their
schistose surfaces like the pleats of a gown. Plication may not imply
flowage. Schistose micaceous scaly structure, with myriads of small
scales imbricating and dove-tailed together in a sensibly flexible
texture, creates an accommodating lithic material which might not
inaptly be compared to minute mail armor. Compression would here
induce a slipping over each other of the component scales, and it is
further conceivable that if the compression occurred at the time when
the scales were forming, it would enormously facilitate the process of
almost indefinite mobility, so that, without fracture and without
flowage, the rock would become pliable. Even a quartzite might in this
way, along innumerable microscopic slip planes, assume complete
curvature. In such a process of accommodation the steps would be very
gradual, and the time consumed in folding a low and broad area of
sediments would be very considerable. The evidence of this slowness is
furnished in the small linearly extended wavelets of plication that
can be seen in the mica-schist of Manhattan Island. These were probably
deeply seated. There were doubtless also axes of maximum plication,
where the strata were squeezed into steeper and longer folds; such axes
would assert themselves along the less resistant beds, or even where
there was the deepest accumulation of deposits, and so more pressure
and more material for unlift and compression. The minor folds evolved
over surfaces a few square feet in width must be distinguished from the
anticlinals which carry the series of beds up in almost vertical sheets
(well seen in the excavation of the Pennsylvania Railroad, Fig. 2). The
examination of the folded and contorted strata of Manhattan Island is
well calculated to impress and instruct the observer.
By
way of a supplement to the foregoing pages, the reader may be
interested in a quite contrasted theory proposed by Dr. T. Sterry Hunt
(a very distinguished investigator in the physics and chemistry of our
earth) on crystalline schists,* which he called crenitic, and by which he intended, and ex-
♦Nature, Vol. xxxvii, p. 519; also Dr. Hunt's book, "The Schists."
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