288 DEPARTMENT OF THE INTERIOR 



2 GEORGE V, A. 1912 



About 200 miles to the south-southeastward a huge batholith of a somewhat 

 similar granite has been described by Lindgren. The two batholiths have been 

 tentatively correlated by the present writer. The correlation is based entirely 

 on petrographical likenesses; it is thus important to review in 'actual quotation 

 the principal facts and conclusions reached by Lindgren : — 



' Granitic rocks prevail in the Bitterroot range and in the Clearwater 

 mountains, and form a central mass of vast extent, bounded in the four 

 corners of the region covered by this reconnaissance by smaller areas of 

 different sedimentary series. To the north of this region the extent of the 

 granite is not well known. But as the granite is absent in the Coeur 

 d'Alene section it is probable that the main area does not continue far 

 north of Lolo ridge except as detached masses. Southward this granite 

 'continues through all of central Idaho as a broad belt, and finally disap- 

 pears below the sediments of Snake River valley. It does not reach Snake 

 river at any place between Huntington and Lewiston. It forms on the 

 whole an elongated area 300 miles from north to south and 50 to 100 miles 

 from east to west, constituting one of the largest granitic batholiths of 

 this continent. 



' On the whole, this extensive area of granite shows great constancy 

 in it's petrographic character. It is a normal granular rock sometimes 

 roughly porphyritic by the development of large orthocl'ase crystals up to 

 3 cm. in diameter. The colour is almost always light gray, the outcrops 

 assuming a yellowish-gray colour, which in glaciated districts changes to 

 a brilliant white or light-gray tone. Biotite is always present in small 

 foils, and over large areas muscovite also enters into the composition ; quartz 

 is abundant in medium-size grains, while the feldspars are represented by 

 both orthoclase and oligoclase, the latter usually in large quantities. Per- 

 thite is also frequently encountered, and more rarely microcline. The rock 

 contains far too much oligcclase to be classed as a normal granite and 

 should be rather characterized as a quartz-monzonite. Modifications more 

 closely allied to granodiorite, diorite, and granite occur in subordinate 

 quantity. 



' The granite is typically developed near the head of Mill creek, 

 Bitterroot range, where it is a light-gray, medium-grained rock, with small 

 foils of biotite and a little muscovite. A few larger crystals of orthoclase 

 reach one-half inch in length. Under the microscope the rock shows much 

 quartz, a little normal orthoclase, and many large grains of microperthite. 

 An acidic oligoclase with very narrow striations is very abundant. Biotite 

 and muscovite occur in scattered straight foils. Few accessories except 

 zircon and apatite were noted, though titanite occurs abundantly in basic 

 concretions in the same granite. The structure is typically granitic; the 

 oligoclase is in part idiomorphic and sometimes included in the perthite.'* 



* W. Lindgren. Prof. Paper No. 27, U.S. Geol. Survey, 1904, p. 17. 



