358 Wyville Thomson— General Ocean Circulation. 
but I believe that all deviations from a horizontal arrangement 
are due to changes taking place in the icebergs themselves. 
I think there can be no doubt, from their shape and form, 
and their remarkable uniformity of character, that these great 
table-topped icebergs are prismatic blocks riven from the edge 
of the great antarctic ice-sheet. I conclude, therefore, that the 
upper part of the iceberg, including by far the greater part of 
its bulk, and culminating in the portion exposed above the 
surface of the sea, was formed by the piling up of successive 
ayers of snow during the period, amounting perhaps to cen- 
turies, during which the ice-cap was slowly forcing its way 
over the low land, and out to sea over a long extent of gentle 
slope, until it reached a depth considerably beyond 200 fathoms, 
when the lower specific weight of the ice caused an upwar 
strain which at length overcame the cohesion of the mass, and 
portions were rent off and floated away. 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° 
south, the bottom of the berg, the surface which forced itself 
glacier-like over the land, just reaches the layer at which the 
temperature of the watcr distinctly rises; and is rapidly melted, 
and the pebbles and land débris 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 that the 
matter brought up by the sounding instrument and the dredge 
is entirely composed of such deposits from ice; for diatoms, 
foraminifera and radiolarians are present on the surface in large 
numbers, and unless the deposit from the ice were abundant 1t 
would soon be covered and masked by the skeletons of surface 
organisms, - 
The curious question now arises, what is the cause of the 
uniform height of the southern icebergs—that is to say, what 1s 
the cause of the restriction of the thickness of the free edge of 
the ice-cap to 1,400 fathoms? I have mentioned the gradual 
or so in thickness, although of a white color and thus indicating 
leila quantity of air, are very hard, 
rg. 
The upper layers have been manifestly produced by falls of 
snow after the berg has been detached. 
