56 BRITISH FOSSIL SPONGES. 



exhibit between crossed Nicols the same shades of colour as chalcedony and 

 quartz, and in many instances in the same spicule there is a gradual passage from 

 the chalcedonic to the crystalline state of the silica. The outer surface of the 

 spicular skeleton in this condition of the silica is rarely smooth, but generally 

 much eroded, and apparently covered over with minute pitted depressions, which 

 give the skeleton a very ragged appearance under the microscope, and the delicate 

 extensions of the spicular rays appear as if largely worn away by erosion. The 

 axial canals also of the spicules can rarely be detected, probably owing to the 

 fact of their having been infilled with silica of the same optical character as the 

 wall of the spicule, and therefore indistinguishable from it. In the majority of 

 siliceous Sponges in which the skeleton has been preserved the silica is now in the 

 condition of chalcedony. In some, however, the change has reached a further 

 stage, and it is altogether crystalline. The experiments of Mr. Hannay 1 on the 

 siliceous fossilization of the Sponge- spicules from the Lower-Carboniferous Rocks 

 of Scotland show that the change from the amorphous silica of recent Sponges to 

 the cryptocrystalline and crystalline silica of the fossil forms is mainly owing to 

 the loss of chemically-combined water, which causes crystallization to set in. 



The alteration in the spicular skeleton of the Sponges just referred to is 

 mainly limited to the condition of the silica of which it is composed, and the 

 detailed form of the skeletal mesh and spicules is retained as in existing Sponges ; 

 but in many fossil Sponges, notably in those from the Upper Chalk, the skeleton 

 is still of silica, but the skeletal tissues have lost their distinctive form, and the 

 place of the regular spicular meshwork is taken by shapeless fibrous masses of 

 chalcedonic or crystalline silica, which present an appearance as if the original 

 silica of the skeleton had been fused. This alteration is well exemplified in the 

 Sponges from the Upper Chalk of Flamborougk, Yorkshire, which, when freed by 

 acid from the chalky matrix, retain for the most part their complete outer form and 

 the fibrous character of the skeleton, but the delicate spicules of which the fibres 

 were originally composed have altogether disappeared, and the fibres are now of 

 shapeless granular particles of silica. 



In other cases, as in many of the Sponges from the Upper Chalk of Wilts and 

 elsewhere which have been enclosed in flint, the spicular structure of the outer 

 surface of the Sponge is occasionally still preserved, and consists of crystalline 

 silica of a snowy-white tint, but the structure of the interior is usually changed to 

 a mass of botryoidal or porous chalcedonic silica, in which even the course of the 

 canals has been obliterated. These masses form, as it were, cores within the 

 flints, and are frequently entirely free from the outer casing of flint. Not infre- 

 quently between the Sponge and the flint there is a very fine, white, siliceous 

 powder, oftentimes containing detached spicules. 



1 « Mem. Lit. and Phil. Soc. Manchester,' vol. vi, S. 3, p. 234. 



