Flow-structure is exhibited by the parallel arrangement or orientation of minerals in lines which indicate the direction in which the fluid rock had been moving before solidification took place, or while it was in a viscous state.
Classification of Igneous The classification of igneous rocks as at present widely accepted is based upon three things: (1) chemical composition; (2) mineralogical composition; (3) texture. (The proposed 'Quantitative Classification of Igneous Rocks,' published jointly by Cross, Iddings, Pirsson and Washington, Chicago 1903, is the most elaborate' attempt at a thoroughly scientific classification of the igneous rocks yet proposed. It has not, however, as yet received general acceptance).
Chemical chemical com position of igneous rocks is primarily ex pressed in terms of silica percentage. Rocks which contain from 60 to 80 per cent of silica are called acid. Most of these rocks contain free quartz; those which range from 40 to 60 per cent in silica are called basic and rarely possess free quartz in any considerable amount; while those which fall below 40 per cent are termed very basic, and practically never contain original free quartz. Generally speaking, as the silica percentage declines the percentage of the bases (lime, iron, magnesia) increases. This change in chemical composition from acid to very basic is indicated by change in color, the acid rocks being light ashy gray, while the very basic are dark.
Mineralogical Composition.— Quartz is more or less abundant in rocks of the add di vision, but soon disappears in passing toward the basic end of the series. Orthoclase is the dominant feldspar among the acid rocks, but even here the plagioclases are more or less common. Among the basic rocks the plagio clases prevail, and in a general way change from the albite to the anorthite variety as the more basic rocks are approached, but fail en tirely in most rocks of the very basic division.
The ferromagnesian minerals are subor dinate, almost negligible, in the very acid rocks. They become dominant in the basic and con stitute the bulk of the very basic rocks.
The ores, chiefly magnetite, chromite and ilmenite, are found more or less sparsely dis seminated through all three divisions, but they become notable constituents among the basic and very basic rocks and may even segregate to form important bodies of ore.
Texture.—As already explained, magmas which solidify deep within the lithosphere usually form coarse (to fine) evenly-granular rock-masses, having the granitoid texture, al though they may be more or less porphyritic or idiomorphic in character. These deep-seated rocks are railed plutonic. Where eruption takes place those rocks winch solidify somewhere be tween the magma-basin and the surface (as intrusion-sheets, dikes, laccolites, etc.) are called intrusive, while those which cool subaerially (or it may be beneath bodies of water) are called effusive or extrusive. While neither the plutonic, intrusive nor extrusive members are limited to any one kind of texture, it is true that the plutonic rucks are more frequently granitoid, and the intrusive and extrusive mem bers even-fine-grained, felsitic, asphanitic or porph3ritic in texture, while the textureless vol canic glasses are nearly always extrusive.
Rock-families: I. Acid Division; Granites. — The granites are plutonic rocks occurring fre quently as huge irregular masses which have slowly melted their way up from some deep seated region into the more ancient and then deeply-buried portions of the lithosphere; or they may occur as apophyses or dikes given off from some parent mass. Mineralogically they consist of quartz, dominant feldspar of the orthoclase variety (with subordinate amounts of soda-orthoclase, microcline or oligoclase), and muscovite or biotite mica, or both. Typical granite has muscovite. Where biotite alone is present it is called granitite. Where horn blende or tourmaline replaces the mica, the rick is called hornblende- or tourmaline-granite. Where feldspar fails and only quartz and mica remain, the rock is called greisen. As access ory constituents might be mentioned, zircon, allanite, cassiterite, rutile, titanite, magnetite. By a gradual decrease of quartz the granites pass inperceptibly into the syenites (to be de scribed later), and by similar changes into the diorites.