MINERAL CHARACTERS. 59 
recorded by Zittel’ in the Sponge-beds of the Upper Jura of Switzerland, 
Wurtemberg, Bavaria, and Poland. In certain localities the entire siliceous 
skeleton is replaced by calcite, whilst in others the original silica remains, though 
in the condition of chalcedony. On the other hand, many instances occur in 
which the calcitic replacement is only partial, even in the same Sponge, and by 
treatment with acid the calcareous portions are dissolved, whilst fragmentary 
portions of the siliceous structure remain behind. 
The replacement of silica by glauconite is of much less frequent occurrence, 
but the change has taken place in detached spicules in the Sponge-beds of the 
Upper Greensand. The glauconite in these specimens appears first to have 
infilled the axial canal of the spicule, and afterwards to have been deposited 
gradually, in proportion as the silica of the wall of the spicule has been removed. 
In other instances the siliceous spicules are replaced by a mineral of a greenish- 
white tint, apparently allied to glauconite, and nearly entirely transparent when 
viewed under the microscope in Canada balsam. This replacement is also fre- 
quently accompanied by a very peculiar distortion and contraction of the spicules. 
It occurs in spicules preserved in cavities of Upper-Greensand chert at Ventnor 
and at Warminster in Wiltshire. 
Siliceous Skeleton replaced by Perowide of Iron and Iron-pyrites.—-This latter 
mineral but seldom takes the place of the silica, but the former is of frequent occur- 
rence; and Sponges thus replaced are found in strata from the Silurian upwards, 
and they are more particularly abundant in the Upper Chalk of the South of 
England. ‘he peroxide occurs as a rusty, reddish brown, often powdery and in- 
coherent, material. As a general rule the finer structures of the skeleton are not 
preserved in this material, and they are usually so confused as to be unrecognisable. 
Rarely, however, as in the case of some of the Ventriculites from the Upper Chalk, 
so carefully worked out by Toulmin Smith, the peroxide is sufficiently firm to 
allow of the chalky matrix being removed, and the skeletal structure stands out 
by itself, but it is nevertheless very delicate and perishable. It is somewhat un- 
fortunate that the majority of the siliceous Sponges in the Upper Chalk of the 
South of England, in which the matrix is the Chalk itself, should have had their 
skeletons replaced by this incoherent peroxide, since, though their entire forms 
are retained, and their canal-systems can be made out, the minute characters of 
their spicular tissues cannot be determined with precision. Not infrequently 
also the peroxide has replaced the silica in Sponges enveloped in a matrix of solid 
flint ; and the replacement is particularly well shown in specimens of Ventriculites 
and Plocoscyphia, whose delicate convolute or labyrinthine walls appear in section 
on the surface of fractured flint lke narrow reddish bands. In some cases these 
bands show the minute spicular structure, but more frequently it is altogether 
indistinct and unrecognisable. 
! «Studien tiber fossile Spongien,’ i Abth., p. 13, 1877. 
