Publisher Summary The classification of igneous rock bodies in terms of their form and structural relationship to adjacent rocks is discussed in this chapter. These aspects are the most relevant to the appearance of igneous rocks on geological maps. Igneous rocks are formed by the solidification of molten rock or magma. They are described separately because they come into position in a way that is quite different from the orderly “layer cake” manner typical of sedimentary rocks.. They are classified as either intrusive igneous rocks or extrusive igneous rocks. Intrusive igneous rocks are bodies formed by the injection of magma into an existing sequence of rocks. They can be concordant bodies or discordant bodies, for example, sills, dykes, volcanic necks, and batholiths. Extrusive igneous rocks are formed from the products of volcanoes.