256 J. MILNE CURRAN. 
igneous rocks. The opal is found in Upper Cretaceous Sandstones, 
lining joints and filling fissures both vertical and horizontal. 
Shells and other fossils are found completely opalised. As might 
be supposed only a part of the opal found is of any value as a gem. 
The dull, milky, and opaque stones are called “potsh” by the 
miners. Messrs. Hoffnung are large buyers of the better class of 
the opal found here. Much of the stone is cut at their Sydney 
establishment, but the greater quantity is exported to Europe 
and America. I have at various times received opaline stones 
from other localities within the cretaceous area, as far north as 
Mount Brown.! Mr. Slee gave it as his opinion some years ago, 
that opal would be found over a large area of the far west, and 
there is every reason now to believe his opinion is correct. The 
drawback to opal-mining is that there are no surface indications 
to guide one where to begin operations in the search. Some of 
the best stone at the White Cliffs is come upon almost by 
accident. 
When sent to the market the Wilcannia stone can be always 
separated from the Queensland opal. The matrix of the Wilcannia 
opal is so friable that it is easily removed, and the gem is sent 
down in the rough. *In fact they are picked out of the “face” in 
the mines directly they are seen. This is done with a short blade 
knife, and without removing any of the matrix with the gem. I 
very often noticed a clear space above the opal along the whole 
length of the layer. The character of the gem as brought to the 
surface is shown on Plate 17, fig. 2. 
The Q land gems on the contrary are in a matrix of siliceous 
itvoulés or bioniitite; or a sandstone cemented by those iron com- 
pounds. The matrix can be safely removed by the lapidary’s 
slitting disc only. Exceptions are known, but generally there is 
the difference shown in Plate 17, figs. 1 and 2. 
1 Having gone over a cee deal of the north-western districts 5 of of the 
Colony since the above was written, I am not now so hopeful as to the 
large extent of country indicated by Mr. Slee, being found to carry © 
