Page:South African Geology - Schwarz - 1912.djvu/88

 this kind are common in Java, and we have evidence of their having been very frequent when the volcanoes of the Drakensberg were active, for we find a volcanic sand, the Cave Sandstone, with all the characters of such a mud-rush, and embedded in it are the giant reptiles (Dinosaurs) which browsed once on the rich grass on the flanks of the volcanoes.

Mud volcanoes, such as occur in the Bay of Bengal and throw up islands of hot mud, are due to the combustion of oil or oil gas in the interior of the earth. As Brun finds marsh gas, which is oil gas, in all true volcanic explosions, the essential difference between mud volcanoes and true volcanoes, which was formerly insisted upon, is not now so great.

The action of a volcano may be divided into several stages.

''1st Stage. Clearing the Throat.'' — The gas liberated from molten rock, raised to the explosion point by some source of heat within the earth, tends to burst outwards, and should a crack lead to the surface the gas tears up it, drilling a cylindrical chimney. The material thrown out may be fine dust torn from the sides, when the rocks traversed are solid granite; or, if the rocks admit of "plucking", as in the case of sedimentary beds especially, blocks are hurled forth. The fine powdered rock or the boulders, angular as they have been torn off, or rounded by their grinding against each other, are strewn round the orifice as non-volcanic ash, breccia, or agglomerate.

''2nd Stage. The Emission of Ash.'' — Into the chimney so formed the lava rushes; but the gases imprisoned in it expand, now there is free space for them to do so, and the liquid rock is blown into the finest powder, which is