ELEMENTS AND MINERALS OF THE EARTH'S CRUST. Relatively few of the seventy or more elements form an important percentage of the crust. Oxygen, the most abundant element of the outer portion of the earth, constitutes 86 per cent. of the ocean, 21 per cent. of the air, and 47 per cent. of the crust. Nitrogen, though form ing about three-quarters of the air, is of little importance in the ocean or the rocks. Silicon forms 27 per cent. of the crust, and aluminum 8 per cent., so that the three elements oxygen, silicon, and aluminum together constitute 82 per cent. of the crust. Next in importance are the following: iron, 5; calcium, 4; sodium, potas sium, and magnesium, each about 2.5; carbon, 0.22; hydrogen, 0.21; phosphorus, 0.1; sulphur, 0.03; and chlorine, 0.01 per cent.
These elements, combined according to definite chemical laws, form minerals. A great variety of different combinations are known, making, in all, over 2000 mineral species. Most of these are rare, and only a very few form prominent contributions to the crust. Of these common minerals, by far the most abundant is quartz, made of the two common elements silicon and oxygen. Its hardness and indestructibility make it a factor of strength in rocks. Probably next
in abundance is the group of feldspars, of which a number of different kinds are recognized. Al though hard minerals, the feldspars disintegrate in the weather, forming clay and certain soluble substances. Calcite is a third common mineral, composed of calcium, carbon, and oxygen. Being both soft and easily dissolved, calcite is an ele ment of weakness in rocks. Dolomite, the mag nesium carbonate of lime, has similar character istics to calcite. Other common rock-forming minerals are the micas, amphiboles, and py roxenes, all complex silicates of aluminum with potassium, magnesium, iron, etc. Gypsum, the hydrous sulphate of lime, and the several oxides of iron—limonite, hematite, magnetite—the car bonate of iron, siderite, and the sulphide of iron, pyrite, are other common minerals. Of these or their decayed products the great part of the rocks of the crust are made. These minerals are of high geological importance; the others are of interest especially to the mineralogist and the petrographer. See MINERALOGY; QUARTZ; FELD SPAR, etc.