182 REVIEWS 



A Geological Reconnaissance Across the Bitterroot Range and Clear- 

 water Mountains in Montana and Idaho. By Waldemar 

 Lindgren. (Professional Paper No. 27, U. S. Geological 

 Survey, 1904.) Pp. 122, XV plates, and 8 figures. 



The area embraced in Mr. Lindgren's reconnaissance contains about 

 12,000 square miles, of which 6,000 are included in the Bitterroot Forest 

 Reserve. It lies between the 113th and 117th meridians and between 

 the parallels of 45 and 47 . Roughly speaking, one-fifth of the area is in 

 western Montana, and the remainder extends across Idaho to the Wash- 

 ington boundary. The whole area lies in the watershed of the Columbia 

 River. The Snake River is the largest stream which has its source in the 

 region. 



From east to west the characteristic topographic features are, in order, 

 the following: (1) the Bitterroot Valley, (2) the Bitterroot Range, attaining 

 an elevation of some 11,000 feet, and merging westward into (3) the great, 

 dissected, high plateau of the Clearwater Mountains, and still farther 

 westward (4) the Columbia River lava plateau, to which the Clearwater 

 plateau descends rather abruptly. In this great plateau such streams as 

 the Salmon, Clearwater, and Snake are deeply incised. 



The geology is fairly simple, according to Mr. Lindgren's statement. 

 The main Bitterroot Range is a quartz-monzonite mass, the northward 

 continuation of the central Idaho bathylith. This is an intrusive mass of 

 probably post-Carboniferous age. 



The eastern slope of the range is a fault plane that dips about 18 to 

 the east. The rocks of the fault zone are both gneissic and schistose. In 

 addition to these igneous and metamorphic rocks, there are areas of sedi- 

 mentaries, quartzites, and slates, supposed to be of Cambrian or pre- 

 Cambrian age. Into this series the granite is found to have quite extensively 

 intruded. Other areas of sedimentaries, supposed extensions of the Seven 

 Devils' series, are presumably Mesozoic. The granite of the Clearwater 

 Mountains is intrusive into these in many places. These sedimentaries are 

 confined, as a rule, to the flanks of the central granite mass, which is the 

 prevailing rock in both the Bitterroot and Clearwater Mountains. 



The Columbia River plateau is formed of essentially horizontal lava 

 flows, in which are intercalated shallow water deposits which contain 

 Miocene plant remains. In the discussion of the lava flows, Mr. Lindgren 

 mentions some interesting facts that go to show the existence of differential 

 uplifts and subsidence in the plateau region. 



Glaciation, whose effects are to be seen down to about 4,000 feet in 

 some places, is also given some considerable mention. This must have been 

 one of the most extensively glaciated regions in the whole Cordilleran system 



