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.
The Cambrian formations have not been notably metamorphosed, except in a few regions where dynamic metamorphism has been effective.
A Treatise on Metamorphism (1903) and several works with other authors on the different iron regions of Michigan.
It has been variously attributed to metamorphism, consequent upon igneous intrusion, earth movements and other kinds of geothermic action, greater or less loss of volatile constituents during the period of coaly transformation, conditioned by differences of permeability in the enclosing rocks, which is greater for sandstones than for argillaceous strata, and other causes; but none of these appears to be applicable over more than limited areas.
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.
These systems are separated one from another by unconformitics in most places, and the lower systems, as a rule, have sufferetl a greater degree of metamorphism than the upper ones, though this is not to be looked upon as a hard and fast rule.
In that region the Silurian rocks have been invaded by large bosses of granite and have undergone a variable amount of metamorphism which has in some places altered them into hard crystalline schists.
In the Southern Uplands, owing to the greater softness and uniformity of texture of the rocks, rock-tarns are comparatively infrequent, except in Galloway, where the protrusion of granite and its associated metamorphism have reproduced Highland conditions of rock-structure.
To the east of the dislocation of the Great Glen these puzzling rocks may also be met with, though in that tract most of the surface comprises sedimentary and igneous rocks, the metamorphism of which has varied much.