237 



FOSSILIFEROUS SILICEOUS DEPOSIT FROM THE 

 EICHMOND EITER, N.S.W. 



By Akchibald Liveesidge, Professor of Geology and Mineralogy 

 in the University of Sydney. 



IJRead before the Royal Society of N.8. JF., 6 December, 1876.] 



Most of the specimens of this material which I have had the 

 opportunity to examine, exhibit many of the appearances which 

 are usually presented by the deposits thrown down from hot 

 springs or geysers. 



Although no such hot springs or geysers are known to exist at 

 the present day in the Colony, yet I understand from Mr. W. 

 "Wilson, of Monaltrie, to whom I am indebted for my specimens, 

 that the district in which they occur presents many features 

 which lead him to consider that it has been the scene of com- 

 paratively recent (i.e. in a geological sense) active volcanic 

 phenomena. 



The district has not, I believe, been examined in detail by any 

 trained and experienced geologist ; but judging from Mr. Wilson's 

 account it must be one of remarkable interest. 



Basaltic and trachytic rocks are the principal surface rocks 

 occurring in the neighbourhood. The basalt is remarkable for 

 containing very large and well developed amygdaloids of 

 chalcedony, agate, arragonite, and certain of the commoner 

 zeolites. Of the amygdaloidal and other minerals, together with 

 specimens of the matrices, Mr. Wilson sent a large series to 

 the Commissioners for the Philadelphia Centennial Exhibition — 

 the collection of which must have entailed the expenditure of 

 much time and labour. 



In the interior of the mass, the siliceous deposit is usually of 

 a more or less pale wax colour, and in certain respects closely 

 resembles wood opal. Wood opal is actually present, and in parts 

 streaks of true opal, although not of the precious variety, occur. 

 Occasionally, on breaking open a specimen, jet-black patches 

 are met with; the colouring matter apparently contains carbon, 

 as it is slowly burnt off in the blow-pipe flame. 



On the surface, the mineral weathers white, and the decom- 

 position passes in to a depth of from i to ^ inch. 

 2b 



