There are 2 classes of this stone. The first consists almost entirely of small shells cemented together, but shows no crystals on fracture : Purbeck is an example. Stones of the second class consist chiefly of shells, but break with a highly crystalline fracture : Hopton Wood and Nidderdale stones are examples. Stone of this class is chiefly used for paving. The weight is about 157-169 lb. per cub. ft., and the absorption is very small, generally much less than 2 per cent. of the weight.