712 THE JOURNAL OF GEOLOGY. 



analyzed by Dr. H. N. Stokes of the laboratory of the U. S. 

 Geological Survey. No. 8589 contains a large proportion of 

 diallage and olivine, while No. 8786 is more nearly of the average 

 composition of the entire mass. 



8s8q 878b 



Si0 2 

 TiO s 

 P 3 5 



A1 2 3 - . - 



Cr 2 3 - 



FeO .... 



Fe 8 3 - 



NiO .... 



MnO ..... 



CaO - - - 



MgO .... . 



K 2 



Na 2 . . . . . 



H ¥ at 105 



H 2 above 105 



Total, 100.03 100.75 



The larger percentages of A1 3 3 and of CaO in 8786 as 

 compared with 8589, and the smaller percentages of FeO 

 and MgO, substantiate the results of the microscopical study. 

 An increase in the proportions of A1 2 3 and CaO indicates 

 an increase in labradorite, and a decrease in FeO and MgO, 

 a decrease in the iron-bearing minerals olivine and diallage. 

 The variations are somewhat larger than was to be expected 

 in a rock so uniform in structure and so monotonous in 

 composition as that of this great mass, but they are easily 

 accounted for by the local accumulation of certain of its heavier 

 constituents. So far as known there are no " schlieren" in the 

 normal rock nor any other evidences of a differentiation (" spal- 

 tung") of its magma before cooling, so that the variations in 

 mineralogical and chemical composition must be looked upon as 

 due purely to accidental causes. Moreover, the differences are 

 not great enough to effect any material impression upon the 

 rock as a whole. Its characteristics are practically identical 

 throughout an area of several thousands of square miles, and are 



