GEOLOGY AND PETROGRAPHY OF THE PROSPECT INTRUSION. 503 



it is primary. 1 These rocks are so closely related to the 

 Prospect essexite in mineralogical and chemical composition 

 (see analysis of analcite-basalt of Bondi, Analysis No. II) 

 that were their analcite proved to be primary it would be 

 strong prima facie evidence for a like origin at Prospect. 

 Mr. Card's proof that the isotropic mineral is indeed anal- 

 cite is incontrovertible; but his only grounds for believing 

 in its primary origin are contained in the sentence: "The 

 manner of occurrence of the analcite, and the freshness of 

 the rocks as a whole, quite preclude the possibility of a 

 secondary origin." With this conclusion we are unable to 

 agree. The manner of occurrence is interstitial, except 

 for occasional irregular patches. An interstitial disposition 

 is exactly what we should expect in a very fine-grained 

 rock where the analcite was secondary after felspar, and 

 the few irregular patches would occur where alteration 

 had proceeded further. Our experience at Prospect, too, 

 is that analcite, undoubtedly secondary, is one of the very 

 first products of decomposition, and may occur in a rock 

 where the other minerals are very little altered. 



13. Consideration of the Analyses. 

 By the kind permission of the Government Geologist of 

 New South Wales, Mr.E.F. Pittman,two complete analyses 

 of the Prospect rock were specially made for our investi- 

 gation by Mr. J. 0. H. Mingaye, Analyst to the Deparment 

 of Mines, in the Government Laboratory. The samples 

 submitted were parts of specimens I (essexite) and B (pallio- 

 essexite), and the analyses of these are stated in columns 



I and III respectively of the following table. In column 



II is quoted an analysis of the Prospect rock previously 

 made by Mr. Mingaye from a chemical study of the basic 

 rocks of the Sydney district. The specimen, of which no 

 part has been preserved, probably came from the Emu 



1 Eec. Geo]. Surv. N.S.W., Vol. vn, (pt. 2), p. 93. 



