192 PORTLAND STONE QUARRIES. 



A layer of flint from six inches to a foot thick separates it from 

 the lower whit hed, which varies in thickness from two to five feet ; 

 helow it again is another flint seam succeeded by a bed of whitish 

 stone intermixed with large black flints. This bed, too, rests upon 

 a seam of flint fifteen inches thick. In some parts of the island 

 these beds are absent, and the whit bed rests on the curf, with a 

 thin seam of flint or shelly bar intervening. The curf is usually 

 sand, and capable of being squared up into blocks. It is very 

 white, compact, and not oolitic, but is useless when underlying the 

 whit bed and flint. From the curf downwards the large Ammonites 

 and Pleurotomaria occur, but they predominate in the curf. The 

 base bed differs widely in different parts of the island. When 

 protected by the Purbeck and Portland beds the stone is white with 

 a fine oolitic grain. On the west side of the island, however, it is 

 soft and in a rotten condition, and quite useless for quarrying. In 

 the quarries when the base bed underlies the whit bed, which is 

 not adapted for the market and is unprotected by the Purbeck bed, 

 it is of good quality, eight feet thick, and harder than the pro- 

 tected base bed. Although this stone is good for building purposes, 

 it cannot be relied on like the brown whit bed, and owing to its 

 more compact material is liable to be affected by frost. The joints 

 are more open than those of the whit bed, and it is cut up to size 

 with greater facility. It is the lowest bed that is of any economic 

 value, and is often called the Base bed. There are some twenty or 

 thirty beds on the west side of the island intervening between it 

 and the Portland clay and sands, fifty-five feet thick in the 

 aggregate. Some of these are quarried by the prisoners for building 

 and for the fortifications of the Verne Citadel. Two of them, 

 termed flat beds, were used for the Admiralty works at the Break- 

 water. They are about three feet thick, of a white to a bluish- 

 gray colour, with hard close grain containing a high percentage of 

 silicate. It is a good building stone when defended from the 

 weather. This and all other close-grained stone is unfit for use 

 when the sea bottom is muddy, being liable to the attacks of boring 

 worms. 



