There may be said to be two classes of this stone.
The first consists almost entirely of small shells cemented together, but shows no crystals on fracture.
Purbeck is an example of this class.
Stones of the second class consist chiefly of shells, but break with a highly crystalline fracture.
Of this class Hopton Wood stone is an example.
This is given in the Table, pp. 67-73.
Stone of this class is chiefly used for paving.
The weight of this class of stone is from about 157 to 169 lbs. per cubic foot, and its absorption is very small, generally much less than 2 per cent of its weight.