CHEMICAL CONSTITUTION AND MODE OF PRESERVATION. 55 



tion are present in a diatomaceous rock of the pre-Miocene age from Oarnaru, New 

 Zealand, and in marls from St. Peter, Hungary. 



In fossil siliceous Sponges the silica may be either (a) amorphous or in the 

 colloid state ; (b) chalcedonic or cryptocrystalline, or (c) crystalline. It may also 

 be replaced by glauconite, or other mineral silicate ; or dissolved so as to leave 

 empty negative casts of the spicular skeleton in moulds of limestone, chalk, or 

 silica; or the moulds may be infilled and the silica replaced by crystalline calcite, 

 iron-pyrites, or iron peroxide. The siliceous skeleton may, further, be entirely 

 dissolved without leaving any mould or trace of its presence, whilst the form of the 

 Sponge is retained, and the canals infilled with a solid mass of silica. The Sponges 

 may still consist of silica, but the minute structure of the skeleton may be partially 

 or entirely obliterated, and the silica re-deposited as chalcedony. In this state the 

 Sponges in the Chalk are frequently enclosed in an outer casing of solid flint. 



(a) The silica amorphous. In this case the silica of the Sponge skeleton is in 

 a colloid condition, like that of recent Sponges, and it presents the same reaction 

 to heated caustic potash, and it is likewise negative to polarized light, but it has 

 lost the brilliant glassy aspect present in the siliceous skeleton of recent Sponges. 

 The skeleton of Sponges in this condition has a porcellanous, milky- white appear- 

 ance by reflected light ; and in transmitted light, when mounted in Canada balsam, 

 it is nearly transparent, and the silica appears to be in the form of minute granular 

 particles with a pinkish reflection, and not infrequently of minute spherules or discs. 

 If mounted in glycerine the spicules become so transparent that nothing more 

 than faint outlines of their forms can be seen. As a rule the surface of the 

 spicules, in which the silica retains the original amorphous condition, is smooth and 

 even, and the axial canals are usually wider and therefore more distinct than in 

 recent forms. These axial canals appear in many instances to be now filled with 

 chalcedonic or crystalline silica, and they frequently remain after the more soluble 

 silica of the spicular walls has been dissolved away. Sponges in which the silica 

 remains amorphous are comparatively rare, and in this country I have only seen 

 examples from the Upper Greensand of Wiltshire ; but they are common in glau- 

 conitic marls of Senonian age in North Germany. Detached spicules in this con- 

 dition are present in considerable numbers in the interior of masses of chert from 

 the Upper Greensand of Wilts, and in nodules in the so-called malm or firestone 

 rock, of the same geological horizon, at Merstham, Surrey. 



(6) The silica is cryptocrystalline or (c) crystalline. The spicular skeleton, in 

 which the silica is in either of these states, has by reflected light either a pure 

 snowy white tint, or a dull glassy appearance, not unlike that of ground glass. 

 When mounted in Canada balsam, and viewed by transmitted light, the spicules 

 become so transparent that but little more than their contours can be distinguished, 

 but in glycerine their forms stand out very distinctly. In polarized light they 



