the
invaded beds. Granites appear in the rocks of Manhattan Island, and are
referred to igneous protrusions, though it is not inconceivable that
minor veins and tracks of so-called peg-matized gneiss have resulted
from a refuson of the metamor-phic gneiss in the development of heat
from frictional movement, under stress and plication, upheaval,
distortion or compression, and through the action of included water.
In this way the gneiss became saturated with granitic lenses, fillings
and streaks drawn out in parallelism with the enclosing gneissoid
envelopes, upon the folding or elevation, under pressure, of the
entire complex. Some granite veins suggest segregation or
water-filling.
The
crystalline rocks of Greater New York have been deeply compressed, and
their original horizontal extension may have undergone a contraction of
more than a mile, which was the result of crustal shortenings over the
whole of the earth's sphere. Of course the effect of such a contraction
would be to throw up the more or less horizontal beds into hills or
mountains of inclined strata, bringing about at the same time internal
mineral changes and structural changes connected with the metamorphism
of the beds. (See section on Meta-morphism.) Such horizontal
contractions have been considerable in the geological history of the
earth. De la Beche has shown that contorted and inclined beds would
require, if reduced to a level surface, much more room, and that if
stretched out into flat sheets they would invade adjoining areas.
Professor Heim, of Zurich, computed that the Alps, toward the north,
have been thus compressed, and the horizontal shrinkage of the
superficies of the earth at this point has been something like
seventy-four miles, or "one-half of the original horizontal extent of
the component strata, which have been corrugated and thrown back upon
each other in huge folds, reaching from base to summit of lofty
mountains " (Geikie).
Such
a contraction in the New York area has resulted in the upward
projection of high hills, or monticules, which may have attained
altitudes of over four thousand feet, and which
