106 Geology of Wiitshire. 
soft for building stone, but sometimes passing into a more solid 
state. It consists almost wholly of carbonate of lime. The strati- 
fication is often obscure, except where rendered distinct by layers 
of flint a few inches thick, sometimes in continuous beds, but oftener 
in separate nodules. The matter of which chalk is composed was, 
no doubt, once a soft white mud, such as is now being slowly de- 
posited in the depths of tranquil seas wherein corals abound and 
shell-fish browse on them; and the flints, probably, resulted from a 
slow separation of the siliceous particles contained in the mass, 
which being heavier and finer than the chalk, filtered through it to 
the bottom of each bed, and settled. The flinty particles seem to have 
arranged themselves by preference about the sponges, which while 
living, contained much silex. Hence in many gravels composed of 
flint from chalk, nearly every pebble will be found to contain a 
specimen of some such zoophyte. Among the fossils of chalk, be- 
sides corallines, Echinoderms are very numerous, as well as Pectens, 
Terebratule, Belemnites, and many forms of oyster. Teeth and 
palates of fishes also are frequently imbedded in it, but no bones of 
land animals, nor any terrestrial or fluviatile shells. All the ap- 
pearances lead to the conclusion, that white chalk was deposited in 
an open sea of considerable depth. Figs. 18 to 30, represent some 
of the most characteristic fossils of the chalk. 
Fig. 18 (285). 
Fig. 19 (284). 
A branching sponge in a flint, from the white chalk. 
From the collection of Mr. Bowerbank 
Fig. 20 (277). 
Ventriculites radiatus. 
Mantell. 
Ostrea carinata. Chalk marl, Upper, and 4 ; 
wer greensand. ; Syn. Goel Joa 
