278 SUPERFICIAL DEPOSITS 



The mineral has a feebly saline bitter taste. 



Heated in the closed tube it gives off water in abundance. 



Gave an intense yellow colour to the flame. 



Readily soluble in water. 



A very pure sample of mirabilite (NajSO^lOH.jO). 



Material from Atmospheric Weathering of Mirahilite-Exanthalose {?). 



Practically a dehydrated sodium sulphate. 



The general appearance of the mirablhte deposit and of the mii-abilite crystals 

 is shown on Plate XC. 



PEAT DEPOSITS 



Deposits of a remarkable type of alga -were observed by us in nearly all the 

 lakes of Ross Island, on the floors of dried-up lakes in the same area, as -well as 

 in the neighbourhood of Dry Valley on the coast of South Victoria Land. The 

 alga grows to a considerable size, individual specimens being fully 2 to 3 feet in 

 diameter. They are of a brown to red colour, almost vermilion in places.* There 

 can only be a few weeks available for the growth of this organism during the year, 

 that is, the period of time when the ice has thawed in the lakes. This extends 

 from about the middle of December to late in January. The exact period of thaw 

 depends largely on the nature of the rock forming the sides of the lake. Where 

 the rock consists of black lava, the lakes begin to thaw early where the ice is in 

 contact with such black rocks, and thus the algse are set free to grow. Small 

 deposits of this alga are found chiefly on the leeward side of the lakes. The 

 deposits are formed by the wind, which causes rajiid ablation of the ice surface, 

 removing by abrasion with snow crystals the tips of the algse as they come to 

 project with the progressive ablation from the surface of the lake ice. Thus a 

 quantity of small fragments of the algse form an algous drift of insignificant proportions, 

 as is the case near Green Lake at Cape Royds. During the period of thaw a kind 

 of algous slush is produced by the action of the blizzard winds on the surface 

 waters of lakes, such as Green Lake. At Green Lake, for example, it was observed 

 that during a heavy drift-laden gale coming from the south, in February 1909, 



* The red colour may have been due to the red rotifers feeding upon the algaj in vast numbers. 



