slowly forcing itself over the low land and out to sea over a long extent
of gentle slope, until it reached a depth considerably above 200 fathoms,
when the lower specific weight of the ice caused an upward strain which
at length overcame the cohesion of the mass, and portions were rent off
and floated away. If this be the true history of the formation of these
icebergs, the absence of all land _débris_ in the portion exposed above
the surface of the sea is readily understood. If any such exist, it must
be confined to the lower part of the berg, to that part which has at one
time or other moved on the floor of the ice-cap.

"The icebergs, when they are first dispersed, float in from 200 to 250
fathoms. When, therefore, they have been drifted to latitudes of 65° or
64° S., the bottom of the berg just reaches the layer at which the
temperature of the water is distinctly rising, and it is rapidly melted,
and the mud and pebbles with which it is more or less charged are
precipitated. That this precipitation takes place all over the area where
the icebergs are breaking up, constantly, and to a considerable extent,
is evident from the fact of the soundings being entirely composed of such
deposits; for the Diatoms, _Globigerinoe_, and radiolarians are present
on the surface in large numbers; and unless the deposit from the ice were
abundant it would soon be covered and masked by a layer of the exuvia of
surface organisms."

The observations which have been detailed leave no doubt that the
Antarctic sea bottom, from a little to the south of the fiftieth
parallel, as far as 80° S., is being covered by a fine deposit of
silicious mud, more or less mixed, in some parts, with the ice-borne
_débris_ of polar lands and with the ejections of volcanoes. The
silicious particles which constitute this mud, are derived, in part, from
the diatomaceous plants and radiolarian animals which throng the surface,