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 Destruction of Pompeii.

The famous volcano of southern Italy named Vesuvius, which is now so constantly in eruption, was described by the ancients as a cone-shaped mountain with a flat top, on which was a deep circular valley filled with vines and grass, and surrounded by high precipices.  A large population lived on the sides of the mountain, which was covered with beautiful woods, and there were fine flourishing cities at its foot.  So little was the terrible nature of the valley on the top understood, that in A. D. 72, Spartacus, a rebellious Roman gladiator, encamped there with some thousands of fighting men, and the Roman soldiers were let down the precipices in order to surprise and capture them.

There had been earthquakes around the mountain, and one of the cities had been nearly destroyed; but no one was prepared for what occurred seven years after the defeat of Spartacus.  Suddenly, in the year 79 A. D., a terrific rush of smoke, steam, and fire belched from the mountain’s summit; one side of the valley in which Spartacus had encamped was blown off, and its rocks, with vast quantities of ashes, burning stones, and sand, were ejected far into the sky.  They then spread out like a vast pall, and fell far and wide.  For eight days and nights this went on, and the enormous quantity of steam sent up, together with the deluge of rain that fell, produced torrents on the mountain-side, which, carrying onward the fallen ashes, overwhelmed everything in their way.  Sulphurous vapors filled the air and violent tremblings of the earth were constant.