TRECIOUS OPAL: OCCURRENCE IN AUSTRALIA 383 



about sixty-five miles north-north-west of Wilcannia. The following account of this; 

 occurrence is taken from a recent (1901) description by Mr. E. F. Pittman, Government 

 Geologist of New South Wales : 



The discovery of opals at White Cliffs was accidentally made by a hunter in 1889. 

 Since that time mining operations have been carried on continuously, though sometimes 

 under great difficulties, as in time of drought the locality is very badly provided with 

 water ; opal-mining has, however, now become a settled industry, and a thriving township 

 has been established at White Cliffs. The area within which the mineral has been found 

 in the district is about fifteen miles long and about two miles wide. Prospecting for 

 precious opal is a decidedly speculative business because, as a rule, there are no indications 

 whatever on the surface of the occurrence of the mineral below. It is only in very rare 

 instances that an outcrop of the precious stone can be seen, and the usual procedure is to 

 dig a trench or pit in such a position as fancy may dictate, and trust to luck. Fortunately 

 sinking is easy, as the rock is of a soft nature, and in a fair number of instances the opal 

 has been met with at a very short distance from the surface, though a large majority of 

 attempts are unsuccessful. For several years the belief existed among the miners that it was 

 useless to prospect for precious opal at a greater depth than 12 feet from the surface^ 

 but of late the incorrectness of this view has been proved, and the stones have been 

 discovered at a depth of nearly 50 feet. 



The precious opal occurs in a white siliceous rock, varying from a sandstone to a fine 

 conglomerate in character, of Upper Cretaceous age. It is sometimes met with in thin 

 flat veins between the bedding-planes of the rock ; at other times it forms irregular- 

 shaped nodules, or deposits occupying joints ; occasionally fragments of wood are found 

 converted into common opal, while where cracks have occurred in the wood they are filled 

 by precious opal. Fossil bivalve shells and belemnites, entirely converted into precious opal» 

 are not uncommon, and a fair number of opalised bones of saurians have also been found. 

 Although these opalised fossils are of no intrinsic value as gems, they have acquired 

 very high prices through the competition of dealers in curiosities, and are now extremely 

 difficult to obtain. Another curiosity which is not uncommon is a pseudomorph of opal 

 after groups of gypsum crystals ; bunches of these, several inches in length, are sometimes, 

 found composed of precious opal, though the quality is usually poor. The really valuable 

 opal, however, which is cut and polished as a gem, is found in the irregular nodules and 

 seams in the joints and fissures of the soft siliceous rock. When the miner finds the first 

 indication of such a deposit, he proceeds with gi-eat care to excavate the soft rock from all 

 round it, and occasionally masses worth several thousand pounds have been found in this 

 way. The output of the White Cliffs deposit in 1900 was valued at about ^£'80,000. 



The deposits of opal in Queensland are far richer than those in New South Wales. 

 The occurrence here differs from that of other regions and is similar to that at White Cliffi* 

 described above. The opal occurs in thin strings and veins, and also in larger irregular 

 nodules in a highly ferruginous sandstone or siliceous iron-stone known as the Desert. 

 Sandstone. The strings of opal are sometimes so thin that it is impossible to cut stones 

 en cabochon out of this material ; instead, it is cut in the form of flat plates. The bodv- 

 colour of the opal found here is milk-white and the light reflected from the stone is dark 

 blue, green, or red. As is the case at other localities, there is a considerable amount of 

 common opal, which has to be cut away from the precious variety. It is difficult to obtain 

 detailed accounts of the occurrence of opal at the different localities in Queensland. In 

 recent years Queensland opal has come mainly from Bulla Creek, where it generally occurs. 

 as the nucleus of large nodules of iron-stone. Good specimens are also found on the Barcoo. 



