REVIEW OF VOLCANIC ACTION. 365 
something of its seeming extravagance vanishes upon seeing in a 
corner of Kilauea a single lake of boiling lava one thousand feet by 
one thousand five hundred in diameter; and remembering that were 
Kilauea in general action over its whole surface, the molten interior 
might average a mile in breadth by three and a half miles in length. 
At the top of Mount Loa, the crater, though a mere dot in the summit 
plain, is two miles in diameter; giving this breadth to the fluid in- 
terior, even at a height of nearly fourteen thousand feet, as the 
figure on page 219 illustrates. Thus in these recent times, with the 
crater of its present contracted limits, we find that when in full 
action, Mount Loa bears evidence of a vast amount of matter in fusion 
within the volcano. And if there is any probability in the supposition 
that the interior plains of some volcanic islands ten or a dozen miles 
in diameter were actual craters, (there is no doubt that one hundred, 
and even one hundred and fifty miles is an actual breadth in the moon, ) 
we can no longer deem it an improbable hypothesis that the whole 
interior was once in fusion together, over a breadth as great as any 
facts presented in igneous regions would seem to require. Only 
twice the breadth of the summit crater of Mount Loa would explain 
the facts at Tahiti. We learn from the instructive pools of Kilauea, 
that the boiling centres at times become extended by melting into them- 
selves the adjoining layers, or contract again by cooling; and these 
variations, which would take place in vents of any size, may account 
for many facts presented by these islands. Such a central mass of 
fluidity, covered on all sides from the external air, would cool with 
extreme slowness, and offer the circumstances necessary for crystal- 
lizing the feldspathic rock into syenites, and forming hornblende. 
We shall recur to this topic in the sequel. 
Rapidity of the Formation of Lava Cones.—We know little with 
reference to the rate of increase of Mount Loa: and if correct data 
were at hand for the existing period, we could not assume these as 
data for the past. The whole surface is about two thousand four 
hundred square miles in area. Consequently it would require one 
hundred and sixty such eruptions as that of 1840 to cover the moun- 
tain with a single layer of twelve feet. If eruptions have happened 
no more frequently in former times than at present, one thousand 
two hundred years would be occupied in this increase. Allowing for 
an occasional outbreak from the summit crater, we may set down 
eight hundred years as the time, on this hypothesis, required for 
92 
