An AUSTRALIAN SAU ROPTERYGIAN ( CIMOLIO - 
SAURUS), CONVERTED INTO PRECIOUS OPAL. 
By R. Etheridge, Junk., Curator. 
(Plates v., vi., vii.) 
I was recently favoured by Messrs. Tvveedie and Wollaston, 
Merchants, of Adelaide, through the good offices of Mr. 11. Y. L. 
Brown, Government Geologist for South Australia, with a large 
quantity of opalised material from White Cl ills, representing the 
broken-up skeleton of a Plesiosaur, but unfortunately wanting 
the skull. There are numerous vertebrae in various states of 
completeness, innumerable portions of ribs, a few teeth, phalanges, 
and other bones that will be subsequently referred to. These 
have now become the property of the Trustees of the Australian 
Museum. 
L — Precious Opal as an Agent of Replacement. 
The replacement of the calcareous matter in fossils by Precious 
Opal appears to be a fact but little commented on by Authors. 
The search for opal in the Upper Cretaceous at the White 
Cl ill’s Opal-field on Momba Holding, about sixty -live miles north- 
north- west of Wiluannia, in Co. Yungnulgra, has been signalised 
by the discovery of many beautiful examples of the entire con- 
version of the shelly envelopes of Pelecypoda and Gasteropoda, 
the internal shells of Beleinnites, and Reptilian remains, into 
Precious Opal by a process of replacement. 
Many of these are in the Collection of the Geological Survey 
of N.S. Wales, others have been lent to the same, and through 
the courtesy of Jjtr. E, F. Pittman, Government Geologist, I have 
been permitted to examine them. 
The process of “silicification,” as it is called, or the replacement 
of matter in fossil organic remains, by silica, in one or other 
of its varieties, is too well-known to require more than the briefest 
notice. 
Silicification is said to be primary when organisms have under- 
gone a slow process of alteration in water holding silica in solution, 
each particle of tissue, as it decayed, being replaced by the 
mineral in question, the minute structure of the body thus acted 
on being so preserved. “By far the commonest mode of re- 
placement is that whereby au originally calcareous skeleton is 
replaced by silica. This process of ‘silicification ’ — of the replace- 
ment of lime by silica — is not only an extremely common one, 
A 
