ROCKS. Regarded as a whole, the earth, so f as we can examine it first baud and this know it intimately, consists of three envelopes: (1) an outer gaseous envelope called the at mosphere; (2) a middle aqueous envelope called the hydrosphere; (3) an inner solid envelope called the lithosphere. These en. velopes, considered together, including the lithosphere to a depth of some 10 miles below the sea-level, have been shown to consist quanti tatively of the following elementary substances: Oxygen 47.07 Potassium 2.43 Silicon 28.06 Sodium 2.43 Aluminum 7.90 Magnesium 2.40 Iron 4.43 Calcium 3.44 Total 98.18 All the remain' ing elements together make up less than 2 per osnt of the earth.
The above-named elements rarely occur in the uncombined state in the lithosphere, but are usually united chemically in different pro portions to form those more or less definite chemical compounds called minerals (for ex ample, quartz, SiO,; calcite, CaO.CO,; olivine (MgFe).2SiO.; and minerals in turn are ag gregated in various ways to form rocks. A rock may be defined as any accumulation or aggregation of one or more mineral species or substances, which constitutes a considerable component of the lithosphere; for example, granite, sandstone, limestone, clay, gravel, salt beds. Although the crust of the earth or lithosphere has been shown to contain many hundreds of species of minerals, nevertheless the great bulk of it is composed of compara tively few species, and these have been termed the rock-forming minerals, in contradistinction to the great multitude of mineral species found more particularly in veins, and which, though of great importance, economically, and of pri mary interest, scientifically, are insignificant in amount when compared with the great mass of the more common minerals which go to make up a large portion of the earth. Thus gold, silver, platinum, the sulphide ores and the minerals containing such rare substances as thorium, uranium or radium, are negligible in quantity when compared with the mass of minerals composing the lithosphere.
The science of rocks, in its broadest scope, which considers them in all their relations, is called petrology or lithology, while the system atic and descriptive science of rocks, leading snore particularly to their nomenclature, is called petrography. This distinction is implied
in the etymology of the words. The latter term is used commonly in connection with the micro scopic study of the igneous rocks.
Those minerals which are of especial impor tance as rock-formers can be grouped as fol lows: Quartz.— This group, for present purposes, may be limited to a single species, quartz, which is one of the most abundant of minerals, and is remarkable otherwise for its rather superior hardness and lack of cleavage. It is the most indestructible of all the common rock-making minerals. It has strong crystallizing powers and, where allowed to grow intact, forms those beautiful hexagonal crystals with which all are familiar. Chemically it is an oxide of silicon,' SiO,, that most abundant of all acid radicals, which, when combined with the bases alumina, lime, iron, magnesia and the alkalies, soda and potash, in different ways, forms that most im portant of all classes of mineral substances, the silicates.
Feldspars.— These are the most abundant of all rock-formers. They comprise a rather complex group of silicates, where alumina, the alkalies, potash and soda, and the alkaline, earth, lime, are the bases. On a chemical and crystallographic basis they fall naturally into two series, the orthoclase and the plagioclase. Orthoclase is the type-species of the series. It is a silicate of potash and alumina having the chemical composition ICAISLO., and is other wise known as potash feldspar. It belongs to the monoclinic system of crystallization. A part of the potash of orthoclase may be re placed by soda. In fact most of the analyses of the mineral show only 1 per cent or more of the substance. This replacement of potash by soda can take place until these two constituents are present in nearly equal proportions without altering the monoclinic character of the species. Such an orthoclase is called soda-orthoclase.