s--Their effects on the
bottom when they run aground--Packing of coast-ice--Boulders drifted
by ice on coast of Labrador--Blocks moved by ice in the Baltic.

The power of running water to carry sand, gravel, and fragments of rock
to considerable distances is greatly augmented in those regions where,
during some part of the year, the frost is of sufficient intensity to
convert the water, either at the surface or bottom of rivers, into ice.

This subject may be considered under three different heads:--first, the
effect of surface-ice and ground-ice in enabling streams to remove
gravel and stones to a distance; secondly, the action of glaciers in the
transport of boulders, and in the polishing and scratching of rocks;
thirdly, the floating off of glaciers charged with solid matter into the
sea, and the drifting of icebergs and coast-ice.

_River-ice._--Pebbles and small pieces of rock may be seen entangled in
ice, and floating annually down the Tay in Scotland, as far as the mouth
of that river. Similar observations might doubtless be made respecting
almost all the larger rivers of England and Scotland; but there seems
reason to suspect that the principal transfer from place to place of
pebbles and stones adhering to ice goes on unseen by us under water. For
although the specific gravity of the compound mass may cause it to sink,
it may still be very buoyant, and easily borne along by a feeble
current. The ice, moreover, melts very slowly at the bottom of running
streams in winter, as the water there is often nearly at the freezing
point, as will be seen from what will be said in the sequel of
ground-ice.

As we traverse Europe in the latitudes of Great Britain, we find the
winters more severe, and the rivers more regularly frozen over. M.
Lariviere relates that, being at Memel on the Baltic in 1821, when the
ice of the river Niemen broke up, he saw a mass of ice thirty feet long
which had descended the stream, and had been thrown ashore. In the
middle of it was a triangular piece of granite, about a yard in
diameter, resembling in composition the red granite of Finland.[283]

When rivers in the northern hemisphere flow from south to north, the ice
first breaks up in the higher part of their course, and the flooded
waters, bearing along large icy fragments, often arrive at parts of the
stream which are still firmly frozen over. Great inundations are thus
frequently occasioned by the obstructions thrown in the