74 
RECORDS OF THE AUSTRALIAN MUSEUM. 
On the OCCURRENCE of BEEKITE in connection with 
“FOSSIL ORGANIC REMAINS,” in N. S. WALES. 
By R. Etheridge, Jnr., Paleontologist. 
[Plate XVI.] 
Among the many mineral substances replacing the original 
carbonate of lime composing what are generally known under 
the name of “fossils,” are iron-pyrites, iron-oxide, sulphur, 
malachite, magnesite, talc, and silica of various forms, such 
as Beelcite , chalcedony, and both common and precious opal. 
By far the commonest mode of replacement is that whereby an 
originally- calcareous skeleton is replaced by silica. This process 
of ‘ siliciiication ’ — of the replacement of lime by silica — is not 
only an extremely common one, but 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 percolation by heated or alkaline 
waters holding silica in solution.”* 
“ In a large number of cases of silicification, p continues Prof. 
Nicholson, “ the minute structure of the fossil which has been 
subjected to this change is found to have been more or less 
injuriously affected, and may be altogether destroyed, even 
though the form of the fossil be perfectly preserved. This is the 
rule where the silicification has been secondary and has taken 
place at some period long posterior to the original entombment 
of the fossil in the enveloping rock ; whereas if the original 
fossilisation has been effected by infiltration with silica in the first 
instance, then the minute structure is usually perfectly preserved. 
In secondary silicification, as seen in corals and shells, the 
carbonate of lime of the original fossil is gradually more or less 
completely replaced by silica, the process beginning on the ex- 
terior and gradually extending inwards.”! 
In New South Wales we are at present acquainted with three 
methods of replacement of carbonate of lime — by iron-pyrites, 
*H. A. Nicholson, Man. Pal., 3rd Edit., 1880, I., p. 7. 
t Loc. cit p. 7. 
