The escape of great quantities of steam and other gases from the midst of a mass of fluid or semi-fluid lava gives rise to the formation of vast quantities of froth or foam upon its surface. This froth or foam, which is formed upon the surface of lava by the escape of gaseous matters from within it, is made up of portions of the lava distended into vesicles, in the same way that bubbles are formed on the surface of water. It bears precisely the same relation to the liquid mass of lava that the white crest of foam upon an advancing wave does to the sea-water, from the bubbles of which it is formed. This froth upon the surface of lavas varies greatly in character according to the nature of the material from which it is formed. In the majority of cases the lavas consist, as we have seen, of a mass of crystals floating in a liquid magma, and the distension of such a mass by the escape of steam from its midst gives rise to the formation of the rough cindery-looking material to which the name of 'scoria' is applied. But when the lava contains no ready-formed crystals, but consists entirely of a glassy substance in a more or less perfect state of fusion, the liberation of steam gives rise to the formation of the beautiful material known as 'pumice.' Pumice consists of a mass of minute glass bubbles; these bubbles have not usually, however, retained their globular form, but have been elongated in one direction through the movement of the mass while it was still in a plastic state. The steam frequently escapes from lava with such violence that the froth or scum on its surface is broken up and scattered in all directions, as the foam crests of waves are dispersed by the wind during a storm. In this way fragments of scoria or pumice are often thrown to the height of many hundreds or thousands of feet into the atmosphere, as we have seen is the case at Stromboli and Vesuvius. Indeed, during violent eruptions, a continuous upward discharge of these fragments is maintained, the ragged cindery masses hurtling one another in the atmosphere, as they are shot perpendicularly upwards to an enormous height and fall back into the vent; or they may rise obliquely and describe curves so as to descend outside the orifice from which they were ejected. [Sidenote: FINENESS OF VOLCANIC DUST.]