases where anhydrite is mixed confusedly with gypsum 

 and muddy sediment, or occurs in the centre of isolated balls, the 

 deposition not only of the anhydrite, but of the gypsum and the 

 attendant sediments as well was also irregular. Constant changes 

 in the strength of the various mineral solutions were taking place 

 with the inrushes of water that brought the sediment into the 

 inland basins. Transformation processes at, or near, the surface 

 (often at fairly high temperatures) would thus play a greater part 

 than in those cases where crystallization was more regular. 



Thus the field evidence, on the whole, appears to be in favour 

 of the formation of the greater part of both the anhydrite and 

 gypsum of England as original deposits that have undergone little 

 subsequent alteration since they were buried by later deposits. 



(b) Microscopic Evidence. 



We may now discuss the evidence furnished by the microscope: — 

 (i) In a specimen of the anhydrite from Cocklakes, the mineral 

 occurs in long crystals, less sheaf-like than those of the Gotham 

 and Fauld rock that will be described presently. Plates of gypsum 

 occasionally enclose crystals of anhydrite in ophitic fashion, and 

 with these are minute rhombs of dolomite (measuring ^mm.), 

 smaller than those of the Magnesian Limestone of Nottingham- 

 shire and Yorkshire, referred to below, where the rhombs measure 

 about Jy mm. These rhombs are aggregated in lines and 

 clusters separating variously oriented bundles of anhydrite-crystals 

 (measuring | mm. x -fa mm. average size ; bundles are 2 to 3 mm. 

 across) : they give to a hand-specimen of the rock a pseudo- 

 crystalline appearance which is coarser than the true crystalline 

 structure. The ophitic plates of gypsum imitate a porphyritic 

 structure. 



