VULCANISM 



179 



In this lava plateau, the Snake River has excavated a great 

 canyon (Fig. 49) 4,000 feet deep in some places, and 15 miles wide. 

 The walls of the canyon show the structure of the plateau. They 

 show, among other things, the edges of the successive flows of lava, 

 sometimes separated by beds of sediment, with soils in which the 

 roots and trunks of trees are still preserved. These beds of sedi- 

 ment, and these soils, show that long periods of time elapsed be- 

 tween successive lava-flows. 



An older lava plateau of still greater size and more dissected by 

 erosion occurs in India. Still others, now made rough by erosion, 

 are found on the north coast of Ireland and the west coast of Scot- 

 land, and some of the islands off Scotland are remnants of an old 

 lava plateau. 



Fissure eruptions have occurred in Iceland within historic times. 

 In 1783 lava flowed from a fissure 20 miles or so in length, spreading 

 out in sheets, and advancing down the valleys farther than on the 

 uplands between them. 



While fissure eruptions of lava sometimes build up plateaus or 

 raise the level of the plains on which they spread, they do not 

 commonly give rise to mountains; but mountains are sometimes 

 developed from them, as they are dissected by stream erosion. 



Intrusions of lava. Lava is sometimes intruded from below 

 into the crust of the lithosphere, without rising to the surface. 



Fig. 177. Fig. 178. 



Fig. 177. Ideal cross-section of a laccolith with the accompanying sheet 



and dikes. The black parts are hardened lava. 

 Fig. 178. Ideal cross-section of a group of laccoliths. (U. S. Geol. Surv.) 



In such cases the surface strata may be arched up over the intru- 

 sion, making domes which sometimes reach the size of the moun- 

 tains. The Henry Mountains of Utah are examples (Figs. 177 and 



