LOWER AND UPPER GREENSAND OF THE SOUTH OF ENGLAND. 427 
when examined in Canada balsam, appears to be filled with exceedingly minute 
granules in a transparent ground-mass. These residuary spicules give a very slight 
double refraction between crossed nicols. In some instances one portion of a 
residuary spicule is of glauconite, or at least has the green appearance of this mineral, 
whilst the other portion is of this transparent mineral ; and there is a gradual transi¬ 
tion from one to the other, As nitric acid has no more influence on this mineral than 
it has on the glauconite, it is probably some silicate allied to glauconite. 
These residuary spicules of glauconite and other silicates, which remain after dis¬ 
solving aw r ay the silica by caustic potash are also produced under natural conditions, 
and they are very common in the loose material of the cavities in the chert of the 
upper greensand at Warminster in association with unaltered colloid spicules, and in 
similar positions at Ventnor, where they have been nearly wholly reduced to this 
condition (Plate 45, figs. 15-15e and 1G). 
Fossil siliceous spicules, with the silica in an amorphous condition, are of rare 
occurrence in comparison with the numerous instances in which the silica is crypto¬ 
crystalline or even crystalline, and they have not been previously noticed in this 
country, They are, however, abundant in the sponge-beds of the northern margin 
of the Weald, also at Wallingford, Warminster, and Penzlewood. Both spicules 
and entire sponges of amorphous silica are also common in glauconitic marls of 
senonian age in Westphalia and Hanover, from whence they have been described 
by Professor Ztttel.^ 
The beds in which the spicules of amorphous silica are inclosed, also frequently 
contain an important quantity of silica in the colloid state, amounting in the case of 
the malm at Farnham, recorded by Messrs. Way and Paine,! to as much as 75 per 
cent. In some instances, however, whilst the spicules are of colloid silica, the in¬ 
closing matrix is of chalcedonic and even partially of crystalline silica, and calcite is 
also present. 
The presence in the same beds of silica in an amorphous condition, with the siliceous 
spicules in a similar state, taken in connection with the fact that the beds are filled 
with empty spicular casts from which the spicules have been dissolved, and that many 
of the spicules are residuary forms which have lost all their soluble silica, points to 
the conclusion that the colloidal silica in the beds has been directly derived from the 
breaking up and dissolution of the sponge-remains. 
The colloidal silica of the sponge-beds occurs in the form of minute granules, similar 
to those in the spicules, and also in a globular form, that is to say, in very minute 
bodies with circular outlines, though not strictly of a spherical form. The siliceous 
bodies can be examined in thin microscopic sections from the harder portions of the 
malm of Godstone and other places ; but their characters are still better shown in the 
* “ Ueber Coaloptyohimn.” ‘ Abhandlung der konigl. Akad. der Wiss, zu Miincben.’ XII. Band, 
iii. Ab. Also, “ Studien fiber fossile Spongien.” Id., XTII. Bd., i. Ab. 
t ‘ Journal of the Royal Agricultural Society,’ vol. xiv , p. 243. 
