easily
reached example is in 110th Street, west of Columbus Avenue. (Fig. 21.)
The wrinkled and twisted lines of bedding are most extraordinary along
this section; hard, dense bands of hornblende gneiss, alternating with
a looser mica-schist, or gneiss, are folded into complicated figures of
vertically undulating streaks, and with overthrusts where the
flinty-looking hornblende gneiss is wrapped up in sheets of mica rock,
while strings of quartz, intercalations of granite, with quartz knobs
and insertions, produce an almost damasked surface in its variations
of structure. On the south side of the street the aspect of the twisted
beds is even more instructive. There are portions of the rock which
seem saturated with quartz that, in excess of all possible combinations
with bases, has been expelled in strings of crystalline nodular quartz,
while granite veins, streaks of inclusions dot and, as it were, stream
down the wavy and crumpled faces of the mica-schist. It tells in
unmistakable language of extreme compression, of the rolled-up and
smashed strata, and it seems even to express more legibly a period of
partial mineral fluidity. Not, indeed, that there is seen here flowage,
or that the lines of rock become chaotically mingled in currents and
streams, but there has been plasticity and movement and, if the
language can be pardoned, a secretive action by which the granite and
quartz have at joints, crevices, openings, pits, or loculicidal
slippings, formed within the gneiss itself.
There is, perhaps, on the island nothing more extraordinary and instructive than the sectionized hill at 152d Street
and Central Bridge, mentioned above. This exposure would repay a dozen
visits of inspection. If ever rocks spoke, they speak here.
As
demonstrating a series of disturbances, or shocks of compression at
different times, the faulted veins of granite seem significant, though,
as remarked by Mr. Stanton, these are infrequent. One in 87th Street,
not seen by me, but noted by Mr. Stanton, offered an instructive
example of faulting, and the intersecting veins seen at Mt. Tom
(Riverside Drive and
