LOWER AND UPPER GREENSAND OE THE SOUTH OF ENGLAND. 
411 
(14.) Sand and siliceous rock . 
(15.) Sand. 
(16.) Siliceous rock (sponge-bed) 
(17.) Sand. 
(18.) Siliceous rock (sponge-bed) 
(19.) Sand. 
(20.) Siliceous rock (sponge-bed) 
feet, indies. 
1 6 
3 6 
0 11 
1 0 
0 6 
1 6 
1 0 
38 3 
Thus, in this section of 38 feet 3 in. (11 *475 m.) there are eight distinct zones or 
beds of sponge-remains, with a total thickness of 8 feet 5 in. (2 '52 m.). These 
sponge-beds, being of a much harder nature than the intermediate layers of sand, 
stand out prominently in relief in the face of the cliff, and through the weathering 
away of the loose sand, large masses and blocks fall down, and are piled over each 
other on the beach. The sponge-beds consist of spicules in a matrix of coarse grains 
of quartz-sand and glauconite, cemented by siliceous material. The rock is of a 
porous or spongiose character, harsh and rough to the touch, and readily friable. 
There is in some of the beds a central layer of cherty rock, but this is usually porous, 
though in places it passes into true chert. The spicules in these beds are most clearly 
shown on weathered surfaces, and they appear to the unaided sight as minute white 
rods of a dull porcellanic aspect. They are often sufficiently numerous to give a 
distinct white aspect to the rock surface. In freshly fractured surfaces only empty 
casts of spicules can, as a rule, be detected. By the aid of a lens, gradual transitions 
can be detected between apparently perfect spicules and forms with indistinct outlines, 
in which the spicules are gradually breaking up to form the whitish siliceous cement 
which binds them and the sand grains into one mass. The cavities in the cherty 
portions of the sponge-beds are frequently lined with a smooth layer of chalcedonic 
silica, which show's under the microscope a radiately fibrous structure. When the 
cavities are infilled by successive layers of this silica, which there can hardly be a 
doubt is derived from the sponge-spicules, a true chert is produced. 
Not infrequently the weathered surfaces of the sponge-beds are covered by a raised 
network of sub-cylindrical anastomosing branches, resembling the structures generally 
known as Spongici paradoxica. These branching bodies have been mistaken for actual 
sponges," but they appear to me to be merely the burrows of marine animals into 
which the loose sponge spicules have been washed, and from the partial solution of 
these the burrows have been filled with a strongly cemented material, which resists 
weathering better than the inclosing rock. 
The spicules of these sponge-beds are exclusively of siliceous sponges, and belong 
* Mr. F. G. H. Price refers to the sponge-beds in this section as seams of coarse calcareous sandstone, 
and states that large white tabular masses of branching spouge are met with on them. ‘ Proceedings of 
the Geologists’ Association,’ vol. iv., p. 1?9. 
