EVOLUTION OF MODERN SURFACE FEATURES 



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Fig. 9. Map of northwestern volcanic province, in Washington, Ore- 

 gon, Idaho, and adjacent states. 1, Nevadan basement; metamorphic 

 rocks Hned, plutonic rocks in heavy shading. 2, Inferred margins of 

 Nevadan orogenic belt. 3, Plateau basalts of Miocene and later age. 4, 

 Andesitic volcanics of Cascade Range. 5, Volcanic cones of Cascade 

 Range, mainly of Quaternary age. 6, Other rocks; mainly sedimentary 

 rocks of Mesozoic and Tertiary age, but including some older Tertiary 

 volcanics. 7, Edge of continental area. 



The volcanic rocks and their associates assume diverse geographic 

 forms: along the Pacific, the low Coast Ranges; east of a longi- 

 tudinal depression, the high Cascade Range; and farther inland, the 



