Royal Society of London. 325 



the first-named district the sponge-beds are of a soft greyish- white 

 siliceous, or silico-calcareoiis rock, known under the names of mahn, 

 hearth, or firestone. In this the sponge-spicules principally occur 

 in the negative form of minute empty casts, the presence of which 

 renders the rook extremely light and porous. The beds can be 

 traced nearly continuously along the northern and western margin 

 of the Wealden area, and they are well shown at Godstone, Mers- 

 tham, near Reigate, Betchworth, Farnham, and Selborne. Further 

 northwards they are present at Wallingford, in Berkshire. The 

 beds vary in thickness from 15 feet at Merstham to 60 feet at 

 Farnham. 



In the more typical Upper Greensand of the Isle of Wight and the 

 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 Underclifif, 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 

 recognizable 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 Axraouth 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 Hilsandstein in Westphalia, which is of 

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

 the " Guize de I'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 

 oi'iginal colloidal condition, in which it is negative to polarized light 

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

 of the malm and fii'estone 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 

 throughout 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 

 crystalline 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 



