THE DEEP SEA AND ITS CONTENTS. 341 



Thus neither the general Oceanic Circulation nor the Gulf Stream 

 could alone produce the result which is due to their conjoint action. 

 The Gulf Stream water, without the Polar indraught, would remain 

 in the mid-Atlantic ; and the Polar indraught, without Gulf Stream 

 water to feed it, would he almost as destitute of thermal power as 

 it is in the South Atlantic. 



The transient visit of the Challenger to the Antarctic ice-barrier 

 gave her scientific staff the opportunity of examining the structure 

 of the southern icebergs, which altogether differs from that of the 

 icebergs with which our northern navigators are familiar ; these 

 last being now universally regarded as glaciers, which have 

 descended the seaward valleys of Greenland and Labrador, and 

 have floated away when no longer supported by solid base ; and 

 the information they have gathered is of considerable interest, as 

 helping us to form a more definite conception of the condition of 

 our own part of the globe during the glacial epoch. A number of 

 independent considerations now lead almost irresistibly to the 

 conclusion that the icebergs of the Antarctic are for the most part 

 detached portions of a vast ice-sheet, covering a land surface either 

 continuous, or broken up into an archipelago of islands which 

 occupies the principal part of the vast circumpolar area, estimated 

 at about four and a half millions of square miles, or nearly double 

 the area of Australia. Of this ice-sheet, the edge forms the great 

 southern &quot; ice-barrier,&quot; which presents itself, wherever it has been 

 approached sufficiently near to be distinctly visible, as a continuous 

 ice-cliff, rising from 200 to 250 feet above the sea-level. 



The icebergs of the Antarctic Sea are, as a rule, distinguished 

 by their tabular form, and by the great uniformity of their height j 

 this, in bergs which show least signs of change since their first 

 detachment from the parent mass, seldom varies much from 200 

 feet above the sea-line. The tabular surface of the typical berg is 

 nearly flat, and parallel with the sea-line ; its shape usually 

 approaches the rectangular, and it is bounded all round by nearly 

 perpendicular clifTs. From a comparison of the specific gravity of 

 berg-ice with that of sea-water, it appears that the quantity of ice 

 beneath the surface required to float that which is elevated above 

 it must be about nine times as great \ in other words, supposing 



