52 





IRVING. 







I. 



II. 



SiO., 



72.25 



71-35 



A1.63 



15.01 



12.21 



FeP3 



2.05 



4-53 



FeO 



not det. 



1. 14 



CaO 



2.06 



0.22 



MgO 



trace 



none 



Nap 



not det. 



6.51 



Kp 



not det. 



3.22 



up 



not det. 





Loss 



0.002 



0.33 



MnO 





0.78 



TiO^ 





0.50 



100.79 



It will at once appear that in general the correspondence be- 

 tween the two is quite close except that the alumina and the 

 lime are much higher in the Elk mountain rock than that from 

 Norway. 



3. Terry Peak Type. 



This rock has been described by Caswell in his report on the 

 Black Hills, but, except to call attention to the coarse granitic 

 appearance of the groundmass, he has not gone very far into 

 the description. 



Megascopic Appearance. — The rock is of a mottled appear- 

 ance and a grayish-green, almost white color, the greenish tinge 

 being due to the presence of large phenocrysts of segirine which 

 frequently show a cross section of y^- inch in diameter. 



Microscopic Characters. — Under the microscope the rocks are 

 seen to consist of a mass of sanidine phenocrysts closely packed 

 together, embedded in a groundmass of quartz and orthoclase. 

 Through this are also scattered in greater numbers than in any 

 of the rocks of the type yet described phenocrysts of aegirine- 

 augite. The latter are sometimes perfectly bounded, but in the 

 majority of cases show an irregular outline, due to the grouping 

 of the smaller segirines about them, either in parallel orienta- 



