A PERLITIC PITCHSTONE FROM TWEED RIVER. 



311 



Chemical and Mineralogical Constitution. 

 Sp. gr. = 2-47. The bulk analysis of the rock gave the follow 



ing results : — 



Si0 2 



A1 2 3 



Fe 2 3 



CaO 



MgO 



K 2 



Na 2 



H o 



75-51 per cent. 

 14-30 



1-01 



1-81 



•24 

 2-89 

 1-21 



2-84 



99-81 



The alkalis were further determined in some of the glassy 

 matrix from which the phenocrysts were separated, and gave the 



following : 



K 2 -78 per cent. 



Na 2 1-14 „ 

 showing that the porphyritic felspars are rich in potash, while the 

 glass is relatively rich in soda. 



The relative volumes of the constituents were approximately 

 determined by drawing several sections and cutting out and 

 weighing the the pieces. After multiplying by the specific gravities 

 of the minerals the mean of six sections gave the following result: 



Quartz... ... 5'11 percent. 



Sanidine ... 14*94 



Albite 2-07 



Hypersthene ... 1-90 



Glass 73-14 



Water 2-84 



100-00 

 The water being deducted from the percentage of glass. From 

 these figures we can deduce that the glass contains 80*27% of Si0 2 

 and that its sp. gr. = 2-44. Assuming that the above figures 

 are fairly accurate, it follows that there is not enough K 2 to 

 completely satisfy the sanidine, which will therefore contain some 



