A COMPARISON OF THE COEUR D'ALENE MONZONITE 

 WITH OTHER PLUTONIC ROCKS OF IDAHO 



C. A. STEWART 



University of Idaho 



The rough mountainous area comprising nearly all of the central 

 part of Idaho and adjacent portions of Montana is an enormous 

 bathoHth of quartz-monzonite of late Mesozoic age. In the 

 northern part or "panhandle" of the state the predominating 

 Algonkian sediments are cut by irregular masses of acidic intrusives 

 which are quite generally believed to be outliers of the great 

 bathoHth to the south. It is the purpose of this paper to point out 

 that among these intrusives of northern Idaho is a type distinctly 

 different from the other acidic intrusives of the state, and to pro- 

 pound for future field-workers the problem of its relationship 

 to them. 



The quartz-monzonites of central Idaho and those found in the 

 southern part of the state are light-gray, granular rocks with biotite 

 the chief dark silicate, and often containing muscovite. Porphy- 

 ritic development of the feldspar and variations in the relative 

 amounts of the minerals are common, but taken as a whole they are 

 remarkably uniform rocks. I have seen this quartz-monzonite in 

 place in the vicinity of Moscow, of Elk City, and in the Dixie 

 mining district, and have examined specimens of it from other 

 localities, and in every case it showed the same general characters. 

 Professor D. C. Livingston has traversed great areas of it along the 

 Salmon River; and the various United States Geological Survey 

 reports by Lindgren, Eldridge, and Umpleby all describe the same 

 light-gray, micaceous quartz-monzonite of which some of us have 

 come to speak familiarly as the "Idaho granite." In the extreme 

 north of the state around Priest Lake I have crossed many miles of 

 the same rock. 



Indeed it would be difficult in a pile of mixed specimens from 

 these localities to distinguish the rocks from the various districts. 

 Should there be in this pile, however, a sample of the average 



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