CHAPTER XVIII 



RECENT DEPOSITS 

 MIRABILITE 



A DEPOSIT of this mineral was discovered by one of us (R. E. Priestley) near Cape 

 Barne, at the spot marked on the geological map. After the summer thaw had been 

 in progress for some time, many ridges in sheltered positions, which had remamed 

 snow covered during the winter, were exposed to view through the snow being dis- 

 mantled by the heat of the sun. It was on one of these ridges, at the north end of 

 Deep Lake, near Cape Barne, that a deposit of white salt was found in February 

 1909. The deposit was lenticular in shape, a few yards in extent, and had a 

 maximum thickness of a little over 2 feet. It occvu-red in rough irregular masses, 

 the largest of which were several ])ounds in weight, and was encrusted by a white 

 23owder evidently formed from the dehydration of the salt. These blocks on freshly 

 fractured surfaces showed a perfect cleavage and a vitreous lustre, cleavage flakes, 

 even when several inches in thickness, being quite transparent. The salt was 

 determined by Dr. Mawson to be mirabilite (Na2S04 + 10H,,O), and the efllorescent 

 powder was identified by him as exanthalose (NajSO^H^O). 



The mirabilite at Deep Lake is evidently the result of a process of concentration 

 of the waters of an ancient saline lake. Possibly, during the recent elevation of 

 the land, small ocean basins have been isolated, the salts in the sea water then 

 underwent concentration by evaporation, the sodium salts being further added to by 

 material leached out of the volcanic rocks of Erebus, which are themselves rich in 

 soda. That such leaching process is in progress was proved by the very alkaline 

 character of the waters of the lakes in Dry Valley. The following is an analysis by 

 Mr. J. C. H. Mingaye, F.C.S., &c., Analyst and Assayer to the Geological Survey, 

 Department of Mines, New South Wales, respectively of the mirabilite and of the 

 exanthalose : — 



Mirabilite. Clear mineral of glassy translucency from Deep Lake near Cape 

 Barne, Boss Island, Antarctica. 



Water . 

 Soda . 



Sulphur trioxide 

 Chlorine 

 Insoluble matter 



55'420 per cent. 

 19-478 

 25160 



minute traces. 

 006 per cent. 



100-064 per cent. 



2n 



