and
many leagues in length, originate, and are eventually blown out to sea
by off-shore winds. In their interior are inclosed many fragments of
stone which had fallen upon them from overhanging cliffs during their
formation. Such floating icebergs are commonly flat-topped, but their
lower portions are liable to melt in latitudes where the ocean at a
moderate depth is usually warmer than the surface water and the air.
Hence their centre of gravity changes continually, and they turn over
and assume very irregular shapes.

In a voyage of discovery made in the antarctic regions in 1839, a
dark-colored angular mass of rock was seen imbedded in an iceberg,
drifting along in mid-ocean in lat. 61 degrees S. That part of the rock
which was visible was about 12 feet in height, and from 5 to 6 in width,
but the dark color of the surrounding ice indicated that much more of
the stone was concealed. A sketch made by Mr. Macnab, when the vessel
was within a quarter of a mile of it, is now published.[298] This
iceberg, one of many observed at sea on the same day, was between 250
and 300 feet high, and was no less than 1400 miles from any certainly
known land. It is exceedingly improbable, says Mr. Darwin, in his notice
of this phenomenon, that any land will hereafter be discovered within
100 miles of the spot, and it must be remembered that the erratic was
still firmly fixed in the ice, and may have sailed for many a league
farther before it dropped to the bottom.[299]

Captain Sir James Ross, in his antarctic voyage in 1841, 42, and 43, saw
multitudes of icebergs transporting stones and rocks of various sizes,
with frozen mud, in high southern latitudes. His companion, Dr. J.
Hooker, informs me that he came to the conclusion that most of the
southern icebergs have stones in them, although they are usually
concealed from view by the quantity of snow which falls upon them.

In the account given by Messrs. Dease and Simpson, of their recent
arctic discoveries, we learn that in lat. 71 degrees N., long. 156
degrees W., they found "a long low spit, named Point Barrow, composed of
gravel and coarse sand, in some parts more than a quarter of a mile
broad, which the pressure of the ice had forced up into numerous mounds,
that, viewed from a distance, assumed the appearance of huge boulder
rocks."[300]

This fact is important, as showing how masses of drift ice, when
stranding on submarine banks, may exert a lateral pressure capable of
b