MINERAL CHARACTERS. 59 



recorded by Zittel 1 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 Peroxide 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. The 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 labyrinthiue walls appear in section 

 on the surface of fractured flint like narrow reddish bands. In some cases these 

 bands show the minute spicular structure, but more frequently it is altogether 

 indistinct and unrecognisable. 



1 ' Studien iiber fossile Spongien,' i Abth., p. 13, 1877. 



