specular iron, chlorides of the alkalies, realgar, and orpiment are also occasionally observed among the gaseous emanations in old volcanic regions. Some observers testify to the existence of inflammable gases in sufficient quantities to produce flames, these gases being hydrogen and sulphuretted hydrogen; but there are other observers, equally distinguished, who have had frequent opportunities to examine volcanoes, both in action and at rest, and who have never seen any indication of flame. What is generally called fire, in eruptions, is, of course, simply the light or the reflection of the lava, which is intensely heated but not actually undergoing combustion.

During the solfataric condition of a volcano, its crater becomes blocked up with congealed lava, perhaps overgrown with forests and dense vegetation, and the signs of activity die out, until, as the last relic of former life, only a thermal spring may be found here and there, an indication of the mighty forces slumbering beneath. Such is the present condition of nearly all the great volcanic cones on our own coast, from Arizona to Oregon.

Midway between the conditions of solfataric repose and of paroxysmal violence is another stage of activity, in which some volcanoes remain during long periods, while a few appear never to pass out of it into more violent action; others, however, remain in this condition of partial repose during the intervals between violent outbursts. At such times the crater and the channel connecting it with the interior remain open, and the lava can be seen in them maintaining a mobile condition, while occasional explosions of the surface of the melted mass take place, fragments of slag and cinders being thrown up and mostly falling back into the abyss from which they were hurled. This was the condition of Vesuvius when visited by the writer in November, 1843. At that time there had been no eruption of lava overflowing the lip of the crater since 1839, when the cavity was cleaned out, and left as a funnel three hundred feet deep, accessible to the bottom. From this time a smaller cone began to grow inside the large one, and in 1843 it was about fifty feet high, and could be reached by clambering down the walls of the old crater, the whole bottom

of which, around the foot of the new cone, was covered with lava, which was red-hot a few inches beneath the surface, but could in most places be safely walked on. From the vent a shower of cinders was thrown up every fifteen or twenty minates; and although it was possible to climb to the summit of the cone on the windward side, with occasional calls for skill in dodging the projectiles, the orifice was too much occupied with ascending vapors to permit of anything below being clearly seen. This interior cone kept on growing by additions made to it from the falling materials, and finally, in 1847, the crater became filled, and the lava overflowed, running down on three sides at once. From that time forward Vesuvius became very uneasy, and finally a great eruption took place in 1850. This lasted about twenty days, and when it was over the summit of the mountain was left much changed in form, the old walls having been broken down, the central cone reduced in size, and a new crater formed, about two miles in circumference, and a hundred and fifty feet deep. The volcano then remained quiet from 1850 to 1855, when it became very active; again a grand eruption occurred in 1858, and slight ones in 1860 and 1861. Since the last-named year Vesuvius has rarely been at rest. During the winter of 1867-68 there was a great outburst of volcanic force, which lasted several months.

In the condition of half-repose just noticed as not uncommon between intervals of paroxysmal activity, observers are able to look down into the throat or channel of Etna, as well as of Stromboli, during the periods of repose between the eruptions, which take place with great regularity every ten or fifteen minutes. At such times the lava is seen to move up and down in the chimney; as it rises, its surface swells up into a great blister, which finally gives way to the tension exerted and explodes with a loud noise, the fragments being scattered and thrown up with great force; the column of melted matter then sinks back into temporary repose, and rises again after an interval of a few minutes. The same phenomena were observed on Sangay, one of the Quito group, a permanently half-active volcano, like Stromboli.

The most gigantic exhibition of this condition of the volcanic forces is to be seen in Kilauea during its quiet periods, when VOL. CEX.NO. 224.

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the crater, which is three miles in its greatest diameter, has in it large pools of boiling and extremely fluid lava, which is continually thrown up in jets of from thirty to forty feet in height, that fall back into the pool before they have time to cool. These lakes of liquid fire vary in size according as the volcano is more or less active, and sometimes cover the whole area of the crater, the wind raising the surface in waves of molten rock, which dash against the encircling walls with an indescribably grand effect. The greater the liquidity of the lava, the less the force with which it is thrown up, for the jets of imprisoned vapors do not have time, in a very fluid material, to accumulate sufficient pressure to act with extreme explosive violence.

The phenomena which, we have seen, thus characterize the semi-active condition of volcanic activity are, in most respects, similar to those of the fully active state, differing rather in the degree of violence with which they are manifested than in kind. It seems, indeed, that the longer and more complete the repose of the volcano has been, the more violent its action when it once breaks out again. This is natural, for the resistance to an outburst must, as an ordinary thing, go on increasing the longer the vent remains stopped, and when this resistance is finally overcome the magnitude of the eruption will be proportionate to the force required to clear the way. The first recorded eruption of Vesuvius was the most violent of any which are known to have taken place; next to this in its destructive effects was that of 1631, occurring, as it did, after several hundred years of entire repose.

