wholly unlike ours. The fact of the internal heat of the earth which
becomes very perceptible even at the moderate depths reached in mines
and deep borings, and in the deepest mines becomes a positive
inconvenience, leads many people to suppose that the surface-
temperatures of the earth are partly due to this cause. But it is now
generally admitted that this is not the case, the reason being that all
rocks and soils, in their natural compacted state, are exceedingly bad
conductors of heat.

A striking illustration of this is the fact, that a stream of lava often
continues to be red hot at a few feet depth for years after the surface
is consolidated, and is hardly any warmer than that of the surrounding
land. A still more remarkable case is that of a glacier on the
south-east side of the highest cone of Etna underneath a lava stream
with an intervening bed of volcanic sand only ten feet thick. This was
visited by Sir Charles Lyell in 1828, and a second time thirty years
later, when he made a very careful examination of the strata, and was
quite satisfied that the sand and the lava stream together had actually
preserved this mass of ice, which neither the heat of the lava above it
at its first outflow, nor the continued heat rising from the great
volcano below it, had been able to melt or perceptibly to diminish in
thirty years. Another fact that points in the same direction is the
existence over the whole floor of the deepest oceans of ice-cold water,
which, originating in the polar seas, owing to its greater density sinks
and creeps slowly along the ocean bottom to the depths of the Atlantic
and Pacific, and is not perceptibly warmed by the internal heat of the
earth.

Now the solid crust of the earth is estimated as at least about twenty
miles in average thickness; and, keeping in mind the other facts just