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 1 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 chalcedonic 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-becls 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 



1 ' Bulletin de la Soc. mineralogique de France,' T. vii, 1884, p. 147. 



