It is otherwise, however, when the calcareous fossils have been filled or injected with some mineral matter different from the matrix, as, for example, silica or some silicate, oxide or sulphide of iron. In this case the texture, colour, or hardness of the filling appear different from those of the limestone, and may be seen in a fresh fracture or polished slice; or when the rock is weathered, the hard mineralizing substance may project from the surface of the specimens, or may be disclosed by treating the surface with a weak acid. The figures here given may suffice to show some of these conditions of 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 _glauconite_ from its bluish-green colour--a name which we shall do well to remember. In such compounds, bases of similar chemical properties 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 enclosed silicate remains as a beautiful cast or core, representing all the forms of the interior, and any pores that may have penetrated the walls, and also perfectly representing the soft gelatinous body of the animal which once tenanted the shells (Fig. 24). (See also Fig. 25 at end of chapter.) [Illustration: Fig. 24.--_Cast of Cavities of Polystomella in Glauconite (magnified)._ After a photograph from Dr. Carpenter, and mounted specimens from his collection.]