OCT 1 1897 



An AUSTRALIAN SAUROPTERYGIAN (CIMOLIO- 

 SAURUS), CONVERTED INTO PRECIOUS OPAL. 



By R. Etheridge, Junr., Curator. 



(Plates v., vi., vii.) 



I WAS recently favoured by Messrs. Tweedie and Wollaston, 

 Merchants, of Adelaide, through the good offices of Mr. H. Y. L. 

 Brown, Government Geologist for South Australia, with a large 

 quantity of opalised material from White Cliffs, 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. 



I. — 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 

 Cliffs Opal-field on Momba Holding, about sixty -five miles north- 

 north-west of Wilcannia, 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 Belemnites, 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 Mr. 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 an 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, 



