The original mineral components of igneous rocks and the essential part of metamorphic schists are quartz, plagioclase, orthoclase, augite, hornblende, muscovite, biotite, rock glass, magnetite and garnet. These are called primary minerals. Rock glass, included among them, is a mineral of variable composition found in certain volcanic rocks which cooled very rapidly. It is extremely brittle and when present in appreciable quantities has a tendency to lower the wearing properties of the rock. Orthoclase and plagioclase are usually called "feldspar" by engineers and biotite and muscovite are called "mica."

Quartz is the most widely distributed mineral known. It has a specific gravity of 2.66 and a hardness of 7 in Mohs' scale.1 When present in large quantities, especially when finely consolidated, as in fine-grained, igneous and massive metamorphic rocks, the resulting material is extremely hard and offers great resistance to wear.

Orthoclase and plagioclase are among the principal ingredients of igneous and metamorphic rocks and some sandstones. Their specific gravity is 2.54 to 2.76 and their hardness 6 to 6.5. Many coarse-grained feldspathic rocks break down readily under impact on account of the cleavage of these minerals. In finegrained rocks the effect of this cleavage is less marked, and some of them are extremely hard and tough.

Augite and hornblende are the chief iron-bearing or dark silicate constituents of basic igneous rocks, commonly called "trap rocks," and the crystalline schists derived from them. Their specific gravity is 2.93 to 3.71 and their hardness 5 to 6.5. Their crystalline shape is such that they interlock very compactly with other minerals, which is one of the reasons for the marked durability of trap rocks.

Biotite and muscovite occur chiefly in granite, gneiss and micaceous schist. Their specific gravity is 2.7 to 3.2 and their hardness is 2 to 3. The flaky character of mica is well known and is largely responsible for the foliated character of many metamorphic rocks and their resulting inferior wearing properties in roads. Magnetite has a specific gravity of 5.18 and a hardness of 5.5. Garnet has a specific gravity of 3.15 and a hardness of 7.5. They occur in only two road-building rocks, peridotite and eclogite, and in some cases materially increase the wearing properties of the rock.

Secondary minerals are produced by the alteration of rocks, mainly by the chemical action of water and carbonic acid on primary rock constituents. The chief secondary minerals are

1 Mohs' scale of the relative hardness of minerals is as follows: 1, Talc; 2, gypsum; 3, calcite; 4, fluorite; 5, apatite; 6, orthoclase; 7, quartz; 8, topaz; 9, corundum; 10, diamond.

calcite, dolomite, kaolin, chlorite, epidote, limonite, serpentine, talc, zeolite and opal.

Calcite has a specific gravity of 2.6 and a hardness of 3. Dolomite has a specific gravity of 2.9 and a hardness of 3.5. These two minerals are the chief constituents of limestones and dolomites, and cannot be distinguished microscopically. They cleave freely and hence many calcareous rocks break down readily when used in roads.

Kaolin is derived to a large extent through the decomposition of orthoclase. It sometimes occurs in small crystal flakes resembling white mica (muscovite), and sometimes as minute grains of very indefinite composition. In the latter form, called 'amorphous," kaolin has a great effect on the binding property of rock powders, for it becomes glue-like when wet, and when dry it binds together firmly the other mineral particles with which it is associated.

Chlorite and epidote are derived from augite, hornblende, biotite and plagioclase and are most abundant in trap rocks and dark crystalline schists. Chlorite is a soft green mineral which occurs either in mica-like flakes or as very fine scales and fibers of indefinite composition. In the latter form it has cementing properties like those of amorphous kaolin. Epidote (specific gravity, 3.25-3.5; hardness, 6-7) occurs as yellowish green crystals which, when present in appreciable quantities, apparently increase the wearing properties of rocks.

The results of a study of several hundred road-building rocks indicate that the effects of their mineral composition on their value for highway purposes are probably as follows:

Igneous and nonfoliated metamorphic rocks, owing to a preponderance of hard silicate minerals combined with greater uniformity in structure, are more durable than other road-making materials, finer-grained varieties offering greater resistance to abrasion than coarse-grained types.

The resistance to wear of igneous and metamorphic rocks, containing an abundance of quartz, hornblende, augite, epidote, and garnet, is greater than that of similar rocks rich in mica, chlorite, serpentine, and calcite.

Foliated metamorphic rocks, owing to the parallel arrangement of their mineral constituents, are, as a rule, deficient in toughness and therefore not well adapted to road construction.

Sedimentary rocks are usually deficient in wearing properties, except in the case of highly indurated sandstones, containing a moderate amount of siliceous clay, cement, and limestones or dolomites rich in quartz and having very little clay.

Rocks for road making break down under impact into fragments, the shape and physical character of which are conditioned by mineral composition and structure.