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PHYSICAL GEOLOGY 







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presence of glacial stria? on the crater rim proves that the sum- 

 mit of the mountain was at one time much higher than now, 

 and that a glacier moved down it over what is now the rim. 

 It is believed that the crater was formed by the sinking in of 

 the top of the mountain, the absence of volcanic ejectamenta 

 about the mountain being proof that the mountain was lowered 

 in this way rather than by an explosion. The craters of the 

 moon are two to twenty times larger in diameter than those 

 of the earth and may have been formed in the same manner 

 as those of the Hawaiian type. 



Steep Lava Cones : Volcanoes of the Chim- 

 borazo Type. — It should not be concluded from 

 a study of the Hawaiian volcanoes that all lava 

 cones have a gentle slope. When the lava is 

 viscous, steep-sided cones are formed. The great 

 Ecuador volcano, Chimborazo (20,498 feet), is 

 composed of lava and is, moreover, craterless. As 

 the lava welled up from the vent, it left upon 

 cooling no depression in the summit of the 

 mountain. 



III. Fissure Eruptions 



When lava is very fluid and in great quantity, 

 it may flow long distances and form compara- 

 tively level plains many square miles in extent. 

 The most remarkable lava plateau of this kind in 

 North America (Fig. 304) covers an area of 200,000 

 to 250,000 square miles in Washington, Oregon, 

 Idaho, and California (p. 583). It is a vast plain 

 of black basic lava over which one may ride for 

 many hours on a level surface. The lava which 

 overspread this region poured out from fissures 

 instead of from volcanoes, with little or no explo- 

 sive action, and since it was very fluid, flowed for 

 long distances, filling the valleys and covering the 

 smaller hills. Some portions of the region were 

 buried hundreds of feet deep, the greatest depth 

 being estimated at 3700 feet. This great plain 

 was not built up by a single great outpouring of 

 lava, but by a number of flows, some of which 

 followed each other in rapid succession. On the 



