THE HEART OF THE ANTARCTIC 



lakes. As some of the lakes are very saline, this eryo- 

 hydrate often bulks large. Some of it freezes at as low 

 a temperature as 50° Fahr. below freezing-point. 



In the case of sea-ice the first stages differ somewhat 

 from those of lake-ice. On account of the wave move- 

 ments, the regular growth of vertical prisms is seldom 

 possible in the initial stages. Instead, delicate glancing 

 scale-like crystals of ice form on the surface and are seen 

 floating about, even at the depth of several feet. The 

 single scales are but an inch or less in diameter, but they 

 soon unite to form freely floating rosettes. Eventually, 

 as these become more abundant, a complete felt-work is 

 produced. In this stage the ice, only one to three inches 

 in thickness, is partially transparent and appears dark, 

 as the colour of the sea shows through it. This ice is 

 peculiarly flexible. Unless a heavy swell intervenes the 

 ice is now sufiiciently strong to hold together and to aid 

 in maintaining the sea in a tranquil state. If, however, 

 the swell increases, the ice is broken into pieces of a foot 

 or more in diameter, depending on its thickness at the 

 time. These fragments, jostling each other continuously, 

 become rounded and develop a turned-up rim. This is 

 called pancake ice. Eventually, with the advent of calmer 

 conditions, the pancake ice may be firmly frozen together. 

 Crystallisation now proceeds by additions from below. 

 Thus, vertical prismatic ice similar to that found in the 

 lakes is produced. These prisms may be half an inch in 

 diameter, and many inches in length. A few hours of 

 still, cold weather goes far to freeze over the sea. If, 

 however, the air temperature rises, the active formation 

 of ice is suspended, to be resumed when the temperature 

 again falls. By a succession of such variations in tem- 

 perature a horizontal banded appearance is produced in 

 the sea-ice; each band representing a separate period of 



358 



