1906. ] 



SEE— THE CAUSE OF EARTHQUAKES. 377 



hardening in its throat ; yet earthquakes which have no surface out- 

 let may successfully maintain a languid existence on a small supply 

 of water, and hence they may continue to be felt in a region long 

 after all volcanoes have been extinguished. 



§ 46. Explanation of immense outHozvs of lava such as are seen 

 in the plateau of Dec can and in Oregon and Utah. 



For a number of years it has been a subject of remark among 

 geologists that the largest lava flows are not the output of volcanoes, 

 but of immense fissures which opened in the earth's crust and per- 

 mitted the welling forth of vast quantities of molten rock. Sir 

 Archibald Geikie (cf. Suess, Vol. I, p. 145) emphasized this view 

 as long ago as 1880. In recent years this theory of the origin of the 

 immense deposits of sheet lava seen in the region of the Columbia 

 River in Oregon and in Utah, as well as in the great tableland of 

 Deccan in India, has been very generally adopted. In all such cases 

 fissures no doubt opened and poured forth the molten rock through- 

 out their length. The subsidence of considerable areas of the earth's 

 crust may have contributed to this outflow, and different degrees 

 of liquidity are invoked to explain the observed phenomena. Reyer 

 suggests (cf. Geikie's "Geology," Vol. I, p. 301) that the degree 

 of saturation with gases and vapors may have influenced the form 

 of eruption, volcanic discharges resulting when the impregnation 

 was strong enough to cause eruption, and tranquil outpourings when 

 the rock is but feebly saturated with explosive gases. 



Major Dutton thinks differences of temperature as well as 

 chemical differences may have been more important in giving the 

 great lava flows their peculiar aspects. In regard to these outflows 

 in general, I believe that the crust cracked open on account of the 

 relative movement of neighboring portions. There are many wavs 

 in which this could occur. If there were any appreciable tangential 

 pressure between two portions of the crust, the outpouring of lava 

 would be less easy ; but since we abandon the contraction theory and 

 deny that the mountains are due to the shrinkage of the crust, there 

 is on this hypothesis no pressure between the two portions of the 

 crust except that due to their weight when resting side by side. If, 

 therefore, the subterranean movements under two neighboring parts 

 should be such as to force a fault apart, there would be nothing to 

 prevent the lava from rising and pouring forth. 



