Page:South African Geology - Schwarz - 1912.djvu/89

 cast out at the surface with terrific violence. Some of it is so fine that it floats for weeks and months in the atmosphere; but the coarser particles fall to the ground in the neighbourhood, according as the wind carries them, covering wide areas with the fine white powder. The still coarser particles fall round the orifice, and gradually build up an ash cone, the funnel-shaped opening of which is called the crater. As the violence becomes less, larger lumps of liquid lava are thrown out, and, after hurtling through the air, fall, more or less cooled, as lapilli or bombs. If there is much gas left in the molten rock when it is thus thrown as lumps in the air, it still expands and blows the liquid rock into a vesicular mass called pumice. Finally, lumps may be belched out which may solidify as solid volcanic glass, that is, molten rock that has cooled so quickly that it has not had time to crystallize.

''3rd Stage. Lava Flows.'' — The head of the column of liquid lava having been blown off, the rest of the lava rises quietly and occupies the chimney. Hydrostatic pressure is at work, and the cone, composed of loose, unconsolidated ash, becomes riddled with dykes of lava where crevices allow the molten rock to enter. Eventually the lava rises so high that a part of the crater breaks away, and a lava stream issues forth and flows down the side of the hill. Subsidiary or parasitic cones may arise by the dykes finding an exit on the side of the main cone.

''4th Stage. Solfataric Stage.'' — The lava has come to rest; no more gas is available for driving it forth, and consequently the lava column solidifies in the throat of the valcano. The gases, however, are still being given off, and work their way along crevices in the shattered