BY H. C. RICHARDS. ^ 193 



posure to weather alters to ferric oxide and silica* and this 

 is probably the source of the hyalite so commonly developed 

 in the flows in vughs, cra,cks, etc. A rough columnar 

 structure is developed in the flows at the north end of Mt. 

 Boorambool . 



Specimen 280, from the top flow of Mt. Boorambool is 

 typical of the series and in the hand specimen it is a fine- 

 grained, grey, scoriaceous rock. It has a density of 2.747. 

 Under the microscope {see Plate xii, fig. 5) the section is 

 seen to be that of a fayalite basalt made up of plagioclase, 

 fayalite, augite and the iron ores, set in a partly-glassy 

 groimdmass. The rock shows a rough fluxion structure. 

 The plagioclase is a basic andesine corresponding to 

 Abes Aiu5 and ranges in length up to 0.6 mm. The augite 

 is strongly violet-tinted and is titaniferous, and is much 

 disconnected by the interlacing felspar laths. The drawn- 

 out augite crystals occur up to 1 mm. in length. The 

 iayalite occurs in rounded grains and averages 0.25 mm. in 

 diameter. Limonite occurs as a secondary product from 

 the alteration of the fayalite while serpentine does not 

 occur at all. 



Chemical Characters of the Volcanic Rocks. 



The chemical anah'sis of the phonoiite shows a very 

 liigh percentage of alkalies with the soda and potash in 

 approximately equal amounts. The lime content is very 

 low as is also the magnesia. Sulphuric trioxide occurs to 

 a slight extent and occurs probably in the nosean. In the 

 norm the alkaline character of the rock is very well indicated 

 as orthoclase and albite together total over 75 per cent. 

 of the rock while anorthite totals less than 2 per cent. 



The chemical analyses of the two basalts are very 

 close to one another, the most marked difference being in 

 the lime content. A comparison with the average composit- 

 ion of the basalts of the world shows they are nearly normal 

 in their compositions, except in the titania content which 

 is very high indeed in both cases. Both rocks are rich in 

 augite which, from its violet colour, appears titaniferous, 

 but, in addition, a considerable portion of the iron ore must 

 be ilmenite. 



♦Iddings. Rock Minerals, 1911 Edtn., p. 381. 



If 



