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ROCK SPECIMENS FROM CENTRAL AND WESTERN AUSTRALIA. 309 



I. Oavanagh Range. 

 II. "Norite," Norseman, W. A., Bull. 21, Geol. Surv. W.A. 

 1906, p.. 119. 



III. " Diabase," Barina District, British Guiana, Rep. Geol. 



N.W. District, n, p. 6, 1898. 



IV. Hunnediabase, Launceston, Tasmania, Quoted in Ann. 



Rep. Dept. Mines, 1908. 

 V. Molecular proportions of I. 



The analysis is interesting in the first place as adding to 

 our information of the chemical composition of the Hunne- 

 diabase. Typical quartz-dolerites (Kongadiabas) show in 

 general a lower proportion of alumina than normal rocks 

 of the same silica percentage, whereas this rock possesses, 

 if anything, a higher figure. These peculiarities are due, 

 in the former to the presence of free silica, in the latter to 

 the abundance of a very basic felspar, for anorthite is 

 relatively richer in alumina than albite. It is somewhat 

 surprising to find a pyroxene poor in lime crystallising from 

 a magma so rich in that element, but a calculation shows 

 that after apportioning lime to alumina to form anorthite, 

 there is relatively little non-felspathic lime. The mineral 

 composition may be calculated as follows : — 



Quartz 6*23 



Orthoclose 0*49 



, Felspar 54*12 

 Albite 5-34 ] ) 



Anorthite 48*29 I ^ g ' 53 bS 



OaSi0 3 8*93 



MgSiQ 3 17*72 ) Pyroxene 38*16 



i 



MnSi0 3 0*24 [ 29*23 J 



PeSi0 3 



Magnetite 



Ilmenite 



H 2 Q 



Iron ores 1*47 



100*13 



