5^4 



GEOLOGY. 



Massive outflows of this class constitute by far the greatest phe- 

 nomena of the extrusive type, though they are not now the dominant 

 type. It has been sometimes thought that the more local volcanic 

 type of extrusion followed the more massive fissure type as a phase 

 of decline; but this has not been substantiated. 



b. Volcanic eruptions. — In the types of eruption prevailing at the 



Fig. 457. — Lava-flow near the Jordan craters, Malheur Co., Oregon. Though not of 

 the gigantic order, it illustrates the general aspect of massive lava-flows. (Russell, 

 ,. U. S. Geol Surv.) 



present time, the lavas are forced out through ducts or perhaps short 

 fissures or sections of fissures, and build up cones about the vents, the 

 eruptive action maintaining craters in the centers of the cones. The 

 essential feature of a volcano is the issuance of hot rock and gas from 

 a local vent. A mountain is the usual result, but the mountain is 

 secondar}^ and not usually present in the first stages; the localized 

 eruption is the primary and necessary factor. The amount of rock 



