part 3] CHELLASTON GYPSUM BHECCIA. 197 



in longest diameter, with well-defined outlines, somewhat rounded 

 or subangular. Some much larger crystals are occasionally twinned. 



The anhydrite occurs in sheaves or fascines of elongated crystals 

 (averaging I mm. in length, and from jfc to ~ mm. in breadth) 

 with good rectangular cleavage and brilliant interference-tints in. 

 sections of normal thickness. The anhydrite bundles having 

 different orientation, in some cases appear in cross-section. Cracks 

 in the anhydrite have often been initiated in the anhydrite mass 

 parallel to the long axis of the crystals of one of the bundles, and 

 have then cut across the differently oriented crystals of an adjacent 

 bundle. These cracks have probably been caused by the expansion 

 consequent upon the conversion of anhydrite into gypsum : for, in 

 certain cases, it is clear that there has been replacement of the 

 anhydrite on each side of the original crack, giving rise to a vein 

 of gypsum in which the crystals of this mineral rilling the gap 

 have taken on the habit of the contiguous crystals of anhydrite 

 where they lie parallel to the direction of the vein (PI. XVIII, 

 fig. 2). and have formed well-shaped pseudomorphs — quite different 

 from the habit of the gypsum of the mosaic. 



Where the anhydrite-crystals lie transversely to the vein the 

 gypsum occurs as an irregular mosaic, and the anhydrite frequently 

 shows a moth-eaten appearance as if it were being destroyed in an 

 irregular manner. 



In more than one case, again, crystals of anhydrite, in optical 

 continuity, appear on both sides of a vein of gypsum, and the vein 

 may enclose a fragment of anhydrite, also in optical continuity 

 with the two walls of the vein. 



The isolated patches of anhydrite exhibit similar features. They 

 appear to be in process of being ' eaten up ' piecemeal, and the 

 resultant gypsum often shows a sheaf-like arrangement in imitation 

 of the sheaves of anhydrite, but this effect cannot be distinguished 

 in the gypsum \ inch away from the average line of the junction. 

 In the main mosaic of the gypsum these pseudomorphs have undu- 

 lose extinction, similar to those of a strained quartz-crystal in. 

 an igneous rock. 



•&' 



JB. At the contact between the bottom of the anhydrite and the 

 lower bed of gypsum at Cocklakes (PL XVIII, fig. 3), there is 

 clear evidence, as in the previous case, of the destruction of isolated 

 remnants of anhydrite, which have a brown-stained margin and 

 surroundings in which the original structure of the anhydrite can 

 still be made out. The ends of the long anhvdrite-crvstals are 

 often pseudomorphously replaced by brown-stained gypsum. Other 

 anhydrite-crystals show moth-eaten boundaries, and have been 

 replaced by gypsum with a minutely fibrous structure. The matrix 

 consists of gypsum of the same type, with larger crystals of clear 

 selenite in places. 



Where anhydrite is predominant it is often finely granular and 

 occasionally sheaf-like, as in the last slice, but sometimes occurs in 

 bigger and broader crystals. Pseudomorphs of gypsum occur most 



