186 MR. B. SMITH ON THE [vol. lxxiv,. 



subsoil-water in the formation of channels and pot-holes in the 

 seam, and lend no support to Groodchild's theory of ' downward 

 percolation 1 (p. 188). 



At Acorn Bank Mine, about three-quarters of a mile north-east 

 of Temple Sowerby, the gypsum overlies a bed of anhydrite of 

 fairly regular distribution, but wanting at intervals. The complex 

 seam rests upon shales, etc., overlying a lower bed of gypsum, no 

 longer worked. The whole of this mass is probably the equivalent 

 of the seam at Thistle Plaster Quarry. 



The Weald. — In the "Weald of Kent the evidence is in favour 

 of the gypsum of the Purbeck Peds having been formed in situ, 

 much in its present condition. 



The roof and floor of the seam at the Subwealden Mine,. 

 Bobertsbridge, consist of alternations of stratified shales, silts and 

 limestones with gypsum, like the ' cap ' of Kirkby Tliore, the 

 limestone being a new component. The obvious stratification and 

 lamination of the sediment points to deposition in shallow water, 

 in which considerable agitation is indicated by ripple-marks and 

 current-bedding. The gypsum in these beds occurs either in little 

 thin layers of brownish tint, or as a cement, or sometimes in 

 fibrous veins. 



Fig. 6. — A pillar of white ay p sum' in tlie Subwealden JSLine. On 

 the left is a sliclcensided surface with fibrous gypsum. 



Roof „ , 



Roof 



[Scale : 1 inch = 3 feet.j 



The main seam of gypsum averages 4 feet in thickness, but is 

 rather irregular, although its central plane maintains a fairly 

 constant horizon. Where the bed is thickest it is purest, 

 where thinnest it is more mixed with coarsestone : the bed, in fact, 

 is a complex of gypsiferous shales and limestones with irregu- 

 larly scattered lenticles and masses of white gypsum. The latter 

 seem to represent ' pillars ' like those of Chellaston or Aston Glebe. 

 It must be noted that the white masses are often basin-shaped 

 below, and very frequently the horizontally-bedded shales 

 in the coarsestone abut against their curved surfaces, 

 like the marls against the Chellaston pillars (fig. 3, p. 179). Sub- 

 sequent movements of all the Purbeck strata have caused a certain 

 amount of internal readjustment in the seam, and occasionally the 

 solid masses of gypsum have been driven through the more yielding 



