686 C. A. STEWART 



The Coeur d'Alene rock, therefore, falls in a separate order, 

 emphasizing its smaller quartz content. The striking difference in 

 the habit of the ferromagnesian constituents is, of course, not shown 

 by the quantitative classification. Rock of the Coeur d'Alene type 

 is known also from the following localities: 



1. On Vermillion Creek in Montana, where Calkins^ found a 

 rock ''distinctly different from the intrusive masses to the west, and 

 showing marked affinity with the masses of monzonite and syenite 

 exposed to the southwest in the Coeur d'Alene district." The ferro- 

 magnesian mineral is either aegirine-augite or masses of hornblende 

 and biotite pseudomorphic after pyroxene. 



2. At the junction of Black Prince Creek with the St. Joe River 

 where Pardee^ found "a. porphyritic monzonite" of ''marked 

 resemblance to the dominant rock in the intrusions of the Coeur 

 d'Alene." Biotite, in addition to the distinctive hornblende, is 

 reported to occur in this rock, but the quartz is in rather small 

 amount. 



3. On Gold Hill in the northern part of Latah County I have 

 found a porphyritic, hornblendic monzonite that can in no way be 

 distinguished from specimens from the Coeur d'Alene. Only 

 twenty miles south of this locality are the Thatuna Hills, composed 

 of the micaceous, quartzitic monzonite described as characteristic 

 of the great bathohth of central Idaho. In fact, it was the striking 

 difference between the rocks from these two places that first called 

 my attention to this problem. Unfortunately the intervening 

 country is covered by Tertiary basalt, so the relationship between 

 the rocks cannot be determined. 



The other intrusives in northern Idaho, although showing con- 

 siderable variation from the central Idaho type, nowhere, as far as 

 I can learn, resemble this characteristic hornblende monzonite, and 

 in most places they show marked affinities with the central Idaho 

 type. 



Reference to the map (Fig. i) shows that the occurrences of the 

 Coeur d'Alene type all fall in the northeast-southwest line that has 

 already been noted by Calkins for the Coeur d'Alene and Vermillion 



' Bull. 384, U.S. Geol. Survey, p. 47. 



^ Bull. 470, U.S. Geol. Survey, Pt. I, p. 46. 



