PLAINS AND VALLEYS OF EASTERN WASHINGTON 535 



rounded hills, in which the lava is decayed to great depth, and stony 

 tracts and basalt bluffs along the streams and main valleys. There are 

 few extensive flats, and such as occur are floodplains of the streams or 

 areas in which the decomposed basalt has been swept away down to 

 some especially resistant lava sheet. The latter is probably the origin 

 of a relatively flat plain, about 20 miles from Spokane, on the Palouse 

 branch of the N^orthern Pacific Pailroad, made up of very low flat-topped 

 rocky tables of basalt separated by broad swales, many of which have 

 shallow lakes in the rainy season. The larger streams, such as the Co- 

 lumbia, Spokane, and Snake rivers, generally flow in steep-walled valleys 

 or canyons trenched beneath the floor of the uplifted and tilted basin 

 plain except where, in the west central portion of the basin, the plain 

 descends nearly to the level of the Columbia Eiver. On the north and 

 east sides outliers of older mountains rise like monadnocks from the 

 plain; but I am not certain of the relation between the Cascade Eange 

 of central Washington and the plain of the Columbia Basin. 



The Columbia River (Yakima) lava has been shown by Smith, Mer- 

 riam,® Sinclair, and others to be mainly of early Miocene age, as that 

 term is used by the students of vertebrate paleontology, although else- 

 where in the Pacific Coast country the outpouring of basalt continued 

 into the Pliocene period. The highest sheet in the basin is probably 

 middle Miocene in age. The earlier lavas may not have attained such 

 an elevation along the eastern border of the basin as to have affected the 

 drainage of the valleys in the Idaho mountains; hence the deep valley 

 filling under the 600-foot terrace in the Coeur d^Alene Yalley may be 

 entirely middle Miocene in age. The gravel of the 1,150-foot terrace at 

 Kellogg may be rather early Miocene. There may have been an interval 

 between the close of the volcanism and the orographic disturbance 

 which, by tilting the lava plain, inaugurated the erosion of the canyons. 

 The larger canyons, as already pointed out in the case of the Clearwater, 

 are comparable with the Pleistocene canyons of the California moun- 

 tains and probably of the same age. They make it reasonable to attribute 

 the uplift and tilting of the lava plain to the period of orographic dis- 

 turbance of wide extent in the Pacific Coast country which opened the 

 Quaternary Era. 



Summary 



The Tertiary history of the region discussed opens with the erosion of 

 deep, broad valleys in the Clearwater region of Idaho and deeper and 



"Tertiary faunas of the John Day region. Bull. Dept. Geol., Univ. Cal., vol. 5, p. 193. 

 XXXVIII— Bull. Geol. Soc. Am., Vol. 23, 1911 



