Pumice

Pumice, a general term for the cellular, spongy-filamentous, or froth-like parts of lavas. This highly porous and froth-like structure is due to the abundant escape of vapours through the rock while it was in a state of fusion. Under the microscope the rock is seen to be a glass, crowded with minute gas or vapour cavities and abundant crystallites. Owing to its porous structure pumice readily floats in water. It is usually a form of some highly acid lava, such as obsidian; but now and again basic lavas give rise to pumice (Canary Islands, Hawaii). The latter is dark brown or black, and often shows metallic tarnish; the former, which is much the more common, is white or gray, and sometimes yellow. It is a hard but brittle rock, and is much used for polishing wood, ivory, metals, glass, slates, marble, lithographic stones, &c., in preparing vellum and parchment, and for rubbing away corns and callosities. Great quantities are exported from the Lipari Isles; and that from the quarries in the Peak of Teneriffe, 2000 feet above sea-level, is better and cheaper. Pumice occurs as the crust of some kinds of lava, and is often ejected in the form of loose cinders during volcanic eruptions. Sometimes immense quantities are thrown into the sea and are often floated for great distances. Eventually the cinders get water-logged and sink to the bottom. Abundant fragments were dredged up from abyss- mal depths by the Challenger expedition. After the eruption and earthquake in the Straits of Sunda in 1883, the seaport of Folok Batoung was closed with a barrier of pumice 19 miles long, two-thirds of a mile broad, and from 13 to 16 feet deep.