IGNEOVS ROCKS are those which have been produced from materials fused by heat. They differ from the sedimentary rocks in their origin. structure, and position. They invariably come from below upwards, breaking through the older rocks. The materials .rof sedimentary strata are fragments of pre-existing rocks, worn, by the action of water, either into a fine mud or into rounded particles, of greater or less size; whereas igneous rocks exhibit either a vitreous structure, as when they have been quickly cooled; or a granular structure, composed of more or less minute crystals, according to the rate of cooling or a vesicular structure, when they have been expanded by the contained gases, or by being brought into contact with water. Some rocks are erroneously called igne ous, whose materials, though originally obtained from volcanoes or other subterranean source, have yet been ultimately arranged by water, like the materials of Grahame's island (q.v.). When this fact receives due consideration, many igneous rocks, whose position is now a puzzle, will be better understood. Some of the rocks composing Arthur's seat, near Edinburgh, are undoubtedly of this character, and before a right theory of the ldll can be constructed, these must be separated from the truly igneous rocks. In position, also, the igneous may be distinguished from time sedimentary rocks, for they seldom occur regularly stratified, in with a parallel upper and under surface, but are generally local, thinning out into wedge-shaped beds, or having that irregular strati fication which may be seen n modern lava. They also occur as upright, walls or dikes,
filling up cracks in the seditnenta•y" strata The most satisfactory classification of the igneous rocks is based upon their age. The three divisions thus established are each characterized by peculiar mineral and structural differences. The oldest or granitic series (see GRAN1TE) are generally associated with the paleozoic strata, but are sparingly found in the secondary, and even in the tertiary formations. The special peculiarity of the granitic rocks is the great abundance of silica contained in them; it forms not only a considerable amount, of the constituents of the hornblende and feldspar, but crystallizes free in the rock-mass as rock crystal. The trappean rocks (q.v.) are associated with the paleozoic and secondary strata, and are composed of crystals of feldspar and hornblende, varying in their character according to the predominance of the one or other of these ingredients. The volcanic (q.v.) are the newest igneous rocks; they belong to the present period, or the tertiary strata. The chemical ingredients are the same as those that constitute the trappean rocks, but somewhat differently built up, augite being the peculiar form the silicate of mag nesia and lime assumes in the newer rocks, while it appears as hornblende in the older or trappean series.