2IO 



WILLIAM H. EMMONS 



If the pyroxene border is a portion of the aggregate, then its outline 

 is very irregular, but if the inner border of the pyroxene is considered 

 as limiting the aggregate it has a definite crystal form. In other 

 ferromagnesian aggregates rows of magnetite and mica are en echelon, 

 and others are composed entirely of magnetite and feldspar, and 

 still others of biotite and feldspar. All of them are surrounded by 

 the fringe of minute pyroxenes. 



The magmatic alteration shown by variety B is not unusual in 

 igneous rocks of intermediate composition and is commonly regarded 

 as showing a change in conditions after the phenocrysts formed and 

 before the rock solidified. It is quite possible that some feldspar 

 and hornblende had formed before the lava rose, and that when pres- 

 sure was relieved they were partially or wholly dissolved, and under 

 the new conditions recrystallized, their elements going into a number 

 of other minerals. The mineral composition of the aggregates is 

 not constant and could not have resulted merely from recrystallization 

 of the original minerals without substitution. 



An analysis of B taken from a point near the edge of the stock 

 about 30 rods north of the wagon road from Cowles to the Yellowstone 

 National Park, is given below. 



ANALYSIS OF VARIETY B. 



SiOa 64.09 



A1203 16 . 20 



Fe^Oj 2.61 



FeO 2 . 40 



MgO 2.06 



CaO 4-Si 



Na^O 3.88 



K2O 2.51 



I. Norm 



Quartz 19 . 20 



Orthoclase 15 -oi 



Albite 33-°^ 



Anorthite 19.18 



Hypersthene 5 . 82 



Diopside 2 . 08 



Magnetite 3.71 



Ilmenite 0.91 



Apatite o • 34 



Water 0.66 



Total 99-92 



GEO. STEIGER, ANALYST 



H2O— 0.22 



H2O+ 0.44 



TiO^ 0.49 



P2O5 0.24 



MnO 0.09 



BaO 0.15 



SrO 0.03 



Total 99-92 



II. Mode 



Quartz 21.8 



Orthoclase 11. 7 



Albite 30 . 1 



Anorthite 15.1 



Pyroxene 11. 2 



Biotite 4-8 



Magnetite and Ilmenite 4.9 



Apatite 0.4 



Total 100. o 



