ZONES OF ROCK STRAIN
In
the contraction, or assumed contraction, of the earth's crust, the
stupendous strains developed will affect different rocks in a different
way as they are brittle or tough, thickly bedded or in thin layers, and
it will also affect them differently as they are removed at
comparatively great distances from the surface. Van Hise, who has
developed this principle, assumes that at a depth of 30,000 feet below
the earth's surface the overlying pressure is so great that the rocks
cannot break or crack under compression, but that they How, that their particles adjust themselves to new positions of stability under the strain, by movement, that
indeed they behave like soft or mobile bodies, like wax, for instance.
Such movements must be infinitesimal, but as every component molecule
of the rock shifts its position to accommodate the stress, the whole
mass of rock has been welded and moulded like a plastic body. This is
called the Zone of Flowage.
Above
this zone the pressure is relieved or diminished, and the stronger rock
layers will break or crack when the thrusts of compression take place;
they will not then How, while the weaker, less resistant rocks
will flow, and the resultant mixture of effects, where some rocks
yield without losing continuity, and other rocks break, represents the
so-called Zone of Fracture and Flowage.
Above
this zone again, and now in closer proximity to the surface, all the
rocks break, and faults, joints, crevices, are formed, shatterings
occur, crushings and the familiar results of smashing and even
pulverization. This is the Zone of Fracture.
The
rocks of New York Island suggest a complementary hypothesis. They have
not been in the zone of flowage, perhaps very near it; they have
participated in the strains of the zone of fracture, but they have been
bent and folded, crumpled, and thrown into rippling undulations, which
may be seen on
