AN AUSTRALIAN SATJROPTERYGIAN — ETHERIDGE. 21 



specimens hardly any structure at all is to be observed, beyond 

 the outward form of stem or branch, as the case may be. Not 

 infrequently radial cracks are filled with Precious Opal, when the 

 play of colour is very fine. 



The Animal Remains occur under the following conditions : — 



1. As external or internal casts in kaolin, without opalization 

 of any kind. 



2. Entirely converted into hydrous silica or Common Opal, 

 white and opaque, but occasionally with traces of the coloured 

 variety scattered through. 



3. Wholly or partially converted into translucent-glassy to 

 vitreous semi-opaque Precious Opal, displaying a fine range of 

 colour. 



The colours visible by reflected light are principally blue, red, 

 green, and yellow, with their various shades and combinations, 

 not the least pleasing being an ever-varying degree of red and 

 blue-tinted purple. 



When the fossils are in the form of kaolin casts, specific 

 identification, with a very few exceptions, is almost unattainable. 

 Those in which opalisation, however, has taken place, are always 

 determinable, more or less, and the substitution of the original 

 carbonate of lime has been very thoroughly carried out. Frag- 

 ments of these opalised remains, chiefly shells, are freely scattered 

 throughout some hand-specimens of the opaline kaolinised con- 

 glomerate, from the bed B of Mr, Jaquet's section.* The kaolin 

 casts are either white or tinged with iron-oxide, arising from the 

 highly ferruginous clays that Mr. Jaquet says the kaolin passes into. 



The opalised fossils comprise Crinoid remains, the shells of 

 Pelecypoda and Gasteropoda, portions of Belemnite guards, and 

 Sauropterygian bones. The preservation of some of these fossils 

 is excellent, although all are not alike in this respect, and the extent 

 to which the opalisation has at times been carried is remarkable. 

 In some Pelecypoda, the external growth laminae, and inter- 

 mediate sculpture striai are fully preserved, whilst the shell 

 substance is completely changed, and by transmitted light the 

 valves of many are almost transparent. On the fractured edges 

 of one of these bivalves the glassy opal is quite translucent by 

 reflected light. When such valves are met with in apposition, 

 the interiors are often found to be filled with soft kaolin, and 

 no better examples of the complete change that has taken place 

 can be examined. 



The replacement of the fibrous calcite of the Belemnite guard, 

 when viewed in cross-section, presents a far less translucent, and 



*Ann. Eep. Dept. Mines and Agric. N.S.W., 1892 [1893], p. 141. 



