rial which is called scoria. If the lava is glassy it becomes pumice, a mass of minute glass bubbles drawn out in one direction by the movement of the mass while it was still in a plastic state. Fragments of scoria and pumice are often thrown by a violent escape of steam to a height of hundreds or thousands of feet into the atmosphere. While going up and coming down, they encounter each other and wear each other away by their frequent rubbing, with a noise which is one of
Fig. 3.—Minute Liquid-Cavity in a Crystal, with a moving Bubble. (The path of the bubble is indicated by the dark line.)
the most noteworthy accompaniments of volcanic eruptions. Mr. Poulett Scrope, who watched the Vesuvian eruption of 1822 for nearly a month, remarks that at first fragments of enormous size were thrown out, but that they were gradually reduced by constant re-ejections, till at last only the most impalpable dust issued from the vent—a dust which was so excessively finely divided that it went everywhere, even into the most closely fastened boxes. Mr. Whymper estimates that no less than two million tons of dust must have been ejected during a single slight outburst of Cotopaxi which he witnessed; and Professor Bonney calculates from actual examination that it would take from four to twenty-five thousand particles of this same dust to make up a grain in weight!
The temperature and consistency of lava-streams vary greatly, and the variations give rise to differences in the appearance of the cooled mass. The surface of the stream cools rapidly in the air, so that it appears dull-red at night and black by day—like a great mass of rough cinders—while all is of a white heat beneath, and may be so seen at night shining through the rough, cindery masses. Some streams are very liquid, resembling rivers and filling every channel in their course; while others, cooler and stiffer, might be more fitly compared to glaciers, creeping along so slowly that the fact of their movement can be established only by the most careful observation. The stiff lavas leave a crust wrinkled and folded like coils of rope, and are then frequently called "ropy lavas." The very liquid, fast-flowing lavas leave a surface covered with rough, cindery masses presenting jagged projections. Admirable examples of the ropy lava are afforded by the Vesuvian lava-stream of 1868 (Fig. 4), and by the lava-cascade of the Island of
