Igneous rocks formed at relatively high temperatures of approximately 650° to 1200°C and sediments deposited at the earth’s surface represent extreme ends of the temperature range realized in the processes of rock formation. In the course of later geological events such rocks may become part of a region in the earth’s crust where intermediate temperatures prevail; thus they are subjected to different temperatures. Similarly, the pressure of their new environment will, in general, differ from the pressure existing at their formation. Many minerals in these rocks are no longer stable at the newly imposed conditions of temperature and pressure; they will react and form mineral assemblages in equilibrium, or tend toward equilibrium, at the new conditions. Accordingly, the chemical composition of a rock is expressed by a new mineral assemblage; it has been transformed—for example, the conversion of clay or shale to micaschist.