218 



STRUCTURAL GEOLOGY. 



is outpoured, one on another, until masses 2,000 to 3,000 

 feet thick are piled up (Fig. 125). 



The extent and thickness of some of these lava-floods 

 are almost incredible. The great lava-flood of the North- 

 west covers the whole of northern California, north- 

 western Nevada, and a great part of Oregon, Washington, 

 and Idaho, and extends far into Montana and British 

 Columbia. Its area is supposed to be 150,000 square 

 miles, and its thickness, where cut through by the Co- 

 lumbia River, is at least 3,000 feet. There are about a 

 dozen extinct volcanoes dotted, at wide intervals, over 

 this vast area. It seems certain that the lava came up 

 through fissures in the Cascade and Blue Mountains, and 

 spread as sheets which covered the whole intervening 

 space. Afterward eruptive activity continued, in a more 

 feeble form as volcanoes, almost to the present time. 

 The great Deccan lava-field, described by the geologists 

 of India, covers an area of 200,000 square miles, and is in 

 places 6,000 feet thick, and there is no evidence of any 

 crater-eruptions at all. 



These very extensive sheets are usually basalt. In 

 some parts of the Utah and Nevada Basin region, how- 

 ever, rhyolitic and trachytic lavas are found 7,000 feet 

 thick, but these are far less extensive. As a general rule, 

 the basic lavas, like basalt, were very liquid (superfused), 

 and spread out in thin sheets, while the acidic lavas, like 

 trachyte, have been stiffly viscous- (semi-fused), and were 

 squeezed out dome-shaped. 



FIG. 126. lutercalary beds. 