In regard to the precursors of a violent eruption, or the symptoms by which the approach of one may be detected, there is much uncertainty. It may be said, however, that a great outbreak is to be expected when the internal forces begin to show signs of uneasiness and the usual phenomena of halfrepose to be intensified in their action. It seems a well-authenticated fact, that previous to an eruption of Vesuvius the wells and springs adjacent to the mountain begin to dry up. When volcanic cones are covered with snow it is not uncommon for the eruptions to be preceded by devastating floods, caused by its melting, the natural result of the gradual warming up of the mountain mass.

The following are the ordinary phenomena of violent eruptions: an appearance of fire; lightning; subterraneous noises, or thunder; ejection of ashes, cinders, or blocks of lava; the pouring out of melted lava; and, in connection with earthquake shocks, fissures in the earth and permanent changes in the level of the adjacent country.

Great volcanic paroxysms are often preceded by more or less violent earthquake shocks, which are both frequent and prolonged, but usually limited to the mass of the volcano itself or its immediate vicinity. Tremendous underground detonations are heard, sounding like the firing of heavy cannon or repeated volleys of musketry. These sounds are heard at all points at the same instant of time, showing that they are propagated through the crust of the earth and also that they come from a great distance beneath the surface. These explosive sounds have been heard simultaneously over areas of many thousand square miles. Thus the noise of the outbreak of the eruption of Temboro, on the island of Sumbawa, was heard all over Java, and everywhere supposed to come from some point in the immediate vicinity. It was distinctly audible at points two thousand miles apart. As the shocks and sounds continue, people become more and more alarmed and excited, and imagine that they see every kind of portent in the sky or in the conduct of animals. It is generally thought that an oppressive stillness pervades the atmosphere just before the moment of the great outbreak, and that dogs, swine, and geese exhibit peculiar indications of fear. How much reliance can be placed on the statements of the sensitiveness of animals to impending catastrophes, it is not easy to say; but it is evident that the circumstances of a great eruption are eminently favorable to a highly imaginative condition of the mental faculties.

The earthquake shocks preceding volcanic outbreaks take place while the internal conflict is going on between the imprisoned lava, seeking to find a vent, and the resistance offered by the weight and tenacity of the superincumbent crust. When the internal pressure which seeks relief in bringing up to the surface the material on which it is acting at last has its own way, the explosion is tremendous, the mass of the volcano

being shaken to its very foundations. As soon as the channel of communication with the interior is opened, which channel usually communicates with the bottom of the old crater, although not unfrequently opened through some new side fissure, the pent-up vapors and gases begin to escape with tremendous force, carrying up in the air, torn into fragments, rocky masses, which then fall and are thrown out again repeatedly, and thus, by friction against each other or by actual explosion, through sudden changes of temperature, are rapidly reduced to powder and carried off with the gases or vapors which rise from the chimney of the crater.

unless spreads

The ejection of vapor and ashes, as the comminuted fragments of lava are called, is thus,described by Scrope, who was an eye-witness of one of the grandest eruptions of Vesuvius,that of 1822. He says: "The rise of the vapor produces the appearance of a column several thousand feet high, based on the edges of the crater, and appearing from a distance to consist of a mass of innumerable globular clouds of extreme whiteness, resembling vast balls of cotton rolling one over the other as they ascend, impelled by the pressure of fresh supplies incessantly urged upwards by the continued explosions. At a certain height this column dilates horizontally, and driven in any particular direction by aerial currents on all sides into a dark and turbid circular cloud. In very favorable atmospheric circumstances, the cloud with the supporting column has the figure of an immense umbrella, or of the Italian pine, to which Pliny the younger compared that of the eruption of Vesuvius in A. D. 79, and which was accurately reproduced in October, 1822. Strongly contrasting with this pillar of white vapor-puffs is seen a continued jet of black einders, stones, and ashes, the larger and heavier fragments falling back visibly after describing a parabolic curve. This jet of solid fragmentary matter often reaches a height of several thousand feet, while the vapor pillar rises still higher. Forked lightnings of great vividness and beauty are continually darted from different parts of the cloud, but principally its borders. The continual increase of the overhanging cloud soon hides the light of day from the districts situated below it, and the gradual precipitation of the sand and ashes it contains contributes