J. D. Dana— Volcanic Action. 103 
1. General. 
1. The more prominent phenomena of volcanoes.—In the upward 
movement of liquid lavas, two operations apparently distinct, 
are concerned: (1) the ascending movement of the liquid rock 
in the subterranean conduit of the volcano from the deep-seated 
crustal or sub-crustal region of fusion; (2) the projection of the 
lava aerially upward from the surface of liquid lava in the 
vent. Outflows accompany the upward movement whenever the 
height reached by the lavas is such that they have an outlet 
either over the walls of the crater or through an opened 
fissure. . Hach of these operations, the upthrow and outflow, 
involves an expulsion of material from subterranean regions ; 
and a usual consequence (3) is a subsidence, or, Anglicé, a 
down-plunge, of the overlying rock, or the material of portions 
of the cone especially the more central. The upthrow, the out- 
throw, and the down-plunge are the three most universal and 
most marked of volcanic phenomena. Connected with these 
operations there are: (4) The escape of vapors from the crater, 
and (5) displacements and the opening of fissures. 
2. The ascensive action.—The actual ascension of lavas to a 
height, in many volcanos, of more than 10,000 feet above the 
sea-level is evidence of (1) the great power of the agency con- 
cerned. In Mt. Loa, a volcanic mountain of frequent extensive 
summit eruptions, the height is 13,000 feet (or six miles reck- 
oned to the sea-bottom on the east.) The observations of Mr. 
G. K. Gilbert on the laccoliths of the Henry mountains, proving 
that strata 10,000 feet thick were arched upward to a height of 
three to five thousand feet, by the ascending and zntrusive lavas 
indicate that the ascensive force at work (2) is deep-seated as 
well as of great power. ‘This inference is sustained also by the 
long continuance of the heat in the lava-conduit of many vol- 
canoes, as shown by centuries of activity, in spite of the various 
cooling influences to which the upper part is subjected. 
_ 8. The deep-seated ascensive action not catustrophic.—Dr. Gil- 
bert’s valuable observations illustrate another point of much 
interest: the slow action of the force concerned, for catastrophic 
violence would have opened large fissures to the surface, and 
produced a surface discharge. A quiet mode of ascent is prob- 
ably the common fact also in the larger fissure eruptions. The 
movement appears to be very slow in the supply conduit of a 
volcano. For the epochs of high level in the lavas of a crater 
—flood level, we might say—are separated by years, years being 
required for a rise in level of a few hundred feet. Three great 
eruptions in Kilauea were eight years apart, and, in the inter- 
val, the lift in each case was about 400 feet. The last of the 
three referred to occurred in 1840; and the eruptions which 
Am. JOUR. ee nae SERIES, VOL. XXXIII, No. 194.—FesruaRy, 1887. 
