whole of these are often swept away by a high tide, or
when the river is swollen by the melting of the snow in Spring. One huge
block of granite, 15 feet long by 10 feet both in width and height, and
estimated to contain 1500 cubic feet, was conveyed in this manner to
some distance in the year 1837, its previous position being well known,
as up to that time it had been used by Captain Bayfield as a mark for
the surveying station.

_Ground-ice._--When a current of cold air passes over the surface of a
lake or stream it abstracts from it a quantity of heat, and the specific
gravity of the water being thereby increased, the cooled portion sinks.
This circulation may continue until the whole body of fluid has been
cooled down to the temperature of 40 degrees F., after which, if the cold
increase, the vertical movement ceases, the water which is uppermost
expands and floats over the heavier fluid below, and when it has
attained a temperature of 32 degrees Fahr. it sets into a sheet of ice. It
should seem therefore impossible, according to this law of congelation,
that ice should ever form at the bottom of a river; and yet such is the
fact, and many speculations have been hazarded to account for so
singular a phenomenon. M. Arago is of opinion that the mechanical action
of a running stream produces a circulation by which the entire body of
water is mixed up together, and cooled alike, and the whole being thus
reduced to the freezing point, ice begins to form at the bottom for two
reasons, first, because there is less motion there, and secondly,
because the water is in contact with solid rock or pebbles which have a
cold surface.[285] Whatever explanation we adopt, there is no doubt of
the fact, that in countries where the intensity and duration of the cold
is great, rivers and torrents acquire an increase of carrying power by
the formation of what is called ground-ice. Even in the Thames we learn
from Dr. Plott that pieces of this kind of ice, having gravel frozen on
to their under side, rise up from the bottom in winter, and float on the
surface. In the Siberian rivers, Weitz describes large stones as having
been brought up from the river's bed in the same manner, and made to
float.[286]

_Glaciers._--In the temperate zone, the snow lies for months in winter
on the summit of every high mountain, while in the arctic regions, a
long summer's day of half a year's duration is insufficient to melt the
snow, even on land just rai