LOWER AND UPPER GREENSAND OF THE SOUTH OF ENGLAND. 
431 
canal resists dissolution more than the spicular walls, and the solidified canal remains 
either wholly or partially free after the enclosing wall has been removed (Plate 45, 
fig. 13). 
II. Spicules replaced by glauconite and other silicates. —Not only do these minerals 
infill the canals of the spicules, but they also replace the spicular walls as well. The 
replacement seems to have been gradually effected from within outwardly ; the dis¬ 
solution of the silica of the spicule taking place by the enlargement of the axial canal, 
and as this progresses the glauconite and allied minerals occupy the place of the 
silica until they constitute the entire spicule. These glauconitic spicules are very 
common in the malm and firestone of Merstham and Godstone, and also in the cavities 
of the chert at Yentnor. In this latter they are very strangely contracted and 
distorted, and assume figures which might have been produced by the gradual 
desiccation of a gelatinous body, The normally straight shafts and rays of the 
spicules are bent and curved in various directions, and even occasionally become 
spiral (Plate 45, figs. 15a, b, c, d). 
III. Spicules replaced by crystalline calcite. —In every case in which this replace¬ 
ment occurs the matrix enclosing the spicules is of calcite, generally in a finely 
granular condition. The change seems to have taken place in the detached spicides 
in the same manner in which it has long been known to occur in entire fossil sponges. 
The original silica of the spicules has been dissolved subsequent to the enclosure of 
the spicules in the calcitic matrix, and the crystalline calcite has then filled up the 
moulds (Plate 40, fig. 4). In some cases the spicular canals have been infilled with 
glauconite before the calcitic replacement has been effected. The change to calcite is 
not complete in all the detached spicules in the same bed, since some occur in which 
the siliceous structure remains unaltered. Sponge-beds, with the spicules replaced 
by calcite, occur in the lower greensand at Maidstone, Tilburstow Hill, and Hythe, 
and in the upper greensand malm at Selborne. 
IY. Spicules represented by empty moulds. —One of the commonest features of the 
sponge-beds is the empty moulds or casts of spicules with which the beds are filled, 
which have been produced by the complete dissolution of the siliceous spicules. In 
some cases the solidified canal has resisted the solvent influence which has removed 
the spicular wall, and now remains as a slender, smooth, delicate rod in the centre of 
the mould. These empty casts are present alike in the soft, friable, siliceous, and 
siliceo-calcareous sponge-beds of the malm and firestone, in the fine sedimentary 
deposits of the lower division of the upper greensand in the Isle of Wight, in the 
porous siliceous rock which generally accompanies chert, occasionally also in chert, 
and in a matrix entirely of calcite. It is to the dissolution and removal of the 
siliceous spicules that the sponge-beds owe their porous character and low specific 
gravity. The silica derived from the solution of the spicules appears usually to have 
been redeposited in the small interspaces between them, or to have accumulated to 
form nodular masses and layers of chert. Where the silica has been redeposited in 
3 k 2 
