108 J. D. Dana— Volcanic Action. 
waters from rains as the chief supply ; and Mr. Coan, who lived 
for years at the foot of Mt. Loa and watched the volcanic 
movements, was the first to suggest that seasons of abundant 
rain, besides multiplying steaming holes, increased activity and 
favored eruptions. 
10. The introduction of outside waters, whether marine or 
fresh, into the lava conduit has often been thought an impossi- 
bility; and still it must be a fact, as this is the only means of 
getting the required water. ‘The waters of the land, especially 
those of great rains, becoming subterranean, would descend 
deeply along the various vertical fissures, and the spaces by the 
sides of dikes, it may be to thousands of feet, since the bottom 
of Kilauea is between three and four thousand feet above the 
sea level, (and the summit of Mt. Loa over thirteen thousand) ; 
and if the copious waters filling the narrow passage ways below 
over a large area, anywhere reached the hot region along the 
conduit, they would encounter the action of the heat under great 
hydrostatic pressure, and be forced by the steam pressure gen- 
erated into the conduit of ascending lavas as the easiest and 
only way of escape. Once within the conduit, they would 
follow the course of the hotter, more liquid, and upward-mov- 
ing central portion, and expand into vapor toward the surface, 
there to make cellular and scoriaceous lavas, and escape as they 
could. The lavas from fissures that tapped the conduit below 
the level where the chief part of the waters were received 
should be feebly cellular, or not at all so; but froth-covered 
streams might be expected from the vents or pools of the crater ; 
and cellular or scoriaceous from tapping near the top. The 
facts at Kilauea correspond precisely with this view of the 
operations, and give the explanation full support. The play of 
jets over the surface may serve to entangle some air and add to 
the volume of the confined vapors. But air alone has very 
feeble expanding power when heated, and could not make cells 
in the heavy liquid and much less make a’scoria.* 
11. Projectile force at Kilawea.—The vapors within the viscid 
lava struggling for escape are the obvious and the only possible 
source of the projectile results. The elastic force of the vapors 
would depend on the resistance to escape, and the resistance on 
the viscidity, as in the ordinary boiling process. The ascending 
spheres of vapors in the upper or more superficial part of the 
conduit would become bubbles at the surface; and the bubbles 
would continue to enlarge there until the elastic force within 
was sufficient to break the upper or weaker part, when a verti- 
* The facts observed by Prof. J. W. Judd with regard to the waters present 
(sometimes over 8 per cent.) in an obsidian, (Geol. Mag. for June, 1886), have 
interest here. See also the important paper by J. P. Iddings on Onna at 
page 36 of this volume. 
