A CONTRIBUTION TO THE STUDY OF ICE-STRUCTURES 5 



may be passed by as such. In the winter-time projecting stones arrest a quota of the 

 flying saline snow. As summer approaches, the water from thawing of snow lying about 

 again freezes as soon as the shadow of the rock is entered. Even in the shadow of 

 the rock ablation proceeds rapidly and in time only the salt remains. The presence 

 of the rock acting as a wind-break further protects the accumulation from being carried 

 off by subsequent gales. The illustration (Plate I, Fig. 2) is a view of a pebbly slope 

 high up in the Green Lake basin ; small patches of white salt can be seen behind many 

 of the pebbles, easily mistaken for snow. 



The saline mineral matter as collected and air-dried contains slightly under fifty 

 per cent, of water, which is driven off at 100° C. It is a mixture of sulphates and 

 chlorides of sodium and magnesium with but traces of other substances. 



2. GREEN LAKE 



For further information refer to Vol. I, p. 148. Green Lake is a shallow lakelet 

 in a rock basin, receiving the drainage of a relatively large catchment area (Plate I, 

 Fig. 1 ). Its mean diameter is about sixty yards ; greatest depth slightly more than 

 six feet, and it is situated some twenty to thirty feet above sea-level. As the water 

 is unusually briny it thaws readily each summer. The common red-brown alga of 

 the Cape Royds lakelets flourishes in the fresher waters of the margin, where thaw- 

 water trickles in from adjacent snow-banks; in the briny central portion there is 

 nothing living excepting bacteria and perhaps other unicellular organisms. The 

 bottom of the latter situation is mantled with a layer of black putrefying mud, from 

 which so much sulphuretted hydrogen is evolved that both the water and the ice 

 have a most disagreeable odour. The gas is doubtless liberated by the action 

 of thio-bacteria, which appear to thrive in the imprisoned waters beneath the 

 surface of the lakelet, existing by reason of their role in robbing the sulphates of 

 their oxygen. 



Three shafts were sunk by Priestley * in this lake during the winter. The sections 

 exposed in each corresponded very closely in detail. A diagrammatic representation 

 of the July shaft is figured herewith (Fig. 1). In June one foot of brine was found 

 unfrozen at the bottom of the lake. In July there was practically no brine left ; none 

 at least where that particular shaft was sunk, distant only twelve feet from the former. 



On examination at the Hut, the brine found liquid in the shaft sunk in June was 

 found to contain much sulphate and chloride with a small amount of carbonate 

 and sulphite. With these acid radicles were combined the following bases in order of 

 relative abundance : sodium, magnesium, calcium, potassium, and ammonium. There 

 was present also free sulphuretted hydrogen and carbonaceous matter which charred 

 on evaporating to dryness. 



The brine, on standing in a corner of the Hut for a few hours at a temperature 

 of about 30° F. (above its freezing-point), developed a distinctly fluorescent capping 

 layer. From this layer botryoidal masses of a white cloudy substance gradually 

 extended themselves, growing downwards into the liquid at the rate of about half 

 an inch a day. This was noted to take place only in open or loosely corked bottles, 

 and was immediately arrested upon excluding the air. In the course of time the 

 growth extended throughout the liquid, and later settled as a slimy precipitate which 

 proved to be chiefly organic matter. There seems to be no doubt but that the growth 

 was bacterial. 



* I wish to record my great indebtedness in widening the scope of these observations on ice- 

 structures to INIi-. R. E. Priestley and all members of the Expedition who assisted him in the winter 

 programme of sinking shafts through the ice of the lakelets. 



