20 
RECORDS OF THE AUSTRALIAN MUSEUM. 
but it is also a readily intelligible one ; since carbonate of lime is 
an easily, and flint a hardly soluble substance. It is thus easy 
to understand that originally calcareous fossils, such as the shells 
of Mollusca, or the skeletons of Corals, should have in many cases 
suffered this change, long after their burial in the rock, their 
carbonate of lime being dissolved away, particle by particle, and 
replaced by precipitated silica, as they were subjected to percola- 
tion by heated or alkaline waters holding silica in solution. 7 ’* 
On the other hand, if the minute structure of the fossils has been 
injuriously affected during this process, or destroyed, notwith- 
standing the preservation of the outward form, the silicification is 
said to be secondary , having taken place at a period long posterior 
to the entombment of the organic remains. “ In the first stage 
of the process,” adds Prof. Ii. A. Nicholson, from whom I am 
quoting, “ the outer layer of the fossil very commonly becomes 
converted into, or covered by, small circular deposits of silica, 
having the form of a central boss surrounded by one or more 
concentric rings (‘orbicular silica/ or ‘ Beekite markings 7 ). If 
the process goes on the whole of the fossil may ultimately become 
converted into flint. 7 ’ 
A third form of silicification may, I believe, exist — the con- 
version of the original calcareous matter into the form of chalce- 
dony, so excellently seen in the shells ( Physa , etc.) of the Lower 
Intertrapean chert beds of the Deccan Tertiary Trap Scries 
at Nagpur, in India, or the chalcedonic Permo-Carboniferous 
Brachiopoda of Point Puer, Port Arthur, Tasmania, 
The mode of occurrence of the Opal at White Cliffs has 
already been so fully described by Mr. W. Anderson and Mr. 
J. B. Jaquet that it need only be briefly referred to. It is met 
with in beds of kaolin and conglomerate forming a portion of the 
Desert Sandstone, but the former author also says in the “vitreous- 
looking” Desert Sandstone itself. Four separate conditions of 
occurrence are detailed f by Mr. Jaquet, viz. : — 
1. In thin horizontal veins, between the bedding planes of 
the kaolin. 
2. As irregular nodules scattered through the kaolin. 
3. As Wood Opal. 
4. As opaline shells, etc. 
We are at present concerned only with the two last. 
The Wood Opal is usually of an opaque milk-white or horn-yellow 
colour, and is simply hydrous silica, although the woody structure 
is still visible and in some instances well preserved, but in other 
* Nicholson, Man. Palaeontology, 3rd edition, i., 1889, p. 7. 
t Ann. Rep. Dept. Mines and Argic. N.S.W., 1892 [1893], p. 141. 
