502 H. S. JEVONS, H. I. JENSEN, T. G. TAYLOR AND C. A. SUSSMILCH. 



which was the last product to crystallise in the Prospect 

 rock, it would be strange if there were not the slightest 

 distinction in structure or inclusions to be found between 

 the primary and the undoubtedly secondary analcite. We 

 therefore conclude that there is no proof of original analcite 

 occurring in the Prospect essexite or any of the rocks 

 associated with it. 



The hypothesis that whilst most of the analcite is formed 

 from felspar some may have resulted from the decomposition 

 of a lenad, e.g. nepheline, which occurs in some of the 

 closely related basalts of the district, is not easy either to 

 prove or disprove. A careful search with the microscope 

 has revealed no trace of nepheline. It is true that the 

 calculation of the mode shows a small deficiency of silica 

 from that making albite with all the soda and alumina; but, 

 if regarded as original, this deficiency would require the 

 presence of less than two per cent, of nepheline. Without 

 microscopic proof of the presence of nepheline, it seems 

 better to conclude that the deficiency of silica is due to 

 decomposition, a little silica being wholly removed (perhaps 

 with lime) in the formation of analcite. That sodalite was 

 originally present would be an hypothesis hard to prove or 

 disprove ; there can hardly be any remaining, if we may 

 judge by the insignificance of the quantity of chlorine found 

 in the analyses, all of which is probably contained in the 

 apatite. Taking everything into consideration we do not 

 believe that any lenad was present. 



Our conclusion, reached in the foregoing discussion, may 

 seem that we have relied solely upon the internal evidence 

 afforded by our specimens of the Prospect essexite. It 

 remains to be seen whether experience gained from similar 

 rocks elsewhere supports our conclusion. Mr. G. W. Card 

 has noted the occurrence of analcite in many of the basalt 

 dykes and necks of the Sydney district, and concludes that 



