20 RECORDS OF THE ATJSTRALIAN MtTSEtTM. 



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 MoUusca, 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."* 

 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 tirst stage 

 of the process," adds Prof. H. A. Nicholson, from whom T 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'). If 

 the process goes on the whole of the fossil may ultimately become 

 converted into flint." 



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 Series 

 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 detailedf 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. Eep. Dept. Mines and Argic. N.S.W., 1892 [1893], p. 141. 



