LOWER AND UPPER GREENSAND OF THE SOUTH OF ENGLAND. 
425 
cemeuts the particles is partly amorphous and partly chalcedonic. It contains spicules 
and fragments of lithistid and hexactinellid sponges, but not sufficiently numerous 
to be deemed a sponge-bed. 
(5.) Chaumont de Porcion, Ardennes .—Marne de Givron.—This rock, which 
occurs at a slightly higher horizon than the gaize, is of a grayish tint and soft 
structure. It consists largely of calcite, with glauconite, and a small quantity of 
colloid silica in the globular form. There are fragments of entire sponges in it, as 
well as traces of detached spicules, but these do not bear sufficient proportion to the 
other materials to constitute it a sponge-bed. In another specimen of the same rock, 
from Givron itself, the sponges have been replaced by iron peroxide, like those of the 
lower and upper chalk of this country. 
II. Mineral Conditions of the Sponge-Remains and of the Beds derived 
FROM THEM. 
With the single exception of the sponges in the lower greensand strata at 
Faringdon, which are calcisponges, the spicular remains forming the sponge-beds 
above referred to, both from the lower and upper greensand, are exclusively those of 
siliceous sponges. These spicular remains resemble so closely those of existing 
siliceous sponges, that the silica of which they are formed may be assumed to have 
been originally in the same colloid, hydrated condition in which it is present in the 
skeleton of recent forms. But no fossil sponge is yet known in which the spicular 
skeleton retains that transparent hyaline condition which is so striking a feature of 
recent siliceous sponges ; the silica, in all, has undergone various molecular alterations, 
and now presents numerous gradations between the amorphous or colloid and the 
crystalline state. Further, the changes in the fossil sponges have not been limited to 
alterations in the condition of the silica merely, but the silica itself has frequently 
been partially or entirely dissolved and replaced by calcite, glauconite, and other 
minerals, or removed, leaving the empty cast of the spicule. These modifications of 
the fossil sponge skeleton were first made known by Zittel and Sollas, and though 
at first regarded with doubt, on account of the supposed stability of silica, are now 
generally recognised. The investigations of # Mr. Hannay, and those of M. 
Thoulet,! have proved that the silica of recent sponges is readily susceptible to the 
influences of the chemical ingredients of sea-water, and has a strong tendency to 
dissolution after the death of the animal, so that the changes in the silica of the fossil 
spicules are only such as may have been anticipated. 
To a great extent the accumulated masses of detached spicules in the sponge-beds 
present the same modifications arising from fossilisation as those of entire sponges, 
which have hitherto been mostly studied, but in the sponge-becls we have also to take 
* ‘ Memoirs of the Literary and Philosophical Society of Manchester,’ vol. vi., 3 s., p. 234. 
t 1 Bulletin de la Societe mineralogique de France,’ tome vii., 1884, p. 147. 
