The lava thus thrown up falls down again, and when there is no wind it falls down close around the opening. Some of it falls into the opening, where it is melted again. The rest falls on the sides, and in process of time it begins to build up a small hill, as it were, all around the opening, though the puffs and explosions of steam that are continually coming out keep a mouth open at the top.
Things go on in this way for some time, until at length, for some mysterious reason which nobody understands, the interior of the mountain begins to moderate its heat, and finally to grow cool—not entirely cool, but cooler than it has been. The puffs and explosions gradually cease. The lava within the bowels of the mountain shrinks as it cools. The sides of the mountain being firm and solid, do not collapse; but the top, being still more or less soft, falls in, not suddenly, but by a slow and gradual motion, corresponding with the progress of the cooling below. So slow, indeed, is this progress, that sometimes the ground continues sinking slowly in this way for several years before the crater is fully formed.
All this time, although the puffs and explosions have in a great measure ceased, the steam continues to blow out, more or less steadily, from a great many small openings, some of them in the bottom of the crater, and some, perhaps, in the sides. This steam is changed into visible vapor when it comes out where the air is cool, and the several streams, mingling together as they rise into the air, form a cloudy column, which is often called smoke. Strictly speaking, however, it is not smoke. It is almost entirely composed of steam.
After continuing in this state for some time, the interior of the mountain begins to grow hot again. Then the steam and hot lava begin to puff out at some one or other of the vents in the bottom of the crater. If the heating goes on, the lava comes out hotter and hotter from the opening, and by melting away the sides of it and blowing it out, it gradually enlarges it. The lava that is blown out, too, falls down all around the hole, and gradually builds up a mound around it, like a little dome, while the successive blasts keep the outlet open all the time at the top. This small cone, rising up gradually thus, in the bottom of the crater formed by the sinking in of the mountain before, and the chimney opening up through the centre of it, gives vent to all the steam from below, while a great many of the other orifices are stopped up by the lava which comes up out of the great opening falling into them. After a time, the lava that is thrown out spreads over the whole floor of the crater in a mass of black, corrugated slag, with the small cone rising from the centre of it, and the opening at the top glowing like the mouth of a fiery furnace, and bursting out every now and then, with explosions of steam, and red-hot stones, and melted lava.
This was precisely the condition of Vesuvius at the time that Rollo visited it. The top of the mountain had fallen in, in two places, some time before, on account of the cooling below, and two great craters had been formed. Now, the furnace had been for some time heating up again, and in each crater a black cone, with a fiery mouth open at the apex of it, was gradually growing up, and covering the whole floor of the crater with the black and molten matter which it was ejecting.
It was to the edge of one of these craters that the party now advanced, and the engraving will give you some idea of the view which it presented.
VIEW OF THE CRATER.
There were several persons, both ladies and gentlemen, standing on the margin of the crater when our party arrived. Mr. George led Rosie to the place, and looked down with her into the abyss. The sides of it were formed of precipitous cliffs of rocks and sand, all beautifully colored, in every shade of red and yellow, by the deposits of sulphur which had accumulated upon them from the fumes of the volcano. The floor of the crater was black as jet, being covered by the molten lava, which had gradually spread over