48 



PHYSICAL GEOLOGY 



and irregular (Fig. 24). Successive fissure eruptions may 

 build up great plateaus. Such a lava plateau, with an area 



FIG. 23. Surface of a comparatively smooth lava flow. Jordan Craters, 

 Oregon. (Russell, U.S. Geol. Surv.) 

 How may the fissures be explained ? 



of some 200,000 square miles, occurs in Washington, Oregon, 

 and Idaho (Fig. 25). The lava is locally 4000 feet in thick- 



Fio. 24. Margin of a lava flow, Cinder Buttes, Idaho. The broken condi- 

 tion of the lava is due to movement after the outside had hardened. Note 

 the steep edge of the flow. (Russell, U.S. Geol. Surv.) 



ness along the Snake River, which flows through the plateau 

 in a deep canon. The canon walls show that the elevations 



