VULCANISM 369 



due to the penetration of surface-waters to hot rocks that have 

 remained uncooled from previous volcanic action. The contact 

 of the water with the hot rock may develop a volume of confined 

 steam sufficient to cause the explosion. A case of this kind occurred 

 in 1888 at Bandai-San in Japan, where there was a sudden and 

 violent explosion which blew away a considerable part of the side 

 of a volcanic mountain which had not been in eruption for at least 

 a thousand years. The mass and violence of the exploded material 

 was such as to fill the air with ashes and debris in a fashion alto- 

 gether similar to a typical volcanic eruption. The eruption was 

 confined to one explosion, and within a few hours the cloud of dust 

 had disappeared and the phenomenon was ended. No lava was 

 extruded. 



Another illustration is perhaps furnished by Coon Butte, Ari- 

 zona. 1 This "butte " consists of a rim of fragment al material encir- 

 cling a crater-like pit from which the fragments were obviously 

 'ejected by an explosion. The pit is in sedimentary strata, and the 

 material of the rim is composed of fragments of the sedimentary 

 rock thrown out of the pit. The volume of the material in the rim 

 is in keeping with the size of the pit. No igneous rocks appear in 

 the pit or about it, though there has been igneous action in the vi- 

 cinity. The cause of the explosion is not demonstrable, and it may 

 be an error to connect it with an intrusion of lava below. Frag- 

 ments of meteorites were found about the butte, but this association 

 may be accidental or causal. It has been suggested that a large 

 meteorite fell at the site of the butte, and penetrating the earth a 

 few hundred feet, exploded. 2 This sequence of events would ac- 

 count for the pit and the rim. 



2. EXTRUSIONS 



When molten rock is forced to the surface' it gives rise to the 

 most intense and impressive of all geological phenomena. The 

 energies acquired in the interior under great compression here find 

 sudden relief. Enclosed gases often expand with extreme violence, 



1 Gilbert, 14th Ann. Kept., U. S. Geol. Surv., Pt. I, p. 187. 



2 Fairchild, Bull. G. S. A., Vol. XVIII, p. 493. 



