2. Metamorphic rocks, formed from rocks originally sedimentary, by subterranean heat in presence of water. Usually crystalline, that is, composed of more or less distinct (large or minute) crystals of one or several of the minerals mentioned above.
3. Eruptive rocks, ejected in the molten state from volcanoes or fissures; crystalline or not, according to slow or rapid cooling.
Sedimentary Rocks.—Sedimentary rocks are forming to-day by deposition from either sea or fresh water, precisely as they were in the most remote geological times; the oldest clearly sedimentary rocks being sometimes undistinguishable in their nature and composition from the very latest immediately preceding our present time. They may for the purposes of the present work be simply classified as follows:
1. Limestones, formed in comparatively shallow seas, or fresh water basins, from the calcareous shells or skeletons of various organisms.
2. Sandstones, and conglomerates (sometimes called pudding-stones) formed from the debris of pre-existing rocks disintegrated by the agencies described above, ([chap. 1-2]), re-cemented by means of solutions of one or several substances, such as silex, carbonate of lime, ferric hydrate and others. Loose sands and gravels are the initial stages of such rock formation as well as the results of their disintegration.
3. Clays, Claystones and Clay shales, consisting of clay substance with more or less sand, and soft or hard according to the nature of the waters or solutions that may have acted upon them, with or without the aid of heat. These rocks can only be formed in comparatively quiet or “back” waters, since clay would not ordinarily be deposited in moving water.
Metamorphic Rocks.—The effects of subterranean heat or metamorphism upon the sedimentary rocks may be roughly stated as follows:
Limestones are transformed into marbles of various degrees of purity, according to the nature of the original rocks.
Sandstones when cemented by silex are transformed into quartzite, of greater or less purity according to the nature of the “sand” entering into its composition. When cemented by materials other than quartz, these also will be segregated in the form of various minerals in the body of the rock.
The clay rocks form the most varied products under the influence of (aqueo-igneous) metamorphism; granites, gneiss, syenite and hornblendic schist are among the most common. The great variations in the composition of clayey materials account for the correspondingly great variations in the nature of the resultant metamorphic rocks.