They are typical products of "regional" metamorphism, and are in nearly all cases older than the fossiliferous sedimentary rocks.
Extensive subsequent metamorphism has been produced by the invasion of great masses of granite.
Not only is it extensive in area, but the stratigraphic break is very great, as shown by (I) the excess of metamorphism of the lower group as compared with the upper, and (2) the amount of erosion suffered by the older group before the deposition of the younger.
Tremolite-schist and anthophyllite-schist are in nearly all cases the representatives of the ultra-basic igneous rocks such as peridotite in regions of high metamorphism.
Archeozoic (Archean) Group.The oldest group of rocks, called the Archean, was formerly looked upon, at least in a tentative way, as the original crtist of the earth or its downward extension, much altered by the processes of metamorphism.
