heat is intense and it glows like a furnace, so that, during the
night especially, these fiery rivers present a grand yet awful
spectacle. The streams spread themselves till they sometimes attain
a breadth of several miles, with a depth of several hundred feet,
and they flow onward till their length sometimes reaches fifty
miles.

Lava, not being so liquid as water, does not flow so rapidly:
nevertheless, when it is careering down the sides of a mountain, or
where the slope of the ground is considerable, it advances with
great speed. Even when at its hottest, it is somewhat viscid, like
treacle, and this viscidness increases as it cools. Hence on a
level plain, and at some distance from its source, the lava-stream
advances at a leisurely pace. In such circumstances the cooling
proceeds so quickly that a crust of considerable thickness is soon
formed on the top of the current, and persons who are bold enough
may cross the stream by means of this natural bridge. Even where
the current continues flowing rapidly, this crust may be formed on
its surface; and a man, whose curiosity exceeds his prudence, may
stand on the top of it, bore a hole through the crust, and see the
lava flowing underneath his feet!

Nothing can resist the progress of the lava-flood; trees, houses,
everything yields to its massive assault, The trees take fire
before its approach, and when it reaches them they emit a hissing
noise almost amounting to a shriek, and then plunging into the
molten flood are seen no more. Even the sea cannot withstand the
lava-stream, but retires on its approach; so that promontories
stretching to a considerable distance from the shore are formed in
this manner, when the molten matter hardens into stone.