1885.] Sponge-remains in the Greensand. 347 



south-western counties, the sponge-beds consist of thick layers of 

 chert and porous siliceous rock at the summit of the series, immedi- 

 ately beneath the so-called chloritic marl; whilst in the lower division 

 the sponge-remains principally occur in loose quartzitic sands with 

 siliceous accretions. The chert or sponge-beds at the top of the Upper 

 Greensand are best exposed at Shanklin, Ventnor, and the Undercliff, 

 in the Isle of Wight, at Warminster in Wiltshire, and Penzlewood in 

 Somersetshire. They vary from 10 to 25 feet in thickness. The 

 sponge-beds of the lower division are scarcely recognisable in the Isle 

 of Wight, but they attain a thickness of 10 to 20 feet on the summit 

 of the Blackdown and Haldon Hills in Devonshire, and at Axmouth 

 in Dorsetshire. The chert here is only present in beds of subordinate 

 importance. 



Sponge-beds of similar characters to those of the Greensand have 

 been described from the Hilssandstein in Westphalia, which is of 

 Neocomian age, and, judging from specimens which I have examined, 

 the " Gaize de l'Argonne," which is largely developed in the Ardennes, 

 and the "Meule de Bracquegnies " in Belgium, are sponge-beds, filled 

 with spicules and spicular casts like those of the Greensand. 



The sponge-remains in the various beds are exclusively those of 

 siliceous sponges. In some the silica of the spicules yet retains its 

 original colloidal condition, in which it is negative to polarised light 

 and readily soluble in caustic potash. The matrix of the sponge-beds 

 of the malm and firestone is also to a large extent of colloidal or 

 amorphous silica, and this material has been deposited in the form of 

 minute globules or disks, and seems to have been derived from the 

 sponge spicules, with the empty casts of which the beds are through- 

 out filled. 



More generally the original amorphous silica of the sponge-remains 

 has been altered to chalcedony, and the chert and porous siliceous 

 rock accompanying it, which is filled with traces of the spicules, are 

 likewise of chalcedony ; occasionally the chalcedony gives place to 

 orystalline silica. 



Glauconite very commonly fills the canals of the spicules, and 

 remains after the spicular walls have been removed ; it also replaces 

 the spicular walls. 



In some sponge-beds the spicules have been nearly entirely replaced 

 by crystalline calcite ; they are embedded in a matrix of granular lime- 

 stone. 



As a general rule the sponge-spicules are inclosed in a compact 

 matrix in which their forms can only be partially studied, but under 

 certain conditions they are loosely distributed in sand, or in fine 

 powder in cavities in chert, from whence they can be obtained quite 

 free from matrix. The sponge-beds appear to be composed of detached, 

 free, spicules of disintegrated sponges; entire sponges are absent. 



