BY H. C. RICHARDS. 141 



similarity amongst these rocks, and that they are all of a 

 decidedly acid nature. The alumina value is a little lower 

 and the total iron-oxides value a little higher than is 

 usual in these rocks. 



This deficiency in alumina and richness in iron-oxides 

 is a characteristic of the rocks of the area as a whole, but 

 is much more pronounced in the basic representatives. 

 The lime value is normal, and is to be contrasted with the 

 much lower percentages in the alkaline types. The alkali 

 contents are a little above normal, and it is noticeable 

 that in the two rocks from the Esk district the soda is in 

 excess of the potash. These two rocks seem to be midway 

 between the sub-alkaline and alkaline types. 



Two analyses have been inserted in the table for 

 comparison, and a somewhat close relationship exists be- 

 tween the Chinghee Creek rhyolite and the perlite from 

 Yellowstone National Park ; while between the Springbrook 

 rhyolite and a biotite-rhyolite from Sugarloaf Hill, San 

 Francisco, especially close chemical relationship exists. 



The exact locality of the pitchstone in Column IV. 

 from MacPherson 's Range is unknown, but Mr. G. W. Card 

 of the Mining Museum, Sydney, kindly sent me the 

 analysis, and it is evident that it is very similar to the Mount 

 Lindsay pitchstone, and in fact it may be from the same 

 locality. 



The analysis of the perlitic pitchstone from Tweed 

 River is not a complete one, but it is richer in alumina 

 and poorer in alkalies than the other rocks. This analysis, 

 which was originally published by Smeeth.^- and which 

 is undoubtedly an analysis of the perlitic pitchstone from 

 Tweed River, has been published in a subsequent paper 

 by E. C. Andrews'*" as the analysis of the Mount Lindsay 

 rock, but there is obviously a mistake in locality. The 

 analysis in Column IV., with which ■Mr. Card of the 

 Mining Museum, Sydney, kindly furnished me, seems rather 

 to be an analysis of the Mount Lindsay rock. 



^' Op. cit., p. 311. 



=^ Eec. Geol. Surv., N.S.W., vol. vii., p. 240. 



