Case 48. 



20 



PRECIOUS OPAL. 



PfiEciors Opal is obtained in considerable quantities, some of 

 the very finest quality, at Whiie Cliffs, to the north-north-west of 

 Wilcannia. The opal is found in sedimentary rocks of Upper 

 Cretaceous age. The district is covered by several feet of 

 drift material, so that there are no surface indications of the 

 existence of opal. The opal occurs in small pipes, veins, &c. 



The exhibit is intended to illustrate the manner in which the 

 opal occurs ; and, although a number of cut stones are exhibited, 

 there are none of very fine quality. 



10110 ; Veins in ironstone. 



The opal is frequently found replacing fossil organisms. Of 

 these the following are noteworthy : — 

 10117, crinoid stems. 

 6299 and others, bivalce sliells. 

 6238, reptilian vertelrce. 

 10113, hclemnites. 

 1331, 10112, and others, silicified wood more or less opalised. 



The Cretaceous strata contain boulders of Devonian quartzite. 

 These boulders are sometimes more or less opalised — see 9127 

 and others, and, in particular, 5800— in which the opalisation 

 has only invaded the boulder to a moderate depth. Prom such 

 specimens as these it is very evident that the opal has been 

 produced by deposition fiom water. 



3934. Beautiful polished slabs of opalised sandstone, in which 

 sections of bivalve shells are very clearly seen. In case 123, 

 near the entrance, will be found a block of opal matrix showing 

 wood and shells replaced by opal. 



Alercromlie District, Ballina, at Eocky Bridge Creek. 



In the Abercrombie District and at Ballina precious opal has 

 been found as amygda'es in basalt, the original vesicular cavities 

 having been filled up by opal deposited by water (2637, 5394 

 andl012G). 



