178 PHYSIOGRAPHY 



IGNEOUS PHENOMENA NOT STRICTLY VOLCANIC 



Fissure eruptions. Lava sometimes rises to the surf ace through 

 great fissures instead of through the relatively small vents of vol- 

 canoes. From such fissures floods of lava spread over the sur- 

 rounding country for hundreds of miles. Such lava floods once 



Fig. 176. Marysville buttes in contour. (U. S. Geol. Surv.) 



occurred in Oregon, Washington, and Idaho, where, by successive 

 flows, the former hills and valleys were buried, and a vast plateau 

 200,000 square miles or more in extent was built up (Fig. 8). In 

 some places, the nearly level surface of the level plateau meets the 

 mountains along its border, somewhat as the sea meets the land, 

 while islands of older rock rise above it. 



