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’ 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 Flamborough, 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. 
* «Mem. Lit. and Phil. Soc. Manchester,’ vol. vi, 8. 3, p. 234. 
