The solid substances emitted by volcanoes are popularly styled ashes, cinders, or scoria and lava. But what is called ashes would be more appropriately named dust; for it is merely finely divided lava, and in no way resembles genuine ashes. Lavas present a general resemblance to the slag and clinkers of smelters and brick-kilns, but vary considerably in appearance and chemical composition. We need not touch this question further than to state that oxygen forms nearly one-half the weight of all lavas, silicon one-fourth of most, and aluminum one-tenth. From fifteen to twenty per cent. is made of various others, magnesium, calcium, iron, sodium and potassium being most common. Hence, the compounds present are always of the class known to chemists as silicates, substances requiring great heat to melt. These, from being long melted, abound more or less in crystals; but if any one re-melts them and cools them suddenly, the result is a simple glassy mass, with no trace of crystals. [Illustration: VIEW IN ACTIVE CRATER.] Scoria or cinders differ from ordinary lava only in the peculiarity of having partially crystallized in some portions and then stiffened or solidified while large bubbles were yet imprisoned or in the act of bursting; thus leaving the mass very ragged and cellular. But if the lava contains no readily formed crystals, the imprisoned bubbles of steam slowly rise to the surface, and being greatly elongated by the flowing of the lava, produce the beautiful material known as “pumice.” It is to lava exactly what froth or foam is to water. Usually it is much lighter colored than the lava on which it floats, for the same reason that well-worked molasses candy is nearly white: they both contain a vast number of minute air bubbles. Pumice floats on water, and its decomposition being generally very slow, it drifts about the sea currents, and is often found thousands of miles from any volcanic region. In the immediate neighborhood of volcanoes it often accumulates on the sea to such an extent that vessels can hardly force their way through it. In the Sunda Islands it has been seen on the sea three feet in depth. During the year 1878 the accumulation of pumice near the Solomon Isles was so great that it took ships three days to force their way through. Sometimes such masses accumulate along the coast line to such an extent that a person can not readily tell where the shore line is. One may land and walk about on the great floating raft of pumice, unable to guess within even a mile of the actual shore. Deep sea soundings show that the entire ocean bottom is covered more or less with the pulverized pumice and volcanic dust. From the wide distribution it is not probable that the layer attains any great thickness.