since filled up with rubbish varying from ten to fifty feet.
But when we inquire into the proofs of the mass having risen or fallen
suddenly on the one side of these great rents, several hundreds or
thousands of feet above or below the rock with which it was once
continuous on the other side, we find the evidence defective. There are
grooves, it is said, and scratches on the rubbed and polished walls,
which have often one common direction, favoring the theory that the
movement was accomplished by a single stroke, and not by a series of
interrupted movements. But, in fact, the striae are not always parallel
in such cases, but often irregular, and sometimes the stones and earth
which are in the middle of the fault, or fissure, have been polished and
striated by friction in different directions, showing that there have
been slidings subsequent to the first introduction of the fragmentary
matter. Nor should we forget that the last movement must always tend to
obliterate the signs of previous trituration, so that neither its
instantaneousness nor the uniformity of its direction can be inferred
from the parallelism of the striae that have been last produced.

When rocks have been once fractured, and freedom of motion communicated
to detached portions of them, these will naturally continue to yield in
the same direction, if the process of upheaval or of undermining be
repeated again and again. The incumbent mass will always give way along
the lines of least resistance, or where it was formerly rent asunder.
Probably, the effects of reiterated movement, whether upward or
downward, in a fault, may be undistinguishable from those of a single
and instantaneous rise or subsidence; and the same may be said of the
rising or falling of continental masses, such as Sweden or Greenland,
which we know to take place slowly and insensibly.

_Doctrine of the sudden upheaval of parallel mountain-chains._--The
doctrine of the suddenness of many former revolutions in the physical
geography of the globe has been thought by some to derive additional
confirmation from a theory respecting the origin of mountain-chains,
advanced in 1833 by a distinguished geologist, M. Elie de Beaumont. In
several essays on this subject, the last published in 1852, he has
attempted to establish two points; first, that a variety of independent
chains of mountains have been thrown up suddenly at particular periods;
and, secondly, that the contemporaneous chains thus t