178 



ME. B. SMITH ON THE 



[vol. lxxiv, 



are usually shaped like basins below, and are sometimes slightly 

 concave above, but more frequently the surface is gently domed 

 and uneven. A large pillar measures about 12 feet in diameter, 

 and from 8 to 9 feet thick. The foulstone occurs chiefly between 

 the more widely separated pillars : when shaly it is seen to be 

 horizontally bedded', and is riddled throughout with minute veins 

 of fibrous gypsum, as well as larger ones which occasionally attain 

 a thickness of 2 inches. It sometimes includes large rounded lumps 

 ('hard-horses') of pale loamy marl measuring up to 2 feet in 

 length. 



Each pillar passes at its sides and bottom into coarsestone or 

 foulstone. The coarsestone is chiefty red gypsiferous marl and 

 gypsum, bound together by plates of selenite and riddled with 

 satin -spar. In places a mass of coarsestone will form by itself a 

 kind of irregular pillar, embedded in foulstone (fig. 2). 



Fig. 2. — Mode of occurrence of alabaster -pillars at 

 Ohellaston . (Length of section = 90 feet.) 



[White =alabast3r; cross-hatching = coarsestone ; horizontal shading 

 = foulstone ; black = swallo wholes.] 



At or near the junction of two or more pillars frequently 

 occur swallowholes that have been leached out along lines of 

 weakness at the sides of the pillars, and carry down surface- 

 waters to a level some 20 feet below the floor of the quarry. 



From an examination of numerous sections (some of which maj r 

 illustrate only a few of the points set out below) a typical pillar 

 may be described as follows : — 



The upper part is formed of pure white alabaster, veined in an 

 up-and-down direction by irregular pale-green bands. The upper 

 surface is rough and pinnacled (fig. 3, p. 179), while evidences 

 of solution are frequent. In plan the green colour-bands form a 

 reticulate network, in which no regular arrangement can be made 

 out like that in the vertical section. The lower surface of this 

 upper white layer is also irregular and overlies a breccia, thickest 

 in the centre of the pillar, which consists, in general, of irregular 

 masses of white gypsum embedded in red gypsum. The masses of 

 white gypsum are sometimes stained with pink patches, but are 

 more often veined with green, and they vary in shape from sharply 

 angular to subangular or rounded, the roundest masses being 

 usually small and occupying the lowest position in the bed. 

 The red gypsum is fairly pure, and will take a high polish, like the 

 white alabaster ; in places, however, it encloses a little marl which 



