PEBBLES AND SHINGLE. 217 



characteristic zoophytes of the white chalk. Small 

 pebbles of pure transparent quartz or rock-crystal, 

 are often found in the shingle in Compton and 

 Sandown ; these have probably been washed out 

 of the wealden strata, for pebbles identical in 

 mineral ogical character occur in the grits of 

 Tilgate Forest and Tunbridge Wells. 



But the shingle contains other siliceous pebbles 

 which essentially differ from those of the chalk- 

 flints. Some of these are composed of a dark 

 brown mottled jasper, and when cut and polished 

 resemble the well-known Egyptian pebbles ; others 

 are of an opaque white jasper, veined with black 

 dendritical or arborescent figures, or marked 

 with zones of rich brown tints. Some have a 

 concentric arrangement of bands of silex of 

 various shades of colour, and resemble agates ; 

 others assume the character of botroidal and 

 mammillated chalcedony. 



The differences observable in the materials com- 

 posing the shingle, did not escape the notice of 

 Mr. Webster, who describes some of the principal 

 varieties of the pebbles, and suggests that those 

 which are not waterworn chalk-flints, may have 

 been derived from veins or nodular masses of 

 silex, in tertiary strata now destroyed. A polished 

 slice of silicified wood found in the shincde of 



