245 



level plain, often flooded in the wet season by high tides or drainage water. The 

 general topography is flat and uninteresting. Passing inland from a muddy, 

 mangrove-fringed shore, one crosses a plain which varies from a few hundred 

 yards to a mile or more in width. This is flooded by high spring tides in the 

 wet season. It bears a flora of low shrubs, "samphires," chiefly Arthrocnemum 

 halocnemoides. This plain rises very gradually to a slightly higher level that 

 is not flooded by sea water. At this level occurs an open dwarf shrubland of 

 bushes of Airiplex paludosum. Behind this again, lies low rolling country. 

 The soil varies from a loam, which is cultivated, to a red sandy soil. The 

 natural vegetation of this area has largely disappeared under the plough, or 

 because of grazing animals, but on the red sand near the head of the Gulf there 

 occur the relics of a saltbush flora, Atriplex stipitatitm being the most important 

 under shrub. 



Gulf St. Vincent, according to Howchin,*^^^ is of very recent origin, and owes 

 its existence to earth movements that are probably still in progress. In Post- 

 Pliocene times the site of the Gulf was a wide valley which became filled with 

 sediments. A bore at Port Wakefield shows fluviatile deposits to a thickness 

 of 310 feet. Still more recently this valley was submerged, with the result that 

 a shallow arm of the sea was formed. The sea bottom and shore at Port Wake- 

 field consist of clays, overlain to a slight extent by wind-blown sands. 



The position of Port Wakefield, near the head of a long shallow arm of the 

 sea, makes the district a suitable one for the evaporation of sea water and the 

 recovery of sodium chloride. The sea at the head of the Gulf has an abnormally 

 high density, especially at the close of the dry summer seasoon. Recent figures 

 from a Bulletin of the Geological Survey of South Australia show this ; the 

 salinity of some other sea waters is given for comparison : — 



SALINITY PER CENT. 



Ocean water Spencer Gulf, Gulf St. Vincent, 



mean of 77 South Australia. South Australia. Suez Canal, Mediterranean, 



analyses. Crystal Salt Price, near Port Ismailia.06) near Carthage.(<5) 



(Challenger) (6) Wharf.(5) Wakefield.<>) 



3-301-3733 5166 4555 5103 3897 



Thus the water, flooding the plains around Port Wakefield, is likely to bring 

 considerable quantities of salts with it. Some of these will be leached out in 

 the wet season by rain, but others, especially the less soluble, will accumulate in 

 the surface layers by evaporation. A soil of this type, rich in soluble salts and 

 lime, was met with. (See sample 5, below.) 



HABITAT FACTORS. 



Climatic. 

 The climatic conditions at Port Wakefield are those of the serni-arid por- 

 tions of South Australia. The rainfall is actually low, being 12 93 inches on a 

 mean of 42 years,*') the highest recorded annual rainfall being 17 65 inches, in 

 1910; the lowest, 649, in 1888. The monthly distribution is shown by fig. 1. 

 It will be seen that the summer is dry, December to February showing only 

 just over "5 inches of rain each. 



'W Howchin, W., "The Evolution of the Physiographical Features of South Australia," 

 Rept. Australasian Ass. Adv. Sci., xiv., Melbourne, p. 176, 1913. 



<5) Jack, R. L., "The Salt and Gypsum Resources of South Australia," Dept. of Mines 

 S. Austr., Bull. 8, p. 37, 1921. 



(6) Clarke, F. W., "The Data of Geo-Chemistry," U.S. Geol. Sur. Bull. 695, p. 123, 1920. 



(7) "Results of Rainfall Observations in South Australia," Commonwealth Meteorological 

 Service, Melbourne, Govt. Printer, p. 339, 1918. 



