forms of stratification led also, on a fuller investigation, to the
belief that sedimentary rocks had been slowly deposited; but it was
still supposed that _denudation_, or the power of running water, and the
waves and currents of the ocean, to strip off superior strata, and lay
bare the rocks below, had formerly operated with an energy wholly
unequalled in our times. These opinions were both illogical and
inconsistent, because deposition and denudation are parts of the same
process, and what is true of the one must be true of the other. Their
speed must be always limited by the same causes, and the conveyance of
solid matter to a particular region can only keep pace with its removal
from another, so that the aggregate of sedimentary strata in the earth's
crust can never exceed in volume the amount of solid matter which has
been ground down and washed away by running water. How vast, then, must
be the spaces which this abstraction of matter has left vacant! how far
exceeding in dimensions all the valleys, however numerous, and the
hollows, however vast, which we can prove to have been cleared out by
aqueous erosion! The evidences of the work of denudation are defective,
because it is the nature of every destroying cause to obliterate the
signs of its own agency; but the amount of reproduction in the form of
sedimentary strata must always afford a true measure of the minimum of
denudation which the earth's surface has undergone.

_Erratics._--The next phenomenon to which the advocates of the excessive
power of running water in times past have appealed, is the enormous size
of the blocks called _erratic_, which lie scattered over the northern
parts of Europe and North America. Unquestionably a large proportion of
these blocks have been transported far from their original position, for
between them and the parent rocks we now find, not unfrequently, deep
seas and valleys intervening, or hills more than a thousand feet high.
To explain the present situation of such travelled fragments, a deluge
of mud has been imagined by some to have come from the north, bearing
along with it sand, gravel, and stony fragments, some of them hundreds
of tons in weight. This flood, in its transient passage over the
continents, dispersed the boulders irregularly over hill, valley, and
plain; or forced them along over a surface of hard rock, so as to polish
it and leave it indented with parallel scratches and grooves--such
markings as are still