Lavas, which have cooled so quickly that crystals have not had time to form, have a glassy appearance, and are termed obsidian. If the color is dark, due to the presence of large amounts of those elements which make dark minerals, this lava is termed _basalt-obsidian_. Obsidian is characterized by its glassy texture, a hardness around 6, and by breaking with a conchoidal fracture, so called because the surface is marked by a series of concentric ridges, something like the lines of growth on a shell. Obsidians vary greatly in color, but are usually red or green to black, and translucent on thin edges. While glassy, all the obsidians contain embryonic crystals, which appear like dust particles floating in the glassy matrix, or there may even be a few larger crystals present, which are often arranged in flow lines. Most all large masses of obsidian have streaks or layers of stony material in them where crystallization has set in, in a limited way. Near the upper surface, obsidians usually have gas cavities scattered through them, and these may be small and few, or large and numerous. Indeed the cavities may be so numerous as to dominate and give the rock a frothy appearance. In this case, if the cavities are small and more or less uniform, the rock is called _pumice_; if they are larger it is _scoria_. If, as often happens when the lava is ancient and has been buried beneath other rocks, the cavities have been filled with some secondary mineral, then the lava is called an _amygdoloid_. Obsidian is found in many localities, especially where there are recent volcanoes, the most famous places being the obsidian cliffs in the Yellowstone Park, those near Mono Lake in California, and many other localities in the Rocky Mountains, the Sierra Nevadas, and the Cascade Mountains. Pitchstone This is very like obsidian in appearance, but differs in that the glassy material contains from five to ten per cent of water in its composition, the most obvious effect of which is to make the luster resinous, instead of vitreous, as is characteristic of obsidian. The colors are commonly red, green or brown. Pitchstone is associated with recent volcanoes, and some fine specimens have come from Silver Cliffs, Colo., and various parts of New Mexico and Nevada. Perlite _pearlstone_ Perlite is a glassy lava, containing two to four per cent of water, which, on cooling, has cracked into numerous rounded masses, with a concentric structure, reminding one of the layers of an onion. Scoria