418 
DR. G. J. HINDB ON BEDS OF SPONGE-REMAINS IN THE 
The upper layers (21 feet in thickness) of this section so abound with spicules that 
they may be considered as a continuous sponge-bed. The chert resembles that which 
has already been described from the lower greensand. It is usually of a light brown 
tint, and in thin sections under the microscope it is seen to be filled with spicules and 
spicular casts imbedded in a translucent matrix of chalceclonic silica. The spicules 
are likewise of chalcedony, and their canals are infilled with glauconite. Another 
variety of chert, also very abundant, is of a grayish or greenish-white tint; it differs 
from the former in that the matrix is of amorphous silica, whilst the inclosed spicules 
are of chalcedony. The chert bands here, similar to those of the lower greensand, 
are enveloped in an outer crust, of varying thickness, of white or yellow siliceous 
porous rock, which is interpenetrated with the empty moulds of spicules. 
In some of the thicker masses of chert there are cavities or pockets filled with 
spicules, loosely mingled in a grayish siliceo-calcareous powder, in which there are also 
numerous well-preserved foraminifera, chiefly of the genus Textularia. The spicules 
in these cavities have undergone a remarkable alteration in structure ; they appear to 
have lost their original silica, which has been replaced by glauconite and some other 
silicate of a greenish-white aspect. The replacing material has only partially filled 
the form of the original spicules, and thus they look like mere shadowy casts of 
complete spicules. These in many cases are peculiarly distorted and contracted 
(Plate 45, figs. 15a— e). 
Spicules in varying quantities are also present in the beds of freestone and other 
siliceo-calcareous layers, in all about 27 feet in thickness, which form the lower part 
of the section at the Yentnor Station quarry. These beds consist principally of fine 
grains of quartz and glauconite with mica and cal cite, and in appearance they resemble 
the malm rock of the northern margin of the Weald, and, in fact, they have thus 
been named by Mr. Parkinson.* They differ, however, from the true malm in the 
small proportion of silica which they contain, and I have not seen in them any silica 
of the globular form so common in the malm. The sponge-spicules in these beds are 
mostly represented by their hollow moulds, and these are so minute that only by 
careful scrutiny can they be detected. They are, as a rule, more readily recognisable 
in the small dark phosphatic nodules which frequently occur in the beds. The 
spicules are not sufficiently abundant, however, for these beds to be considered as 
definite sponge-beds. 
In the lower series of the upper greensand strata, consisting of about 50 feet 
(10'5 m.) of yellowish-gray micaceous sands, usually soft and unconsolidated, but with 
occasional harder bands, I did not detect any well-defined sponge-beds, but spicules 
are fairly abundant in some of the layers, and it is not improbable that a careful search 
would show them presence throughout. 
The sponge-beds of the Isle of Wight, like those of the lower greensand, exclusively 
consist of the detached spicules of siliceous sponges. A few entire lithistid sponges 
* Quart. Jo urn. Geol. Soc., vol. xxxviii., p. 370. 
