122 



IV. Gypsum from the Patawalonga Creek. 



Several years ago Mr. W. Howchiu (2) exhibited crystals 

 of gypsum from a locality near Glenelg. The following are 

 additional remarks on material collected recently, during a 

 combined visit to that locality. The spot is about one 

 mile beyond the Patawalonga Creek, and close to the Glenelg 

 to Reedbeds Road. The ground is there low-lying, elevated 

 only a few feet above sea-level, and consequently swampy in 

 wet weather. Gypsum crystals are met with abundantly, set 

 in the soil a few inches below the surface. Deeper down, they 

 are interwoven together, and set in a shelly mud, evidently a 

 recently raised estuarine mud-bank. 



We excavated to a depth of about 3 feet, and the gypsum 

 formation was there as well developed as above. 



The specimens saved for examination are, in general, 

 bunched aggregates of interpenetrating tabular crystal 

 masses; the units commonly attain lateral dimensions of 12 

 cms. b}'' 8 cms. There is a marked tendency to radial twinning 

 on the h axis. Crystal faces are not defined, but represented 

 by drusy curved outlines. The drusy character of the surface 

 is due to the development of very numerous independent simi- 

 larly-oriental crystal growths. These latter are of lenticular 

 form, bounded by smooth rounded outlines, and have resulted 

 from the exclusive development and rounding off of the {111,' 

 and {103} forms. Inclusions, usually shells and shell frag- 

 ments, are abundant. In fact, the major portion of the occur- 

 rence exposed varied between gypsum beds containing little 

 residual shell matter, and shell beds with occasional develop- 

 ment of enveloping ramifying gypsum crystals ; in the latter 

 case the crystallographic continuity is not affected by the 

 foreign shell-matter. 



Crystals occurring in the topmost layers, near the surface 

 of the ground, are somewhat differently modified. In this 

 upper zone the matrix is sandy, and almost free from calcium 

 carbonate ; foreign included matter in the gypsum consists, 

 therefore, chiefly of sand grains, which may occupy a large 

 bulk in the crystallization. The crystal aggregates themselves 

 are much smaller, more compact, smoother surfaced, and rep- 

 resent simpler combinations. 



The simplest individual is identical in character with the 

 lenticular growths contributing: the drusy character to the 

 larger crystals already described ; they are, however, much 

 larger, some obtained measuring 5 cms. diameter. The len- 

 ticular form tends towards a somewhat irregular dumpy bi- 

 cone. Twinning is almost universal, and frequently a num- 



(2) Trans. Roy. Soo., S. Australia, vol \xvii , pt. ii., p. 311. 



