394 University of California. [vol. 3. 



I II Average 



Si0 2 44.44 44.33 44.39 



AI2O3 36.55 36.55 



CaO 18.57 18.76 18.67 



Na 2 .83 .83 



MgO none 



FeO trace 



100.44 



This analysis shows that the feldspar has a composition corre- 

 sponding to AbiAnio.T and it must therefore be classed as 

 anorthite . 



Chemical Composition of the Orbules. — An analysis of a por- 

 tion of an ortrale representing its mean composition from center 

 to periphery was kindly made for the writer by Mr. James W. 

 Hoavsoii of San Francisco. His results in duplicate are as follows: 





I 



II 



Average 



Si0 2 



40.50 



39.76 



40.08 



A1 2 3 



23.01 



22.71 



22.86 



FeO 



11.96 



11.96 



11.96 



MgO 



12.24 



12.56 



12.40 



CaO 



11.44 



11.39 



11.41 



Na,0 



1.19 



1.33 



1.26 



K 2 



.40 



.37 



.38 





100.74 



100.08 



100.35 



The results of this analysis show a larger proportion of soda in 

 the rock as a whole than is contained in the feldspar alone 

 according to Mr. Sehaller's analysis, and it seems probable, 

 therefore, that the alkalies may be somewhat too high. The 

 analysis is, however, sufficiently near correct to give a good idea 

 of the general composition of the orbules and the approximate 

 relative proportions of olivine and feldspar. This ratio of 

 olivine to feldspar is estimated to be about 8:11, reckoning the 

 hypersthene as an accessory with the olivine; and this estimate 

 involves the assumption that there is a slight excess of alkalies 

 and alumina in the analysis. 



It thus appears from the foregoing description that while the 

 main mass of the rock is a hornblende gabbro with accessory 

 olivine and hypersthene, the orbicular facies is mineralogically a 

 troctolite . 



Origin of the Orbicular Structure. — The foregoing part of 

 this paper is purely descriptive, and if it were to end here it is 



