44 METEOROLOGY 



beneath, on the walls of the ice-floes, raising them slightly at their edges, reminding one 

 of the little mud walls or bunds raised by the agriculturists of India and Ceylon to 

 impound water in their rice-fields. The sea ice there was repeatedly cracked by the 

 heaving of the sea outside the bay during blizzards. Thus while the edges of the floes 

 touched, the cracks between them were kept open, so that vapour was continually 

 steaming up from below. These little walls of circumvallation, from 4 to 6 inches high, 

 are not to be confounded with the upturned edges of thin ice floes, the upturning 

 being due to constant collision' Avhile the ice at the edge of the floe was in actual 

 process of growth. 



In reference to the relation between precipitation in general, in the Ross region 

 of Antarctica, and ablation,* what appears to us to be an exceedingly important piece 

 of evidence is afforded by Sunk Lake near Cape Barne. In the case of very 

 numerous small lakes, near Cape Royds, there is fair evidence of recent and still 

 existing conditions of desiccation. These either still exist amongst the moi'aines or 

 ice-scooped rock hollows on the western flanks of Erebus, some showing traces of old 

 terraces now considerably above their present water level, or have dried up 

 completely, leaving only old shore lines fringed with algal peat and diatomaceous 

 mud to tell of their former presence. Nevertheless, as much of this morainic 

 material at a depth rests on ice, and this ice is from time to time thawing and so 

 causing the morainic material above it to slowly subside, it might be argued that 

 some, at any rate, of these lake hollows have become dry through a process of 

 draining rather than through evaporation exceeding pi-ecipitation.f 



No such explanation, however, will apply to Sunk Lake, which is shown on 

 Plate VIII. Its ice surface is 18 feet below sea level in spite of the fact that it is 

 only about 70 yards from the sea to the nearest point of the lake. 



The lake is certainly wind-swept, so that very little snow lies upon it ; at 

 the same time it is well supplied with snow-drifts, which during the season of thaw 

 contribute a good deal of thaw water to the lake. 



On the whole this lake may be regarded as a large rain-gauge, and the fact that 

 its ice surface is so far below sea level appears to us to bear most important 

 testimony to the fact that at joresent in this part of Antarctica ablation, in this case 

 due chiefly to evapoi-ation, exceeds precipitation. 



* Ablation is here used in the widest sense, to comprise all the natural processes which lead to the 

 removal of a surface of snow or ice, whether upper or under surface, and includes the processes of sublima- 

 tion, surface thawing and melting at the base, removal of snow by wind as well as loss by wind abrasion, 

 and also loss through the breaking away of bergs. 



t The ice under these old moraines is probably fossil ice left by the foi'mer Koss Bai-rier when at its 

 maximum extension. 



