OCCURRENCE OF PRECIOUS STONES IN N.S.W. 959 
points out how joints and fissures are produced in the contracting 
mass “along which percolating waters have caused a partial 
solution of the silica, and its ultimate resolidification into a colloid — 
form. This process is still going on, and the result is an occasional 
band or reticular patch of opal of a waxy lustre and pale yellow 
colour.” 
*Dr. Cooksey! has pointed out that when opal replaces calcite, 
the cleavage of the latter mineral is preserved, and that much of 
the reflected light in the noble opal is thrown from these cleavages. 
I noted opal replacing shells, reptilian bones, and belemnites. 
The finest opal found was probably a stone which replaced the 
centrum of some vertebrz of a Saurian—allied to plesiosaurus. 
One interesting form of opal is locally known as “pipe” opal. 
This 1 found to be a replacement of a belemnite. 
*The occurrence of waterworn quartzite boulders in the opal- 
bearing marls and kaolins is a matter not easily explained. They 
are undoubtedly derived from Devonian quartzites, and in litho- 
logical character cannot be distinguished from the sandstones that 
are seen on the Broken Hill road about nine miles from Wilcannia, 
or from the vitreous-like sandstone overlain unconformably by 
cretaceous beds on the Tallywalka Creek to the east of Wilcannia. 
Many of these boulders are ten to fifteen hundred weight, and in 
some cases are completely encrusted by a skin of opal, evidently 
formed in situ. Tsawan example where a boulder was surrounded 
by a layer of gypsum, the latter mineral being replaced in places 
by opal. They are also seamed with veins of opal. Sometimes 
specimens of this sandstone are seen to be dotted throughout with 
Specks of fire opal, and these when cut into slabs are magnificent 
objects for the cabinet of collectors, or for ornamental purposes. 
*Apart from accounting for the source whence the boulders were 
derived, it is not easy to see how they came to be transported with 
such fine sediments as surround them. The current that could 
carry along such boulders would not be likely to deposit fine clays. 
ares AUC NR aN 
1 Records, Aust. Museum, Vol. 1, p. 111. 
