A Treatise on Metamorphism (1903) and several works with other authors on the different iron regions of Michigan.
Extensive subsequent metamorphism has been produced by the invasion of great masses of granite.
They are typical products of "regional" metamorphism, and are in nearly all cases older than the fossiliferous sedimentary rocks.
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.
The Cambrian formations have not been notably metamorphosed, except in a few regions where dynamic metamorphism has been effective.