58 BRITISH FOSSIL SPONGES. 
tendency of the silica to pass from the unstable colloid to the stable chalcedonic 
and crystalline condition. Under favorable conditions this chemical change has 
taken place without destroying the form of the spicular skeleton, but in other cir- 
cumstances the colloid silica of the skeleton has been wholly dissolved away and 
redeposited, usually in the chalcedonic condition, so as to form solid beds of chert 
and bands of nodular flints. Some recent experiments of M. Thoulet' on existing 
siliceous Sponges show that the silica is readily susceptible to the solvent influences 
of the chemical ingredients of salt water, and fossil Sponges have been exposed to 
similar influences, during the interval of fossilization, from the action of water, 
charged with chemical substances, percolating through the rocks. 
Siliceous Skeleton replaced by Calcite and Glauconite-—The form of the Sponge is 
usually retained in such cases, but instead of the original silica the skeleton now 
consists of transparent crystalline calcite. As a rule, the Sponges in which this 
replacement occurs are embedded in a calcareous matrix, and by the action of 
dilute acid both the matrix and the replaced skeleton are equally dissolved. The 
replacement does not seem to have been produced by the molecular substitution of 
the calcite for the silica, but it is probable that the calcite has been deposited 
from solution into the hollow moulds left by the removal of the original silica. 
Where the spicular mesh is of an open character, or in the case of large spicules, 
the original form can be recognised in the replaced structures; but when the 
spicular mesh is minute and closely arranged, as in rhizomorine lithistids, the calcitic 
replacement is indefinite and confused, and the distinctive form of the original 
spicules can no longer be made out. It is thus very difficult to determine the 
original character of Sponges in this condition. On the other hand, in cases where 
the calcite has infilled the moulds formed in a matrix of chaleedonic silica, the 
replaced skeleton presents all the details of the original siliceous skeleton, with such 
even and clearly defined outlines that it has been mistaken for the original sub- 
stance of the skeleton, and the Sponge has been described as calcareous. Typical 
examples of this replacement are shown in specimens of Astylospongia from the 
Silurian strata of Gothland and of Hindia from nearly the same horizon in New 
Brunswick. 
This replacement of the siliceous skeleton by crystalline calcite oftentimes 
takes place in the Sponges of definite horizons in particular localities, whilst 
at other horizons but little removed the Sponges retain their siliceous structures. 
Thus, for example, in certain Sponge-beds in the Grey or Lower Chalk near 
Dover and Folkestone the siliceous structure of the Sponges has been entirely 
replaced by calcite, whilst the Sponges in the underlying chloritic marls of the 
same locality and in the Isle of Wight retain the silica. Similar instances are 
«Bulletin de la Soc. minéralogique de France,’ T. vii, 1884, p. 147. 
