Calcite then is readily soluble in water, and streams flowing along crevices and fissures in limestone dissolve out great cavities or caves, like the Mammoth Cave of Kentucky. Other water, percolating through the limestone, comes to these cavities saturated with lime in solution and drips from the roofs and walls; then as part of the water evaporates, it deposits part of its lime in icicle-like masses, hanging from the roof. Such masses of non-crystalline calcite are called _stalactites_. Below on the floor of the cave, conical masses are built up in the same manner where the dripping water falls on the floor. These are _stalagmites_. In these limestone caves and in smaller cavities many of the most beautiful crystals grow. Somewhat similarly, when hot water from deep springs comes to the surface, it cools and can not carry as much lime, and so around the spring is laid down layer after layer of non-crystalline calcite making a mass known as _travertine_. Sometimes this is colored by iron or other impurities and a banded effect results. Such travertine as the “Suisun marble” from California, “California onyx,” “Mexican onyx,” and “satin spar” all belong to this class. The coral animals, especially in tropical waters precipitate an enormous amount of lime, until whole reefs are built of lime in this non-crystalline form. In places it is hundreds of feet thick and hundreds of miles in extent. Some of this coral has become popular for personal adornment. This is particularly a small, fine-grained variety, _Corallum rubrum_, which lives almost exclusively in the Mediterranean Sea. This coral is red in color, varying all the way from a deep red to white. It grows in small masses, three pounds being a good sized mass, in water 60 to 100 feet deep, requires some ten years to develop a full-sized mass. The making of this into beads and ornaments is an Italian industry. The demand is growing, while at the same time the supply is diminishing, and search is being widely made for more such coral, but up to the present time with little success. This precious coral is much worn as a protection against the “evil eye” and is widely imitated, apparently with as much protection to the wearer. When coral beads are offered cheap, they are probably something else, red gypsum being much used. This and all imitations can be readily detected by trying a drop of acid in the bead. Coral will effervesce, but gypsum and other substitutes will not. The bulk of the shells of most molluscs is made of lime, but the mother-of-pearl layer inside is usually aragonite. The chalk of the cliffs on either side of the English channel is lime, and composed of the shells of single celled animals. See p. 213. When lime is deposited in loose porous masses, as around grass, etc., and below hot springs, this mass is termed _calcareous tufa_.