; making the contour line of the Falls to-day about 2,900
feet.

By reason of that shortening of that Fall, two scientific questions are
brought up in regard to those deposits of gypsum, or "Petrified Spray
of the Falls."

First--to what extent has that concretion formed behind the falling
water? Has it formed there in greater quantities than it has where the
face of the cliff has been open to the air? In greater quantities might
have been expected, on account of the greater amount and absolute
continuity of the moisture on the rocky face. The 400 feet length of
cliff, from which the waters have now been permanently shut off,
furnishes the answer. Practically, none of that concrete has ever
accumulated in the crevices of the rock on the face of the cliff
immediately behind the Falls. The currents of air, and the furious
blasts of water which they create, rush constantly away from the under
surface of the falling sheet, and continuously against the face of the
cliff. These scour and cut away the rock, even as a sand blast would
do, though more slowly. They allow no chance for deposits. The strata
of the Clinton Formation (which commences at about the level of the
water in the Gorge, and of the Niagara shale, which overlie it--the two
combined having a depth of about eighty feet) are eaten away the
faster. The eighty-feet-deep layer of Niagara Limestone, which overlies
the shale, being harder, is eaten away slowly; its lower layers being
attacked by the winds and waters from below (as the underlying shale
disappears) and also on its face, yielding faster than the upper ones.

[Illustration: AMERICAN FALL--NIAGARA. CAVE OF THE WINDS IS
BEHIND SMALL FALL.]

That this concretion has always formed in the limestone, back from the
face of the cliff, behind the falling sheet (where the blasts of wind
and water cannot reach nor effect it) is certain.

That it forms under the river bed, and back from the face of the gorge
on both sides of the river, and wherever the water percolates through
the upper layer of rock, is also certain.

It is so found in the limestone (but not in the shale) wherever deep
excavations have been made near the river in the vicinity of the Falls
and wherever tunnels have been driven through the limestone--in the
crevices and especially where a pocket or hollow space exists in that
formation.

This process of eating away the lower rocks, undermining the upper
limestone, which, as its suppor