BY W. N. BENSON. 



611 



scope, however, nothing definite can be learnt as to its nature, 

 and frequently it is not recognisable except in hand-specimen. At 

 other times, there appears to be a slightly greater amount of kaoli- 

 nisation, and the development of a little prehnite. It is evidently 

 due to some radial diffusion, but does not appear to be connected 

 with any process of albitisation. Prehnite is quite frequently 

 developed in tuffs in patches that are not directly associated with 

 cherty inclusions. So far as can be ascertained, the felspar in 

 these pyroclastic rocks is almost entirely acid; no grain has been 

 noted, of which the refractive index is greater than that of Canada 

 balsam. The analysis (B., p 602) does not indicate any great 

 amount of soda, and the composition of the felspar calculated 

 therefrom is that of labradorite. The entry of alumina into vari- 

 ous decomposition-products, probably accounts for the difference 

 between the calculated and observed compositions of the felspar. 



Limestones. 

 The macroscopic features of the limestones of this district 

 have already been described. The following analyses illustrate 

 their chemical composition. These were made by the chemists 

 of the Geological Sur\ey, for a memoir on the limestones of New 

 South Wales, now being prepared by Messrs. J. E. Carne and 

 L. J. Jones. The writer is much indebted to Mr. Carne for his 

 kind permission to use them. Assays 1645-6 were specially 

 made by Mr. Mingaye from specimens chosen by the author. 



Spectroscopic test. 



