to envelope the atmosphere in gloom, and adds to the consternation of the inhabitants of the vicinity."

If the volcano is one which emits lava, this rises gradually in the crater and finally overflows it at the lowest point, unless it succeeds in forcing its way through some side fissure. The molten mass finds its way down the declivity with a rapidity proportioned to its fluidity, overwhelming and destroying everything which it encounters. Clouds of vapor rise from the flowing mass, visible during the day, the exterior soon becoming covered with a dark crust of scoriæ, occasional fissures in which reveal, especially at night, the presence of the intensely ignited material beneath. The flow of lava from the volcanic vent indicates that the crisis of the disturbance is passed, and that there will thenceforth be a gradual slackening in the violence of the eruptive action.

Not a few volcanoes, however, never send out lava, but only ashes and cinders; these are usually the very large ones, as, for instance, the great cones of South America. It is also true that large volcanoes are less frequently than smaller ones the seat of great disturbances. The frequency of the eruption seems to be, in a measure, in inverse proportion to the height of the volcanic cones from which they proceed. Thus the lofty volcanoes of South America have rarely had more than one eruption each in a century; the Peak of Teneriffe had only three between 1430 and 1798. This is very natural, since the higher the cone the greater the resistance offered to an outbreak by the weight of the column. But the rule is not of universal application. Closely connected with the last-mentioned fact is another, previously suggested, namely, that the most fearful eruptions may be expected to occur after long intervals of repose. Both circumstances indicate very clearly the accumulation of force necessary to overcome increased resist

ance.

At night the column of vapor and ejected solid material appears red, not because it is actually a column of flame, but partly because it is illuminated by the reflection from the redhot lava below, and also because the fragments carried up in it are themselves intensely heated. The fact that the column remains perpendicular all the time is a proof that it is not a

flame, for, if that were the case, it would be swayed by the wind; but one of the most characteristic features of an eruption is, that the pillar of fire seems to stand immovable amid the "wreck of matter" around it."

The electrical phenomena of a great eruption are extremely interesting. The upward rush of heated vapor gives rise to furious disturbances in the condition of the atmosphere, as is also the case, on a small scale, when steam escapes from an ordinary boiler through the safety-valve. A constant play of lightning goes on around the ascending column, and the noise of the thunder is mingled with the crash of the projected fragments of rock. Tremendous bursts of rain, or even hail, often occur at the same time, and from the same cause, the electrical disturbance of the atmosphere, of the torrents of water rushing down the sides of the volcano is often more devastating than that of the lava itself. ·

namely, and the effect

The mass of ashes, scoriæ, or cinders thrown out in some volcanic eruptions is prodigious. In that of Vesuvius, in 1794, four cones were formed on a fissure nearly half a mile long, each with its separate crater, throwing up showers of redhot cinders in such rapid succession as to appear like one continuous sheet of fire in the air. These showers really consisted of semi-fluid lava, which expanded in the air like soft paste. This continued for several days, so that the whole space above the crater seemed to be filled with the fragments, which formed a column a mile in circumference and rose to an immense height, then spread out, and seemed to cover a much greater area than the base of the mountain itself. Generally, however, these ejections of cinders are intermittent in character, sometimes following each other in rapid puffs, at others occurring as a succession of explosions at longer intervals.

The size of the fragments thus ejected is variable; often they are as fine as the finest dust, but sometimes the lava is thrown out in great masses. Thus Cotopaxi vomited forth, in 1533, blocks of rock ten feet or more in diameter. The so-called volcanic bombs shelled out by Vesuvius are usually from the size of the fist to that of the head. Generally they are irregu larly rounded or pear-shaped; but in volcanoes in which the

lava is very liquid it comes down in masses which flatten out into cakes when they strike the ground. The finer fragments which in prodigious quantity accompany the larger, and usually vary from the size of a pea to that of a walnut, are now almost everywhere known by the Italian name of lapilli, or rapilli. The finer, sand-like material is called puzzolana, and the finest of all ceneri, or ashes.

One of the most curious features of the eruptions of some volcanoes is the prodigious number of small but perfectly formed crystals which are thrown out among the materials shot up from below. Vesuvius, which is a perfect treasurechamber of interesting minerals, while most of the American volcanoes are miserably provided in this way,—has furnished at times showers of beautiful crystals of augite, leucite, mica, and black garnet, the first-named being the most abundant. They seem to have existed ready formed in the semi-fluid lava, or else to have crystallized out suddenly at the moment of its solidification; which of these suppositions is the correct one is not thoroughly settled, although the first seems by far the most probable.

