66 DISCOVERY REPORTS 



when that constituent is widely distributed in small quantities in numerous living 

 organisms. It is easier, however, to offer reasons for the formation of the tetragonal salt 

 rather than whewellite in very deep water, given a suitable concentration of calcium 

 oxalate. Frey has found that the "envelope" crystals are most stable in alkaline solu- 

 tions containing a high calcium content, and that their stability is also favoured by 

 immersion in viscous media. H. Wattenburg (1933) has observed the variation of 

 specific alkalinity with depth of water in the south Atlantic ocean. It is interesting that 

 his records for many stations show a marked increase in alkalinity between 4000 and 

 5000 m., corresponding with a slight " undersaturation " of calcium carbonate. Hence 

 the formation of tetragonal calcium oxalate crystals at depths of 4434 to 5008 metres in 

 the Weddell Sea may be favoured by a similar increase in specific alkalinity of the ocean 

 bottom water and by the viscosity of the enclosing muds. 



GYPSUM, CaS04.2H20 



The second group of crystals received from Mr Earland consists of two samples from 

 different stations: St. 428, 66° 57' S, 11° 13' W, 2715 fathoms (= 4965 m.), and St. 391, 

 66° 14' S, 31° 18' W, 2630 fathoms (= 4809 m.). These are lenticular crystals up to 

 2-0 X i-o X 0-5 mm. in size which are readily shown by optical properties, specific 

 gravity and chemical tests to be gypsum (Plate II, fig. 2). The crystal forms present are 

 (ill) and (no), only the latter being transparent. The faces (m) are corroded and the 

 crystals resemble much larger crystals of similar form observed by Baret (1888) and 

 others from saline deposits. Crystals identical in form but smaller in size have also been 

 separated from ocean bottom samples brought back by the Discovery Expedition from 

 the Weddell Sea in 1925. These latter crystals, which were found by Mr E. Heron-Allen, 

 F.R.S., come from St. WS 553, 63° 33I' S, 60° 33I' W, 5029 m. Gypsum has not hitherto 

 been recorded from ocean bottom deposits and it is to be noted that this constituent, like 

 calcium oxalate, would be removed at least partially by initial treatment of the sediments 

 with acid. Since gypsum is one of the first minerals to be deposited when samples of sea 

 water are evaporated (J. Usiglio, 1849) its formation in deep-sea deposits is of great 

 interest. J. H. van't Hoff(i9i2) has studied the conditions of formation of gypsum in the 

 Stassfurt salt deposits. There beds of gypsum, CaS04.2H20, and anhydrite, CaSOj, 

 form the lowest layers. The temperature of transition of gypsum to anhydrite under 

 ordinary conditions is 63-5° C. The pressure due to 5000 metres of sea-water is about 500 

 atmospheres and the consequent lowering of the transition temperature about 25° C. 

 Since the temperature of sea-water at such depths is approximately —2° C. the formation 

 of calcium sulphate as crystals of gypsum rather than anhydrite is not surprising. The 

 perplexing fact is that so common a mineral should not be found in other contemporary 

 ocean-bottom samples ; that indeed its distribution is governed by conditions, as far as we 

 know, peculiar to the Weddell Sea. A knowledge of these conditions would probably 

 throw light upon other problems of the Weddell Sea and would form a useful addition 

 to oceanography (see I. Igelsrud, 1932). 



