t 60 milliards of square yards.

Stromboli, in the Mediterranean (Fig. 23), though only 2500 feet in
height, is very imposing from its superb regularity, and its roots
plunge below the surface to a depth of 4000 feet.

It is, moreover, very interesting from the regularity of its action,
which has a period of 5 minutes or a little less. On looking down into
the crater one sees at a depth of say 300 feet a seething mass of
red-hot lava; this gradually rises, and then explodes, throwing up a
cloud of vapour and stones, after which it sinks again. So regular is it
that the Volcano has been compared to a "flashing" lighthouse, and this
wonderful process has been going on for ages.

[Illustration: Fig. 23.--Stromboli, viewed from the north-west, April
1874.]

Though long extinct, volcanoes once existed in the British Isles;
Arthur's Seat, near Edinburgh, for instance, appears to be the funnel of
a small volcano, belonging to the Carboniferous period.

The summit of a volcanic mountain is sometimes entirely blown away.
Between my first two visits to Vesuvius 200 feet of the mountain had
thus disappeared. Vesuvius itself stands in a more ancient crater, part
of which still remains, and is now known as Somma, the greater portion
having disappeared in the great eruption of 79, when the mountain,
waking from its long sleep, destroyed Herculaneum and Pompeii.

As regards the origin of volcanoes there have been two main theories.
Impressed by the magnitude and grandeur of the phenomena, enhanced as
they are by their destructive character, many have been disposed to
regard the craters of volcanoes as gigantic chimneys, passing right
through the solid crust of the globe, and communicating with a central
fire. Recent researches, however, have indicated that, grand and
imposing as they are, volcanoes must yet be regarded as due mainly to
local and superficial causes.

A glance at the map shows that volcanoes are almost always situated on,
or near, the sea coast. From the interior of continents they are
entirely wanting. The number of active volcanoes in the Andes,
contrasted with their absence in the Alps and Ourals, the Himalayas, and
Central Asian chains, is very striking. Indeed, the Pacific Ocean is
encircled, as Ritter has pointed out, by a ring of fire. Beginning with
New Zealand, we have the Volcanoes of Tongariro, Whakaii, etc.; thence
the circle passes through the Fiji Islands, Solomon Islands, New Guinea,
Timor, Flores