Vast masses of volcanic breccia occur in regions of eruptive rock, as for instance in California, where beds hundreds of feet in thickness are found covering many square miles of area, entirely made up of angular fragments of lava, of all sizes, which have evidently been ejected in the form in which we now see them. The explosions with which volcanic eruptions begin after long periods of tranquillity, and which sometimes pulverize the whole summit of the mountain mass in which they occur, give rise to prodigious accumulations of these broken masses of rock. The great eruption of Ararat, in 1840, was of this kind, a terrific explosion having torn open the side of the mountain and thrown off an immense mass of fragments, which were projected for miles in every direction, completely burying the town of Arguré. There was no eruption of lava; but frightful earthquakes and torrents of rain followed, washing down the detritus of the explosion in immense floods of mud, which were quite as destructive as lava would have been.

According to Junghuhn, the Javanese volcanoes now emit no lava, but only give rise to streams of brecciated material, which

have issued from the craters in that condition. The same author also gives a most interesting account of the great eruption of Pepandayan, which took place in 1772. At that time such a mass of fragments and blocks of lava was ejected that the upper part of the Garut valley, for ten miles in length, was filled with ashes and angular materials to the average depth of fifty feet, while in places the great blocks were heaped up in conical hills as much as a hundred feet in height. The distances to which such masses are thrown indicate the immensity of the force by which they are hurled into the air. Cotopaxi, for instance, in 1533, threw rocks from eight to ten feet in diameter to a distance of seven miles. The maximum height to which masses of lava have been thrown by Etna and Vesuvius, in different eruptions, is given by various scientific observers as from seven to ten thousand feet.

Towards the end of an eruption the ashes ejected grow finer and whiter, bearing all the marks of having been longer subjected to the triturating process by which the lava is reduced to powder. This is the natural result of the slacking off of the ejecting forces, the sinking down of the column in the chimney, and the consequent longer time that the materials are exposed to friction against each other. Some observers have thought, however, that the lava might in many cases be blown into fine powder by the sudden expansion into steam of the water it contained, at the moment the pressure was removed by its issuing from the crater, and there are some appearances which seem to render this view a probable one.

The finer the ashes thus ejected, the farther away from the volcano they fall. Carried by the wind, they are sometimes spread over vast areas of country, and the exceeding fineness of the material is testified to by the slowness with which it descends, sometimes filling the air so completely that the darkness of night reigns for days in succession. It is stated that, in the great eruption which devastated the island of St. Vincent in 1812, the fall of ashes on the island of Barbadoes, nearly a hundred miles distant, caused so profound an obscurity that a white handkerchief was invisible at five inches from the eye. The fall of ashes in the great eruption of Temboro, in Sumbawa, in 1815, produced so dense a cloud that it

was dark as night over the islands of Java and Celebes. Ashes fell on the islands of Sumatra, Banda, and Amboyna. West of Sumatra a layer of lapilli, two feet in thickness, floated on the sea, so that ships had difficulty in forcing their way through. A careful comparison of all the data, by Zollinger, led him to the conclusion that the ashes fell over an area of nearly one million of square miles, and that fully fifty cubic miles of material was ejected in this one eruption. Junghuhn, also, calculated the volume of the ejected materials of the same eruption to be one hundred and eighty-five times the dimensions of Vesuvius. The area over which daylight was shut off by this fall of ashes was nine hundred by seven hundred miles in extent, that is, equal to the whole space in our own, territory between the Mississippi River and the Atlantic Ocean. Coseguina, in 1835, covered with its falling ashes an area of nearly one thousand two hundred miles in diameter.

The destructive effects of these showers of ashes are fearfully increased by the torrents of rain which frequently fall in connection with great eruptions; these carry down the ejected materials in the form of great flows of mud, which descend the steep slopes with such velocity that they cannot be avoided, and of course completely overwhelm everything they reach. It was by such a lava d'acqua, or water-lava, as the Neapolitans call it, that Herculaneum and Pompeii were submerged and destroyed. For eight days and nights the torrents of mud poured down over those ill-fated towns, accumulating in places to the depth of over a hundred feet. It was the remarkable way in which these cities were overwhelmed that has preserved them so wonderfully for the inspection of people for almost two thousand years. There is no other possible manner in which they could have been thus hermetically sealed up, as it were, all the walls remaining standing, and everything in its place. Had a shower of ashes, for instance, fallen from above, all the buildings would have been crushed in; but the insidious mud-flow crept into everything, filling rooms, and even cellars, so gradually that nothing was disturbed or displaced. Herculaneum was afterwards covered with a layer of solid lava, and then built upon, so that the opening of that town has been much slower and more expensive; although, in proportion to