GEOLOGICAL FORMATIONS. U 



of over 80° and a height of 850 meters (or about 2,800 feet). There are 

 more precipitous stretches on the larger east and west ridges than else- 

 where, and they are almost invariably on the northern slopes. The 

 reason for this appears to be that these divides are situated along 

 great faulting lines. Notwithstanding the steepness of (he ridges, 

 they do not commonly present rocky sides. The solid bed rock of the 

 country is more frequently deeply covered with debris and soil than 

 exposed. The ravines are tortuous and narrow. Even the longest 

 valleys are comparatively narrow when one considers the great number 

 of side ravines which open into them. A widtii of 3.2 kilometers 

 (2 miles) is a rarity, and is reached only in the slackwater portion of 

 the valleys of the Couir d'Alene and St. Joseph rivers. The average 

 width of the valleys of the principal streams is about 1 kilometer 

 (or about live-eighths of a mile). The width of the lateral ravines varies 

 so much that no average can be given, it will often not exceed 10 to 

 15 meters (33 to 40 feet), with the ridges rising 300 to 500 meters (1,000 

 to 1,600 feet) above the floor of the ravine. Such narrow places are 

 deprived of the direct sunlight during several months of the year. 



The geological formations of the region are wholly composed of non- 

 fossiliferous rocks. In the southern portion micaceous, granitic, feld- 

 spathic, and syenitic rocks abound. The southern base of the C(eur 

 d'Alene triangle is almost wholly composed of these primary rocks. 

 In the middle and lower portions of the St. Mary and St. Joseph there 

 are large areas covered with basaltic outflows, which connect to the 

 northward, near Lake Oceur d'Alene, with the basaltic rocks of the 

 plains of the Columbia. The central and northern portions of the region 

 have less of the primary rocks exposed. The prevailing formations here 

 are siliceous magnesian schists, great masses of ferruginous quartz ite, 

 and here and there dolomitic and calcareous rocks. These two latter 

 classes of rocks are especially abundant near Lake Fend Oreille, 

 along the more northern portion of the western mountain rim, and 

 appear to be the final southeasterly extension of the dolomitic rocks, 

 which abound northwesterly toward the Golville region. The basaltic 

 rocks are absent in the country north and northeast of Lake Cu'ur 

 d'Alene, except over a small area which extends 10 kilometers (0 miles) 

 northeast from the lake. The magnesian schists are frequently trav- 

 ersed by various kinds of igneous dikes. This is especially the case 

 in the basins of the North and South forks of the Coeur d'Alene River. 

 The quartzite rocks of the Coeur d'Alenes are prominent features in 

 the geology of the country. 



The thickness of the bedded rocks of the Cceur d'Alenes can not be 

 told with absolute certainty. It is, however, not less than 3,000 meters 

 (or about 9,800 feet). These figures have been computed from a careful 

 measurement of the exposures of bedded rocks which occur along the 

 east shore of Lake rend Oreille and in the North Fork basin. 



The geological age to which they belong has, to my knowledge, never 



