IX 
it is very difficult to distinguish from the Blytliesdale Bray stone (Rolling 
Downs Pormation), or from the still older sandstones (Hawkesbury Sand- 
stones) of the Triassic Coal Measures. The other variety is an intensely hard 
and brittle porcelainised rock ; in all probability it was originally identical 
with the freestone, and has been indurated by having the interstices between 
the sand grains filled by secondary silica, which has given it a more or less 
opaline appearance. This rock rings like porcelain when struck with a 
hammer, and breaks with a conchoidal fracture. Areas of this indurated or 
porcelainised rock alternate with areas of unaltered freestone, and it seems 
probable that the metamorphism has l)ccn caused by tlie action of thermal 
springs. 
The Desert Sandstones cover an enormous area in Australia. In New 
South Wales they attain a thickness of 400 or 500 feet in places, as in the 
Grey Ranges, and at Mount Oxley, near Bourke ; but for the most part they 
form low isolated hills, representing the remnants of a very widespread deposit 
which has been largely removed by denudation. 
Above the Desert Sandstone occur beds of very fine-grained soft white 
rock, more or less allied to Kaolin. In some places near White Cliffs this 
rock is almost devoid of alumina, and judging from its general characters it 
may possibly have had an organic origin. Spherical casts which have been 
observed in it are probably the remains of Radiolaria, though their structure 
is not sufficiently well preserved to admit of absolute identification. Other 
samples of this fine-grained white rock were found, on analysis, to contain from 
15 to 25 per cent, of alumina, and in them Professor David and Mr. W. S. 
Dun detected minute organic remains, which, they were of opinion, were 
probably Diatomaca). Deposits of precious or noble opal occur in these rocks at 
and near White Cliffs, and opal mining has now become a flourishing industry, 
supporting a population of over 2,000 persons in the township of White 
Cliffs. These white rocks also contain numerous marine fossils, as well as 
Sauropterygian bones, many of wliich have been converted into precious opal. 
A remarkable feature of the beds is that occasional waterworn boulders of 
Devonian quartzite, often several feet in diameter, are scattered througn them. 
Many of these boulders contain characteristic Devonian Brachiopoda, which 
are also occasionally found to have been converted into noble opal. In one 
specimen, about nine inches in diameter, which has been secured for the 
Geological Museum, half of the boulder has been opalised, Avhile the other 
half is in its original state, the line of division between the opalised and the 
