74 EEOORDS OF THE AUSTRALIAN MtTSEtTM. 



On the OOOURRENCE of BEEKITE in connection with 

 "FOSSIL ORGANIC REMAINS," in N. S. WALES. 



By R. Etheridge, Jnr., Palaeontologist. 



[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 Beehite, 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 ' silicification ' — of the replacement of lime hy 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," 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. 

 \Lqc. cit„ p. 7. 



