rable than that of air.[536]

We learn from the valuable observations made by Mr. Dana on the active
volcanoes of the Sandwich Islands, that large sheets of compact basaltic
lava have been poured out of craters at the top or near the summits of
flattened domes higher than Etna, as in the case of Mount Loa for
example, where a copious stream two miles broad and twenty-five miles
long proceeded from an opening 13,000 feet above the level of the sea.
The usual slope of these sheets of lava is between 5 degrees. and 10
degrees.; but Mr. Dana convinced himself that, owing to the suddenness
with which they cool in the air, some lavas may occasionally form on
slopes equalling 25 degrees, and still preserve a considerable
compactness of texture. It is even proved, he says, from what he saw in
the great lateral crater of Kilauea, on the flanks of Mount Loa, that a
mass of such melted rock may consolidate at an inclination of 30
degrees, and be continuous for 300 or 400 feet. Such masses are narrow,
he admits, "but if the source had been more generous, they would have
had a greater breadth, and by a succession of ejections overspreading
each cooled layer, a considerable thickness might have been
attained."[537] The same author has also shown, as before mentioned,
that in the "cinder cones" of the Sandwich Islands, the strata have an
original inclination of between 35 degrees and 40 degrees.[538]

Mr. Scrope, writing in 1827, attributed the formation of a volcanic cone
chiefly to matter ejected from a central orifice, but partly to the
injection of lava into dikes, and "to that force of gaseous expansion,
the intensity of which, in the central parts of the cone, is attested
by local earthquakes, which so often accompany eruptions.[539] It is the
opinion of MM. Von Buch, De Beaumont, and Dufranoy, that the sheets of
lava on Somma are so uniform and compact, that their original
inclination did not exceed four or five degrees, and that four-fifths,
therefore, of their present slope is due to their having been
subsequently tilted and upraised. Notwithstanding the light thrown by M.
de Beaumont on the laws regulating the flow and consolidation of lava, I
do not conceive that these laws are as yet sufficiently determined to
warrant us in assigning so much of the inclined position of the beds of
Somma to the subsequent rending and dislocation of the cone. Even if
this were admitted, it is far more in harmony with the usual mode of
devel