This section deals mainly, with petrological terminology and is intended primarily for the nongeologist. It is impossible to cover this subject adequately in a short space, and the reader is referred to standard textbooks on petrography for further details (1, 2, 3, 5). The problem of terminology is complicated by the fact that many samples which have been used for the determination of physical properties of rocks have come from quarries, and terms peculiar to quarrymen have crept into the literature. Since rocks are mixtures of different phases, they can, at least in principle, have a continuous range of composition and of physical properties. Any scheme of classification demands that this potentially continuous series be fitted into more or less distinct and arbitrary compartments. Three main classes of rocks are recognized: igneous, metamorphic, and sedimentary. Igneous rocks are thought to be in large part the products of crystallization of a silicate melt. Sedimentary rocks result from deposition of material by wind or water at the earth's surface and its later solidification into rock. Metamorphic rocks result from the recrystallization of igneous or sedimentary rocks in the solid state at relatively high temperatures and pressures. Much of the early effort of petrographers went into the classification of the igneous rocks. The result was a bewildering list of categories, most of which contained rocks of unusual compositions. Only the principal types of rock are mentioned here. The igneous rocks may be divided into plutonic and volcanic types. Plutonic rocks crystallized at depth