310 University of California Publications. [Geology 



smaller, dull white feldspars and numerous black hornblendes 

 up to 15 mm. in length. Under the microscope the ground mass 

 proves to be very murky. It is largely isotropic but with doubly 

 refracting areas in it. The smaller feldspars can not be advan- 

 tageously studied owing to their decomposition. Apatite is a 

 common accessory. 



The fourth occurrence is about a quarter of a mile to the 

 southeast of the Ruth Mine. Macroscopically it resembles the 

 occurrence on the Spion Kop claim and under the microscope 

 it differs from that rock only in the presence of apatite in the 

 thin slides. 



Chemical Composition. — A representative specimen of the 

 monzonite porphyry taken from the occurrence north of Lane 

 City was analyzed by Mr. Ross with the following results : 



Si0 2 



59. 



79 



A1 2 3 



17. 



70 



Fe 2 3 



2 



42 



FeO 







76 



MgO 



1. 



92 



CaO 



5. 



22 



Na„0 



9 



64 



K = 



4. 



19 



BaO 





21 



MnO 





09 



P 2 5 





37 



H 2 (ig) 







00 





99. 



31 



Relation to Monzonite Batlwlith. — The analysis is noteworthy 

 for its close similarity to that of the monzonite of Weary Flat. 

 This similarity of chemical composition suggests an identity of 

 magma for the two rocks. So far as the field evidence is con- 

 cerned there is little that can be adduced in opposition to this 

 suggestion. The monzonite porphyry may be the dyke facies 

 of the monzonite of Weary Flat, The one doubt that arises is 

 due to the occurrence of a very small mass of the monzonite 

 porphyry within the area of the copper-bearing porphyry near 

 the north boundary of the Blair claim. This, however, is so 

 near the contact of the porphyry with the underlying limestone 

 that it may be interpreted as a portion of the floor upon which 



