STRUCTUEE AND COMPOSITION OF A BASALT FROM BONDI. 225 



Analysis of Bondi Basalt. 



Silica 



Fresh undecomposed 

 Basalt. 



... 43-5 



Decomposed Basalt 

 filling fissures. 



42-0 



Alumina ... 



... 



14-60 



40-2 



Ferric oxide 



... 



5-40 



trace 



Ferrous oxide 



... 



8-28 





Lime 



... 



8-70 



nil 



Magnesia ... 



... 



6-16 



nil 



Soda 



... 



7-34 



4-4 



Potash 

 Titanic oxide 

 Phosphoric oxide . . . 

 Chromic oxide 



... 



2-95 



•10 



trace 



trace 



1-6 

 trace 



Chlorine ... 

 Water 



... 



•37 

 2-50 



12-00 





99-90 



100-2 



Specific gravity 2-94. 



As one would have expected from the preliminary examination 

 of the rock, the notable feature of the analysis is the large per 

 centage of soda. I have already stated my reasons for believing 

 the rock contains sodalite. An analysis of the decomposed basalt 

 is also given, where it will be noted that iron, lime and magnesia 

 have been extracted completely by natural weathering. 



If we assume that the soda and chlorine found are present as 

 sodalite, we may calculate the soda required for sodalite based on 

 the amount of chlorine present. The mineral sodalite requires 

 from 4*6% of chlorine (minimum) to 7*3% chlorine (maximum). 

 Chlorine found in basalt = "37%. '37% of chlorine represents a 

 maximum of 8% of sodalite in the Bondi rock. 



The total soda combined with silica and chlorine, as sodalite is 

 29*9%,* from this we can calculate that the total soda in the soda- 



* Haughton and Hull — Report on the Chemical, Mineralogical, and 

 Microscopic Characters of the Lavas of Vesuvius — Trans. Eoy. Ir. Acad., 

 Vol. xxvi., page 51. 



O— August 1, 1894. 



