J. D. Dana— Volcanic Action. 113 
the ‘“‘minute” guns heard at Auckland were the larger explo- 
sions; and such a periodicity in the detonations, so rapid and 
continuous, shows that they were those of exploding vapor- 
made bubbles; for nothing else in the mechanism of a volcano 
ean produce it. 
The catastrophic effects of such an action, where the jets are 
single or few, are small, because the break of the vapor-enclos- 
ing shell takes place at the top, and the throw is nearly verti- 
cal; the energy is wasted in the throw of the fragments. Where 
many jets rise together in such lavas, as in a time of violent 
eruption, the explosive action is great; but it is still greatest at 
the center of activity, which is that of greatest liquidity and 
easiest movement. A cone of cinders about the vent might be 
destroyed by being added to the projections; but the more dis- 
tant walls could suffer but little from the projectile action, ex- 
cept in the way of fissuring, and this might be all owing to 
subterranean shakings and the undermining; for the force 
from the accumulated elastic vapors in the rising and explod- 
ing bubbles is too much confined by the limits of the area of 
liquidity within the crater. Hence we have reason to doubt 
much that has been written about explosive eruptions pro- 
jecting off the topsof mountains; or at least to question it, un- 
til the piles of great rocks from such projections have been de- 
scribed.* 
The height of the aerial projections in some volcanoes is 
measured by thousands of feet; but how much of this height 
is owing to ascending currents of air is not stated. The fine- 
ness of the dust that goes to great heights makes it of easy 
transport. 
14, The other forces acting at Kilauea, must be alike 
active in all volcanic regions. The third is the one usually 
appealed to by writers in their explanations of eruptions, espe- 
cially 3a, 3b; and that numbered 8c, must have its effects also, 
although less easily detected where the crater is not broad and 
open, like Kilauea. The fourth, hydrostatic pressure, has, no 
doubt, far more common effects than has been recognized, 
especially in high-sloped mountains like Vesuvius. The quiet 
character of many of the fracturings at Vesuvius and of some 
of its large eruptions are strongly in favor of. this conclusion. 
The sixth operation—subsidence or down-plunge of the under- 
mined parts of the mountain—should be doubly more active 
at Vesuvius than at Kilauea, since the undermining in the lat- 
ter is produced only by the outflow of lava, while at Vesuvius 
there is loss both by outflow and upthrow. I have alluded to 
* According to some describers of the New Zealand eruption, the Tarawera 
mountain had its top blown off in the making of the great southwest chasm. 
The place affords an opportunity to ascertain whether the piles of masses of the 
solid rocks which in that case should exist, are to be found to the southwestward. 
