levelling and abrading action of water on rock has an entirely
different character. Tides or currents driven powerfully and constantly
against a rocky shore, and bringing with them hard materials, may
produce blunt, smooth surfaces, such as the repeated blows of a hammer
on stone would cause; but they never bring it to a high polish, because,
the grinding materials not being held steadily down, in firm, permanent
contact with the rocky surfaces against which they move, as is the case
with the glacier, but, on the contrary, dashed to and fro, they strike
and rebound, making a succession of blows, but never a continuous,
uninterrupted pressure and friction. The same is true of all the marks
made on rocky shores against which loose materials are driven by
water-currents. They are separate, disconnected, fragmentary; whereas
the lines drawn by the hard materials set in the glacier, whether light
and fine or strong and deep, are continuous, often unbroken for long
distances, and rectilinear. Indeed, we have seen[A] that we have beneath
every glacier a complete apparatus adapted to all the results described
above. In the softer fragments ground to the finest powder under the
incumbent mass we have a polishing paste; in the hard materials set in
that paste, whether pebbles, or angular rocky fragments of different
sizes, or grains of sand, we have the various graving instruments by
which the finer or coarser lines are drawn. Not only are these lines
frequently uninterrupted for a distance of many yards, but they are also
parallel, except when some change takes place in the thickness of the
ice, which may slightly modify the trend of the mass, or where lines in
a variety of directions are produced by the intermittent action of
separate glaciers running successively at different angles over the same
surfaces. The deeper grooves sometimes present a succession of short
staccato touches, just as when one presses the finger vertically along
some surface where the resistance is sufficient to interrupt the action
without actually stopping it,--a kind of grating motion, showing how
firmly the instrument which produced it must have been held in the
moving mass. No currents or sudden freshets carrying hard materials with
them, even moving along straight paths down hill-sides or
mountain-slopes, have ever been known to draw any such lines. They could
be made only by some instrument held fast as in a vice by the moving
power. Something of th