PEBIDOTITE INTEUSIVES. 



259 



^henocrysts are surrounded by a narrow border of yellowish-green atigite, 

 which correspouds to the small augites in the groundmass, or to those cases 

 which are so common in plutonic rocks — even in this rock described — where 

 hornblende is found surrounding the pyroxene. 



A general explanation which would account for the successive crys- 

 tallization of hornblende and pyroxene in this rock should be applicable to 

 such a zonal growth as occurs around the olivine, taking into consideration, 

 of course, the probability that a factor of slight importance in the one case 

 may be the controlling factor in the other. Such occurrences seem clearly 

 to indicate a change in the chemical composition of the magma as the chief 

 factor in the crystallization of the diiferent minerals, in the pressure, in the 

 temperature, and also in other factors, either one alone or more of these 

 combined. 



ANALYSIS OK PERIDOTITE. 



The peridotite just described was analyzed by Dr. H. N. Stokes of the 

 United States Geological Survey, and his results are here given (No. 1): 



Analysis of peridotite. 



1 (23333). 



2 (22981). 



SiO-2 



TiO.2 



AI2O, 



Cr.20o -. 



FejOj 



FeO 



MnO 



NiO 



CaO 



MgO 



K2O 



Na^O...., 



H5O at 110° .... 

 mo above 110° . 



P.O5 



CO.2 



Total 



