The internal structure of a rock may have much to do with its apparent specific gravity. As an instance of this, it may be stated that an obsidian pumice will float upon water, buoyed up by the air contained in its vesicles, while a compact obsidian of the same composition will sink immediately. A careful discrimination must, therefore, be made between apparent and true specific gravity. In general it may be said that crystalline rocks have a higher specific gravity than those of a vitreous nature. The specific gravity is, therefore, largely dependent upon chemical and crystallographic properties; for instance, among siliceous rocks those which contain the largest amount of silica are the lightest, while those with a comparatively small amount, but rich in iron, lime, and magnesia, are heaviest. =30. Chemical Composition of Rocks.=—Rocks are often classified with respect to the chief mineral constituent which they contain. Rocks which are composed largely of lime are termed calcareous; of silica, siliceous; of iron, ferruginous; and of clay, argillaceous. In respect of eruptive rocks, it is customary to speak of those which show above sixty per cent of silica as acidic, while those containing less than fifty per cent of silica and a correspondingly larger amount of iron, lime, and magnesia, are spoken of as basic. Illustrations of the classification of rocks on the above principles are given below.[23] STRATIFIED ROCKS. ──────────────────────────┬──────────────┬───────────────────────────── Kind. │ Specific │ Composition. │ Gravity. │ ──────────────────────────┼──────────────┼───────────────────────────── Calcareous: │ │ Compact limestone │ 2.6 to 2.8 │ Carbonate of lime. Crystalline limestone │ „ │ „ │ │ Compact dolomite │ 2.8 to 2.95 │ Carbonate of lime and │ │ magnesia. Crystalline dolomite │ „ │ „ │ │ Siliceous: │ │ Gneiss │ 2.6 to 2.7 │ Same as granite. Siliceous sandstone │ 2.6 │ Mainly silica. Schist │ 2.6 to 2.8 │ 60 to 80 per cent silica. │ │ Argillaceous: │ │ Clay slate (argillite) │ 2.5 │ Mainly silicate of alumina. ──────────────────────────┼──────────────┼───────────────────────────── │ │ ERUPTIVE ROCKS. ──────────────────────────┬──────────────┬───────────────────────────── │ Specific │ Per cent silica. │ Gravity. │