slowly
achieved, but marked by intermittent periods of extreme dynamical
intensity. Such folds are seen in the Transverse Road at 79th Street
and Central Park, formerly at the east fend of Washington Bridge, while
they are shown in the most wonderful confusion at 15 2d Street and 7th
Avenue. For a block on either side the eye traverses a face of
distorted gneiss, with contracted bands of alternating feldspathic,
micaceous, hornblendic rock. The strata, as a whole, lifted up in a
line of double curvature, are standing almost " on end," slightly
inclined eastward, while subordinate wrinklings, twistings, kinks in
endless profusion, convert the surface into a " living picture " of
primary forces crumpling the earth's crust, as the hands might flex and
crush a bundle of paper cards. It seems probable that the minor small
waves of plication were produced before the final uplift came which
crowded these into up-and-down shortened bends. (Figs. 14, 15, 16.)
These
facts and suggestions bring before us the problem of the origin of this
whole group of rocks. They sharply interrogate our explanation of
their occurrence. Without starting out with the most simple assumptions
of geology, or involving this sketch in a rudimentary classification of
rocks, it is usually agreed to regard the crystalline schists to which
these gneisses of Manhattan Island belong as originally sediments, or
the accumulation of ancient muds, mingled, doubtless, with detri-tal
matter that was not mud, but sand, both of quartz and other silicates;
the whole was derived from the wear and tear, the attrition and slow
degradation of still older rocks, perhaps, in the case of this island,
those granulites and highly siliceous and ferruginous rocks which form
the Highlands.
However
accumulated, these beds of sediments represented a heavy deposit of
which silica, alumina, iron oxides, lime, magnesia,- potash, and soda,
and more rare elements were parts; and it was a change of hardening,
solidification and chemical combination which slowly ensued, and under
the auspices of certain physical conditions created these beds of
