44 p. B. KING 



as many believe, the present is merely an interglacial rather than a 

 post-glacial period, far-reaching climatic fluctuations in the region 

 may be anticipated in the future. 



Northwestern Volcanic Province (Columbia Plateaus and Cascade 

 Range) 



General Setting. The northwestern part of the United States 

 exhibits one of the most drastic later modifications of the orogenic 

 structure and topography in the Cordilleran region. Elsewhere in the 

 Cordillera, volcanism interrupted or modified other post-orogenic 

 processes; here it dominated the scene. The deformed geosynclinal 

 rocks are covered, in places deeply, by great floods and piles of lava, 

 and by associated breccias, tuffs, and sediments. In northern Oregon 

 and southern Washington no rocks older than these are exposed for 

 300 miles parallel with the coast, or 400 miles inland ; they also ex- 

 tend over an even greater area to the south and southeast where 

 older rocks emerge in places (Fig. 9). The volcanic regime was 

 prolonged and extended, at one place or another, through most of 

 Tertiary and Quaternary times. 



The Nevadan belt of deformed eugeosynclinal strata frames the 

 region on the south, east, and north. It is exposed at intervals be- 

 tween the Klamath Mountains of southern Oregon and the Cascade 

 Range of northern Washington, but describes a great arc eastward, 

 which passes through the highlands of northeastern Oregon and 

 western Idaho (Fig. 9) . The volcanics are confined by the arc on the 

 north and east, but break across it on the southeast, where they 

 extend into southeastern Oregon and southern Idaho. 



This extraordinary localization of volcanic activity provides food 

 for speculation for which there is no certain answer, especially as the 

 substructure upon which the volcanics were built is widely buried — 

 and entirely so within the area enclosed by the Nevadan arc. A 

 question is worth asking, however, whether the area within the arc 

 might not have been an oceanic embayment, floored by simatic 

 crust, until well through the geosynclinal phase (Figs. 2 and 3), and 

 was only added to the continent later, by volcanism and sedimenta- 

 tion. This possibility is suggested by dominance of basalts within the 

 arc, which were seemingly derived without contamination from the 

 underlying simatic layer, and by more varied lavas southeast of the 

 arc, where there was evidently greater mixing of simatic and sialic 

 crustal materials. 



