114 RELICS OF PRIMEVAL LIFE 



mineralization in ordinary limestones, and the effects 

 which they produce (Fig. 23). 



The mineral matters which thus aid in preserving 

 fossils are of various kinds, and the whole subject is a 

 very curious one ; but for the present we may content 

 ourselves with two kinds of mineralization — that by 

 silicates and that by magnesian limestone or dolomite. 



From the bottom of modern seas the dredge often 

 brings up multitudes of minute shells, especially 

 those of the simple gelatinous Protozoa, known as 

 Foraminifera, whose internal cavities and pores have 

 been filled with a greenish mineral composed of silica, 

 iron and potash, combined with water (or, chemically 

 speaking, a hydrous silicate of iron and potassium), 

 which is named glanconite from its bluish-green 

 colour — a name which we shall do well to remember. 

 In such compounds, bases of similar chemical pro- 

 perties often replace one another, so that various 

 glauconites differ somewhat in composition, the iron 

 being in part often replaced by alumina or magnesia, 

 and the potash by soda. The combined water also 

 differs somewhat in its percentage. When minute 

 shells fossilized in this way are treated with an acid 

 so as to remove the calcareous shell itself, the en- 

 closed silicate remains as a beautiful cast or core, 



