HOCKS OF THE EARTH'S CRUST. Minerals, com bined in various ways, form rocks. Sometimes the combination:4 are according to definite chem ical laws; but rocks are usually mere aggregates of several minerals. A three-fold division of the rocks may he made as follows: igneous, or those derived from a molten condition; sedimentary, mainly sediments in water; and metamorphic, or those due to the alteration of other rocks by heat and pressure.
The igneous rocks vary among themselves in two characteristics. one chemical composition, the other texture, and the classification now gener ally recognized is based upon this double varia tion. From different volcanic vents, the lava differs chemically, in the one extreme being very acid, that is, with much silica; in the other being very basic, that is, with a small percentage of silica and a large percentage of the basic ele ments, iron, magnesium, potassium, sodium, etc. These chemical differences give rise to different classes of minerals—quartz and feldspar prevail ing in the acid rocks, micas, amphiboles, py roxenes, and iron oxides in the basic. Accord ing to the conditions of cooling, the igneous rocks vary in texture. Some are blown out by violent explosive expansion of steam, and, cooling quick ly, form glassy, porous pumice and volcanic In other cases flowing lava cools so rapidly that it sets without the formation of individual miner als, forming natural glass, or obsidian. More commonly the lava becomes crystalline, and is either fine-grained or has a fine ground mass inclosing large porphyritic crystals. Many ig neous masses do not reach the surface, but cool in the vent of the volcano, or, being intruded into the rocks, cool in the crust. These cool so slowly that the elements crystallize into good sized individuals, producing coarse-grained rocks like granite, syenite, etc.
The term sedimentary for the second class of rocks is not perfectly satisfactory, since not all the rocks included are sediments. The group
comprises mechanical deposits, such as conglom erate, sandstone, and clay, which are derived from preexisting rocks by the processes of dis integration and erosion, and are removed and deposited by air, water, or ice; chemical deposits, accumulated by the precipitation of materials held in solution, and including rock salt, gypsum, calcareous tufa, etc.; organic deposits, such as limestone, chalk, marl, coal, and bog iron ore, which are formed by the growth and decay of animal and plant organisms.
Either igneous, or sedimentary rocks, under the action of heat and pressure, are subjected to changes which in some cases go so far as to remake the rock entirely. This alteration, or metamorphism, sometimes takes the form of crushing, accompanied by the development of new minerals; in other cases there is a development of new minerals without noticeable crushing. This formation of new minerals may go so far as to destroy entirely all evidence of the original characteristics of the rock, as in many schists and gneisses. The new minerals naturally de velop with their long axes along the lines of least resistance, thus giving to the rocks a paral lel structure; and it is due to this feature that slates split readily in one direction, namely, parallel to the cleavage planes of the micaceous minerals. By metamorphism, also, limestone is often changed from amorphous carbonate of lime to crystalline calcite, forming marble. Sand stone is changed to dense quartzite by the deposit of silica around the grains. Coal is changed to anthracite by the expulsion of volatile substances. causing the concentration of carbon; and in some cases this metamorphism has gone so far as to produce crystalline graphite, which is pure car bon. For details as to origin, composition, and classification of rocks, see PR;TROGRAPHY; PE TROLOGY.