ust allude. By a study
of the evidences presented by coral-reefs, raised beaches, submerged
forests, and other phenomena of a similar kind, it can be shown that
certain wide areas of the land and of the ocean-floor are at the present
time in a state of subsidence, while other equally large areas are
being upheaved. And the observations of the geologist prove that similar
upward and downward movements of portions of the earth's crust have been
going on through all geological times.

Now, as Mr. Darwin has so well shown in his work on "Coral Reefs," if we
trace upon a map the areas of the earth's surface which are undergoing
upheaval and subsidence respectively, we shall find that nearly all the
active volcanoes of the globe are situated upon rising areas and that
volcanic phenomena are conspicuously absent from those parts of the
earth's crust which can be proved at the present day to be undergoing
depression.

The remarkable linear arrangement of volcanic vents has a significance
that is well worthy of fuller consideration. There are facts known which
point to the cause of this state of affairs. It is not uncommon for
small cones of scoriae to be seen following lines on the flanks or at
the base of a great volcanic mountain. These are undoubtedly lines of
fissure, caused by the subterranean forces. In fact, such fissures have
been seen opening on the sides of Mount Etna, in whose bottom could
be seen the glowing lava. Along these fissures, in a few days, scoriae
cones appeared; on one occasion no less than thirty-six in number.

It is believed by geologists that the linear systems of volcanoes are
ranged along similar lines of fissure in the earth's crust--enormous
breaks, extending for thousands of miles, and the result of internal
energies acting through vast periods of time. Along these immense
fissures in the earth's rock-crust there appear, in place of small
scoriae cones, great volcanoes, built up through the ages by a series of
powerful eruptions, and only ceasing to spout fire themselves when the
portion of the great crack upon which they lie is closed. The greatest
of these fissures is that along the vast sinuous band of volcanoes
extending from near the Arctic circle at Behring's Straits to the
Antarctic circle at South Victoria Land, not far from half round the
earth. It doubtless marks the line of mighty forces which have been
active for millions of years.

CHAPTER XXI.

The Famous Vesuvius and the