PHYSICS 



lower portions of such formations. The tabular icebergs 

 typical of the Antarctic which have originated by breaking 

 off from the barriers, are, owing to the nature of their 

 formation, less dense and far more buoyant than the trans- 

 parent glacier-icebergs of the Arctic. The plateau ice, 

 existing under conditions of great cold, sparse snowfall, 

 and continuous strong winds, is predominantly hard. At 

 a short distance below the surf ace it is still finely granular, 

 but very compact. This is seen where the blizzards have 

 abraded the superficial softer formation and exposed the 

 deeper ice in channelled sastrugi; it then resembles pol- 

 ished Carrara marble. The more typical glacier-ice con- 

 sists of interlocking crystals an inch or more in diameter. 



In the vicinity of Cape Royds there are a number of 

 small lakes occupying rocky basins. The smaller of these 

 thaw out each year during summer, others are only partly 

 thawed, or not at all. In these, refrigeration during the 

 autumn proceeds under tranquil conditions. As a result 

 the ice appears prismatic with the longer axes of the 

 prisms parallel and vertical. Winter snow, falling on this 

 surface, slowly consolidates, building at the same time 

 similarly oriented additions to the surface ends of the lake 

 ice prisms. Should several years succeed without thawing, 

 a most interesting structure is produced, the nature of 

 which is made evident by the subsequent removal by bliz- 

 zards of the unconsolidated snow. It then presents a 

 smooth, polished surface exhibiting a mosaic appearance, 

 produced by clear ice prisms separated by lines of white 

 ice containing air cavities. 



As refrigeration goes on in the lakes, the saline con- 

 tents are gradually concentrated in the residual liquid and 

 a continuously increasing cold is required to freeze each 

 succeeding separation. Ultimately a meshwork of ice 

 and cryo-hydrate crystals is formed at the bottom of the 



357 



