344 NATURE AND MAN. 



chiefly basaltic pebbles about the meridian of 80° E., and further 

 to the eastward pebbles and larger fragments of metamorphic 

 rocks. It was probably from the valleys of the great volcanic 

 range that the rock-masses came which were observed on bergs 

 by Wilkes and Ross ; one of which, clearly of volcanic origin, 

 weighed many tons. That the southern circumpolar area is 

 chiefly land, and not water, seems to be further indicated by the 

 absence of any such low temperature of the deeper water as Sir 

 George Nares ascertained to° exist beneath the " palseocrystic " ice 

 of high northern latitudes. For the thermometers lowered 

 through borings in that ice gave 28° Fahr, at all depths, this being 

 the lowest temperature at which sea-water can remain unfrozen 

 under ordinary circumstances. On the other hand, the bottom 

 temperatures taken in the Challenger in closest proximity to 

 the Antarctic ice-barrier nowhere proved to be lower than the 

 temperature of surface-stratum which was cooled by the melting 

 of the berg-ice, thus indicating the absence of any supply of yet 

 colder water from a source nearer the Pole. 



Thus the antarctic " ice-barrier " is to be regarded as the margin 

 of a Polar " ice-cap," whose thickness at its edge is probably 

 about 2000 feet, nine tenths of it lying beneath the water-line. 

 This margin is not permanent, but is continually wasting away 

 like the terminal portion of a land-glacier — not, however, by 

 liquefaction, but by disruption — and is as continually renewed by 

 the spreading out of the piled-up ice of the area within. What 

 may be the thickness of the " ice-cap " nearer its Polar centre we 

 have at present no means of knowing ; but it must doubtless be 

 kept down by the facility of downward flow in almost every 

 direction towards its periphery of 10,000 miles. 



In regard to the animal life of the deep sea, the Challenger 

 researches do not seem likely to yield any new general result of 

 striking interest. Our previous work had shown that a depth of 

 three miles, a pressure of three tons on the square inch, an entire 

 absence of sunlight, and a temperature below 32°, might be sustained 

 by a considerable number and variety of animal types : and this 

 conclusion has been fully confirmed and widely extended. Many 

 specimens have been brought up alive from depths exceeding four 

 miles, at which the pressure was four tons on the square inch, 



