31l! PKOCBEDINGS OF SECTION C. 



couii)letely isotropic. This latter characteristic is of considerable 

 interest, and has not been previously recorded. Normally the nuneral 

 being- tetragonal is anisotropic. An analogy, however, is found in 

 other minerals containing the rare earths — viz., gadolinite, thorite, 

 and allanite, all of which are normally anisotropic but become by 

 alteration abnormally isotropic. Tlie powder is light ash grey in 

 colour. 



The specific gravity of various fragments differed considerably, 

 the following values being- obtained :— 5-82, 6-01, 6'24, 6-4:8, 6-65. 

 This variation may be due either to differences in the water content, 

 or to dift"erences in the relative proportions of tantalic oxide and niobic 

 oxid:e. A crystalline fragment weigiiing about 6 grammes was 

 selected for analysis and coarsely crushed, all jtarticles showing 

 weathered surfaces being then rejected. The unweathei'ed portion 

 remaining had a specific gravity of 6'23G at 22'9°, and a hai'dness of 

 6. Before the blowpipe it was infusible and diil not decrepitate or 

 glow, but turned light yellow in colour. In the closed tube it yielded 

 water. On crushing- it yielded a greyish white powder. Its composi- 

 tion was : — 



Fergusonite, CuogJec/oiKj. 



Tantalum pentoxide 

 Xiubium pentoxide 

 Titanium dioxide 

 Tin dioxide 



Thoria , 



Yttria 



Erbia 



Ceria 



Lime 



Iron protoxide 



Manganese protoxide 



Uranium trioxide 



Ignition loss (mainly water) 



The water is evidently an alteration product and pi-obably vai'ies in 

 dift'ereut parts of the one specimen. Neglecting tliis, we get the 

 usually accepted foiruula for Fet^usonite, viz. : — 



KaOg.TaoO,^. 



This particular example of fergusonite diffei's irdui all others 

 hitlierto described in two intei-dependent factors — viz., in its high 

 specific gravity and in the very high propoi'tion of tantalum relatively 

 to niobium. It is probabl}- one of the two minerals '' allied to 

 euxenite in physical characteristics " alluded to by Mr. B. F. Davis. 



Tlie mineral collection of the Perth Museum having now been 

 amalgamated with that of the Geological Survey, opportunity has 

 occurred for examining- in detail the parcel of euxenite in the former 

 collection referred to in my 1907 paper. This parcel was found to 



* Less than 0-10 prr cent. 



