596 geology: 



c. Intermediats phenomena.— On the border-line between the intru- 

 sive and the extrusive phenomena there are special cases of interest. 

 There appear to be certain instances in which the intrusion comes so 

 near the surface as to develop explosive phenomena without the extru- 

 sion of lava. From the nature of the case this is an interpretation rather 

 than a demonstration. It is certain, however, that occasional violent 

 explosions take place where no lava comes in sight. This sometimes 

 occurs in old volcanic formations, and sometimes in regions of undis- 

 turbed horizontal strata. In the former case the phenomena may be 

 due to the intrusion of a fresh tongue of lava below, or it may be due to 

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

 uncooled from previous volcanic action, and the development, by such 

 contact, of a volume of confined steam sufficient to produce the explo- 

 sion. A case of this doubtful kind occurred at Bandai-San in Japan in 

 1888, 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 altogether similar to a typical volcanic eruption. A large 

 tract of adjacent country was devastated, and many lives lost. The 

 whole action, however, was concentrated in the initial explosion, and 

 within a few hours the cloud of ashes had disappeared and the phenome- 

 non was ended. An examination of the disrupted area revealed no signs 

 of liquid lava. 



An example of the latter class is Coon Butte in Arizona.* This con- 

 sists of a rim of f ragmental material encircling a crater-like pit from which 

 the fragments were obviously forced by violent explosion. The pit is in 

 ordinary sedimentary strata, and the material of the rim is composed of 

 the disrupted fragments of the sedimentary rock ejected from the pit. 

 There are no signs of igneous material, but there was igneous action in 

 the vicinity. Fragments of a meteorite were found on the rim and in 

 the vicinity, but this association appears to be accidental. Computation 

 shows that the volume of the material of the rim closely matches the size 

 of the pit. The source of the explosion is not demonstrable, and it may 

 be an error to connect it with an intrusion of lava below; but since intru- 

 sions rise to various degrees of nearness to the surface, and in innumera- 

 ble cases reach the surface, there is every reason to entertain the concep- 



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



