123 



ber of twinned individuals combine to form crystal aggregates. 

 Interpenetrant twins on the a face are usually developed. 

 Two forms effected by this type of twinning are figured. The 

 flat upper faces appearing in pi. xxi., fig. 1, are of the {103} 

 form ; the faces below, shown in fig. la, are of the {111} form. 



The spiral type (fig. 2) is very commonly occurring ; it 

 is the normal type modified by distortion due to unequal 

 lateral development during growth. 



The origin of the gypsum is, no doubt, due to the action 

 of soluble sulphates on the calcium carbonate of the shell 

 beds. The topmost horizon in the sandy matrix is thought 

 to owe its location mainly to capillary attraction, due to sur- 

 face evaporation, drawing the calcmm sulphate in solution 

 upward from below. 



V. Gypseous Tufa. 



Locality, near Grampus Dam, 12 miles south-east of Para- 

 too siding. 



At this spot, on the flat ground stretching towards the 

 bed of the creek, is exposed a surface deposit of white to yel- 

 lowish friable material, ascertained to be composed of gypsum 

 admixed with a considerable quantity of calcium carbonate. 

 The area covered exceeds an acre, and the depth in places not 

 less than 6 feet. 



VI. Amazonstone. 



Some fair specimens of amazonstone were recently got 

 at a small copper-mine at Mulga Hill '^'' in the Olary district; 

 they do not, however, compare with the better-known material 

 from Broken Hill. 



The ore-formation at the locality cited is associated with 

 a coarse granitic pegmatite in highly-metamorphic rocks of 

 the Lower-Cambrian or Pre-Cambrian age. 



VII. Stolzite 

 •Well-crystallized specimens of stolzite occur in vughs in 

 the oxidized portion of a galena lode situated about 17 miles 

 east of Olary, and one mile south of the Luxemburg Mine. 

 The crystals vary in colour from light honey-yellow to some- 

 what reddish. Molybdic acid was found in the latter variety, 

 indicating an approach towards wulfenite. 



VIII. Chrysotile. 

 Fine specimens of chrysotile, the silky fibrous form of 

 serpentine resembling asbestos, may be obtained in the highly 

 metamorphic Pre-Cambrian ( ?) marble belt near Tumby Bay, 

 Eyre Peninsula. 



(3) Mulga Hill is some six miles south-east ot th-e Luxemburg 

 Copper-mine. 



