THE CCEUR D'ALENE MONZONITE 



685 



intrusive from the Coeur d'Alene district, the most casual observer 

 would note a difference. This rock, according to Calkins/ is 



a medium-grained granular rock of which the constituents megascopically 

 recognizable are feldspar, hornblende, and a little quartz. Of the feldspars, 

 striated plagioclase, opaque and white, in rather small idiomorphic crystals can 

 be distinguished from somewhat larger, grayish, and more transparent imper- 

 fectly formed crystals of alkali feldspar. The two are present in nearly equal 

 amounts. The microscope shows, in addition to the constituents named, a 

 little biotite, some small prisms of light-green monoclinic pyroxene with 

 titanite and magnetite as abundant accessories. 



Variations from this type are noted, especially in respect to the 

 relative amounts of the minerals, but none of them resembles the 

 rock from central Idaho, from which all of the Coeur d'Alene 

 monzonite differs in the smaller amount of quartz, in the presence 

 of hornblende, and in the lack of megascopic mica. The chemical 

 differences are indicated by Table I. 



TABLE I 

 Analyses of Intrusive Rocks from Idaho 



B 



D 



SiO^ 



AI2O3 



FeA.... 



FeO 



MgO.... 



CaO 



NaaO.... 



K,0 



H,0-... 

 H.0+... 



TiO. 



P.Os...., 

 All other . 



Total 



61 .41 



17.99 



2-93 



1-39 



1.30 



4-75 

 4.01 



459 



O.II 



0.68 



0-S3 

 o. 19 

 0.46 



72.07 



15-51 

 0.31 

 1. 01 

 0-35 

 1-93 

 4.02 

 4.09 

 0.03 

 0.30 

 o. 16 



O.II 



68.42 



15.01 



0.97 



1-93 

 1 . 21 

 2.60 



3-23 

 425 



0-54 

 0-73 

 0.50 

 0.13 

 0.38 



69.56 



15-29 

 0.86 

 2.06 

 0.69 

 2.81 

 3-97 

 336 



100. 24 



99.89 



99-95 



100. 17 



65-23 

 16.94 

 1.60 

 1. 91 

 I-31 

 3-85 

 3-57 

 3.02 

 0.18 

 0.88 

 0.66 

 o. 19 

 0.44 



99.78 



A. Quartz-monzonite, near Gem, Idaho (Cceur d'Alene). — Prof. Paper 62, U.S. Geol. Survey, x>. 47. 



B. Quartz-monzonite, Mill Creek, Mont. — ^Lindgren, Prof. Paper 27, U.S. Geol. Survey, p. 18. 



C. Quartz-monzonite, Hailey, Idaho. — ^Lindgren, 20th Ann. RepL, U.S. Geol. Survey, p. 81. 



D. Granite, Shafer Butte, Boise Co., Idaho. — Lindgren, loc. cit. 



E. Biotite granite, Willow Creek Dist., Boise Co., Idaho. — ^Lindgren, i8th Ann. Rept., U.S. Geol. 

 Survey, p. 40. 



The magmatic symbols for these rocks are as follows (calculation of A by Calkins, loc. cit., and of C, 

 D, and E by Washington, Prof. Paper 14, U.S. Geol. Survey): 



A =1 • 5 • 2 . 3, pulaskose. 

 B =1 • 4 • 2 • 3, toscanose. 

 C=I»4«2«3, toscanose. 

 D =1 • 4 • 2 • 4, lassenose. 

 E =1 • 4 • 3 • 4, yellowstonose. 



^ Prof. Paper 62, U.S. Geol. Survey, pp. 46-47. 



