56 VOLCANOES 



above the sea within historic times. In fact, all volcanic islands 

 are merely submarine volcanoes, or groups of them, which have 

 built their cones above sea-level. If the cones are of fragmental 

 products, the islands are but temporary, because when the activity 

 ceases, the waves cut them down into reefs and shoals. The lava 

 cones persist for long periods. 



FIG. 20. — Truncated tuff cone, island of Oahu. (Photograph by Libbey.) 



Fissure Eruptions. — There is much reason to believe that the 

 mode of volcanic eruption from a single vent, described in the 

 foregoing pages, is not the only method by which molten lava 

 may reach the surface. It would seem that in past times lava has 

 welled up through great fissures and overflowed immense areas in 

 successive floods. As an example of this may be mentioned the 

 vast fields of lava which occur in the northwestern United -States, 

 covering more than 100,000 square miles to the depth of several 

 hundred feet. The largest connected area of this field extends 

 along the Snake River in Idaho, southwest from the Yellowstone 



