ce stream descends into the sea or into a large lake, the
depth of which is about as great as the ice is thick, the relative
lightness of the ice tends to make it float, and it shortly breaks off
from the parent mass, forming an iceberg. Where, as is generally the
case in those glaciers which enter the ocean, a current sweeps by the
place where the berg is formed, it may enter upon a journey which may
carry the mass thousands of miles from its origin. The bergs separated
from the Greenland glaciers, and from those about the south pole, are
often of very great size; sometimes, indeed, they are some thousand
feet in thickness, and have a length of several miles. It often
happens that these bergs are formed of ice, which contains in its
lower part a large amount of rock _debris_. As the submerged portion
of the glacier melts in the sea water, these stones are gradually
dropped to the bottom, so that the cargo of one berg may be strewed
along a line many hundred miles in length. It occasionally happens
that the ice mass melts more slowly in those parts which are in the
air than in its under-water portions. It thus becomes top-heavy and
overturns, in which case such stony matter as remains attains a
position where it may be conveyed for a greater distance than if the
glacier were not capsized. It is likely, indeed, that now and then
fragments of rock from Greenland are dropped on the ocean floor in the
part of the Atlantic which is traversed by steamers between our
Atlantic ports and Great Britain.

Except for the risks which they bring to navigators, icebergs have no
considerable importance. It is true they somewhat affect the
temperature of sea and air, and they also serve to convey fragments of
stone far out to sea in a way that no other agent can effect; but, on
the whole, their influence on the conditions of the earth is
inconsiderable.

Icebergs in certain cases afford interesting indices as to the motion
of oceanic currents, which, though moving swiftly at a depth below the
surface, do not manifest themselves on the plain of the sea. Thus in
the region about Greenland, particularly in Davis Strait, bergs have
been seen forcing their way southward at considerable speed through
ordinary surface ice, which was either at rest or moving in the
opposite direction. The train of these bergs, which moves upward from
the south polar continent, west of Patagonia, indicates also in a very
emphatic way the existence of a very st