Crystalline Rocks

Crystalline Rocks, a name given to all rocks having a crystalline structure. The crystalline texture may either be original or superinduced. Thus some crystalline rocks, such as certain calcareous masses, owe their origin to chemical precipitation from water, while others again, such as lavas, have consolidated from a state of igneous fusion. There is another large class of crystalline rocks, the crystalline granules of which present a remarkable foliated character—that is, they are arranged in more or less parallel layers (see SCHISTS). This peculiar schistose structure appears to have been superinduced—the original rocks having been either fragmental or crystalline or both—and the result of great heat and pressure. Such highly altered rocks occur in the neighbourhood of masses of granite, and cover wide regions, where there is abundant evidence to show that the strata have been subjected to enormous compression, crushing, and crumpling—having been folded and fractured and pushed violently over each other for distances of sometimes 15 miles and more. It is therefore believed that pressure and the heat engendered by great earth-movements, and the intrusion of plutonic igneous matter, are among the most potent agencies in the production of schistose structure.