322 



University of California Publications. 



[GEor/Mi? 



ANALYSIS OF ORE-BEARING PORPPIYRY. 



Si0 2 



58.31 



A1 2 3 



16.69 



Fe,0 :! 





FeO 





MnO 



.19 



MgO 



1.27 



CaO 



5.29 



Na,0 



. 20 



K 2 



6.11 



BaO 



.35 



P=O s 



.31 



Fe 



2. 27 



S 



2.79 



CI 



trace 



co 2 



3.47 



H 2 



2.42 





99.67 



Approximately this corresponds to the following mineral og- 

 ical composition : 



Per cent. 



Orthoclase 37. 



Albite 1.5 



Anorthite 2 . 



Biotite (residues) 3.5 



Apatite . 6 



Kaolin 22 . 



Pyrite 5 . 



Quartz 20. 



Calcite 8 . 



99.6 



The analysis agrees with and confirms the description of the 

 rock based on microscopic study (page 318) which was written 

 before the analysis was made. It seems quite certain from the 

 data thus obtained that the original rock was, as it still is. rich 

 in orthoclase and that this mineral, part of the quartz and the 

 apatite are the only constituents which have escaped alteration. 

 It is clear from the analysis that water, sulphur and carbonic 

 acid have been introduced into the rock from outside sources. 

 Since the kaolin is probably chiefly derived from the breaking 

 down of feldspars, it would seem probable that originally there 

 was a notable proportion of plagioclase, the soda of which has 

 been leached out and the lime of which has been converted into 



