is
noted as almost unchanged from its original condition in some places
and frequently metamorphosed in others. This action (here described as
chemical and mechanical) has been carried so far that included pebbles,
and even boulders, have been mashed into disk-like plates, and
the rocks, by re-crystallization, converted into a gneiss. The origin
of the gneiss would be indeterminable except by a study of the gradual
passage of the conglomerate to the gneiss.
Metamorphism
is a chemico-physical process. Stress of mass against mass strains, the
tearing apart through torsion or shearing will bring about molecular
alterations in identical chemical compounds; it might change a pyroxene
into an amphibole. It (metamorphism) means, also, introduction of new
chemical elements or additions of chemical elements already present.
It means developments of crystals, growth of crystals or crystalline
fragments; it generally means recemen-tation and densification.
Metamorphic rocks are crystallized rocks, rocks also usually laminated,
rocks compressed and having parting planes, cleavage, or fissile, in
thin sheets; sandstones can thus become quartzitic schists, dividing
up into parallel slabs, and igneous massive rocks banded
schists or gneisses. In metamorphic rocks there is a prevalent
parallelism of crystals, which, of course, helps division into
parallel planes; apparently a granite may be smashed and remade,
through plasticity, into a granite-gneiss. Compression or pressure
seems indispensable in metamorphism, its most critical agency; the
irruption of igneous rock bringing heat, mineralizing vapors and
fluids, must also, most sensibly, develop pressure.
The geologist Chamberlin has defined an anamorphic and a katamorphic metamorphism,
the former a constructive, the latter a destructive metamorphism. By
anamorphic metamorphism fragmental sediments are made into crystalline
rocks; by katamorphic metamorphism massive rocks, like granite or
basalt, are crushed down into foliated forms.
The common minerals of metamorphic rocks are feldspars,
