This term applies to the reconstruction of rocks upon a great scale, in areas covering, it may be, thousands of square miles, and evidently other processes in addition to those of contact metamorphism are needed to explain such widespread changes. Regionally metamorphic rocks are, with the exception of the slates, thoroughly crystalline and usually have lost all trace of whatever fossils and stratification planes they may originally have had.

The first step in metamorphism consists in a mere hardening of the rock, accompanied with the loss of water and other volatile substances. In the second stage the component minerals already present are crystallized, but new compounds are sparingly formed. This stage is frequently accompanied by cleavage, which, to distinguish it from that of minerals, is often called slaty cleavage.

Fig. 223. - Oblique synclinal fold in slate, showing cleavage planes at all angles to the bedding-planes. (U. S. G. S).