55 



disintegration of a compact and apparently pure opal rock 

 was an unexpected occurrence. On examination it was found 

 that an efflorescence of salt covered all the planes of fracture, 

 and this is suggestive of the cause of the disintegration. The 

 effect of frozen water in disintegrating and fracturing 

 absorbent bodies by expansion is well known. This property 

 of expanding when passing from a state of solution to that 

 of crystallization is characteristic of most solvent substances, 

 and, in the case before us, it appears that a certain amount 

 of salt solution was taken up by the opal at the time of its 

 consolidation. Whilst the opal was buried in the soil it 

 retained its quarry- water, or "sap- water," to use a quarry- 

 man's term, but when placed in the cabinet desiccation 

 followed, the water evaporated, and the salt crystallized out, 

 producing internal stresses that caused a general rupture of 

 the mass. 



This action is known to be operative in all dry regions 

 where the surface waters are mostly mineral solutions and 

 subjected to alternate conditions of imbibition and desiccation. 

 What has occurred in the case of opal may be expected to 

 occur also in most other rocks under similar conditions, especi- 

 ally those of an open texture, such as sandstones, shales, clays, 

 etc., many examples of such chemico-mechanical disintegration 

 were noted in the region referred to. 



IV. Nodular Barytes of Peculiar Forms from 

 Central Australia. 



Barytes (barium sulphate), or "heavy spar," occurs in 

 South Australia under a variety of forms. In Mitcham 

 quartzite quarries it has been obtained in well-formed tabular 

 orthorhombic crystals, and also of lamellar structure and 

 translucent. In some parts of the Mount Lofty Ranges it 

 occurs in veins with a granular crystallization, as in the 

 barytes mine near Blumberg. It not infrequently forms the 

 gangue in mineral lodes, as at the New Burra Copper Mine, 

 south-east from Kooringa. It also occurs sporadically as 

 nodules in limestones and clays. These nodular forms are 

 interesting from the variety of shapes they assume. At the 

 Brighton limestone quarries nodules of barytes are not at 

 all uncommon. Some of these have mammillary forms, white 

 to brownish in colour, columnar and radial in structure, with 

 a smooth porcelain-like surface, and in some examples attain 

 a large size. One such obtained by me from these quarries is 

 hemispherical in shape, with the appearance as though a 

 viscous liquid had been poured out of some vessel — weighs 

 14 lbs. 



