The Quarterly journal of the Geological Society of London . ing at intervals. The complexseam rests upon shales, etc., overlying a lower bed of gypsum, nolonger worked. The whole of this mass is probably the equivalentof the seam at Thistle Plaster Quarry. The Weald.—In the Weald of Kent the evidence is in favourof 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 MinerBobertsbridge, consist of alternations of stratified shales, silts andlimestones with gypsum, like the cap of Kirkby Tliore, thelimestone be
The Quarterly journal of the Geological Society of London . ing at intervals. The complexseam rests upon shales, etc., overlying a lower bed of gypsum, nolonger worked. The whole of this mass is probably the equivalentof the seam at Thistle Plaster Quarry. The Weald.—In the Weald of Kent the evidence is in favourof 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 MinerBobertsbridge, consist of alternations of stratified shales, silts andlimestones with gypsum, like the cap of Kirkby Tliore, thelimestone being a new component. The obvious stratification andlamination of the sediment points to deposition in shallow water,in which considerable agitation is indicated by ripple-marks andcurrent-bedding. The gypsum in these beds occurs either in littlethin layers of brownish tint, or as a cement, or sometimes infibrous veins. Fig. 6.—A pillar of white ayp sum in tlie Subwealden Mine. Onthe left is a sliclcensided surface with fibrous cjypsum. Root „ , Roof. [Scale : 1 inch = 3 The main seam of gypsum averages 4 feet in thickness, but israther irregular, although its central plane maintains a fairlyconstant 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 latterseem to represent pillars like those of Chellaston or Aston must be noted that the white masses are often basin-shapedbelow, and very frequently the horizontally-bedded shalesin 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 certainamount of internal readjustment in the seam, and occasionally thesolid masses of gypsum have been driven through the more yielding part 3] CHELLASTON GYPSUM BBECCIA. 1
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