Rock from the Highwood Mountains of Montcma. 321 





I. 



II. 



III. 



lY. 



la. 



SiO, 



_. 46-06 



47^28 



46-73 



44-35 



•767 



AIA--- 



.. 10-01 



11-56 



10 05 



10-20 



-097 



FeA ... 



._ 3-17 



3^52 



3-53 



8-20 j 



13-50 



020 



FeO 



_. 5-61 



5 71 



078 



MiiO .... 



_. 14-74 



13-17 



9-68 



12-31 



•391 



CaO 



.. 10-55 



9 20 



]3-22 



11-47 



•188 



m,o--_. 



.- 1-31 



2-73 



1-81 



3-37 



021 



K,0..... 



.. 5-14 



2-17 



3-76 



4-42 



•054 



H,0.._.. 



.. 1-44 



2-96 



1-24 



9 



•080 



TiO,..-... 



•73 



•88 



-78 



? 



•009 



PA..... 



•21 



•59 



1-51 



? 





Mi.O .... 



. - trace. 



•13 



•28 









BaO 



•32 



? 



? 



9 





SrO 



•20 



? 



? 



? 





SO3..... 



•05 







none. 









CI 



•03 



•i8 



-18 











99-57 



100-08 



100 97 



9y-62 





C1 = 



•01 

 99-56 



•04 



•04 









100-04 



100-93 







I. Missourite, head of Sbonkin Creek, Ilighwoocl Mountains, 

 Montana. E, B. Hurlburt, analyst. 

 II. Leucite absarokite (Hague, this Jour. vol. xxxviii, p. 43, 1889. 

 Iddings, Jour. Geol., vol. iii, p. 938, 1895. J. E. Whit- 

 field, analyst. 



III. Shonkinite, Square Butte, Highwood Mountains (Bull. Geol. 



Soc. Amer., vol. vi, p. 414, 1895). L. V. Pirsson, analyst. 



IV. Leucite basalt, Bonj^sberg bv Pelm Eifel (Hussak 77 Bd 



Sitzb. K. Akad Wiss. Wien I Abt. 1878). E. Hussak, 

 analyst. 

 la. Molecular ratios of No. I. 



This analysis brings out strongly the leading characteristics 

 of the rock, its very high lime, iron and magnesia, which have 

 compelled the formation of such quantities of pyroxene and 

 olivine ; the predominance of potash over soda, which with the 

 low silica have conditioned the formation of the leucite, and 

 which explains also why no feldspars have formed. 



The endeavor to compare this rock chemically with the effu- 

 sive leucite basalts, of which it forms the plutonic representa- 

 tive, has not been entirely satisfactory owing to the lack of 

 accurate and complete analyses of them. A number of analyses 

 exist but are deficient in important determinations, and in some 

 cases it is clear, from what is stated concerning the mineralogi- 

 cal composition, that the separation of the magnesia and alumina 

 is inaccurate, the magnesia being in part thrown down with the 



I 



