121 



as they have not previously bean accorded their due import- 

 ance in scientific literature. 



Crystal Gypaum. — Gypsum is found in large clear tabular 

 forms, as much as three inches in diameter, very generally oc- 

 curring in the saline mud below the salt crust of the lakes. 

 The individuals are often separate and are well-formed. The 

 ■{111} form is always well-developed, and the crystal closed 

 by uneven planes, approximately in the position of the 

 {001} form. No other faces were observed. The individuals 

 are therefore much flattened in the vertical direction. They 

 occur set in the mud, with the tabular dimensions fairly regu- 

 larly horizontal. The specimens examined contain only slight 

 inclusions of foreign matter. 



Seed Gypsum. — What is locally designated by this name 

 attains a wonderful development, notably along the south 

 side of Lake Fowler. There, some distance in from the ori- 

 ginal lake boundary, an immense dune about one mile in 

 length, and as much as 30 feet in height and 50 yards in 

 breadth, is formed exclusively of this substance. 



The grains are seed-like, and average 0'25 cm. long, and 

 about one-third of this amount in diameter. Only sufficient 

 impurity is present to impart to it a slight yellowish tint. The 

 whole mass, throughout the dune, is readily friable. Scant 

 vegetation exists along the ridge. 



With regard to the origin of this deposit, it must, un- 

 doubtedly, have been heaped up by hot dry northerly sum- 

 mer winds, sweeping before them the small particles, chiefly 

 cleavage fragments, of gypsum from off the saline crust of the 

 lake. 



"Decompo.'' — The third variety forms a surface zone over 

 the seed gypsum ; it is much whiter than the latter, and is in 

 the form of an extremely finely-crystalline powder. Quali- 

 tatively tested, it proved to be composed of calcium sulphate, 

 with a very small quantity of calcium carbonate admixed. 

 The local term is a contraction of decomposed, as it is regard- 

 ed to have originated from the decomposition of other forms 

 of gypsum. 



Corrsctly speaking, this is not a decomposition product, 

 but has resulted from solution and re-precipitation of pre- 

 viously-existing gypsum by surface percolating waters. This 

 explanation accounts for the widely-varying thickness of the 

 zone, from a few inches to several feet and even more where 

 an easy passage for descending waters has been available. 



Though the present market does not allow of more than 

 desultory exploitation of the Yorke Peninsula gypsum, these 

 enormous deposits, conveniently situated and easily won. must 

 ever be an increasingly valuable asset to the State. 



