Coral Rock is made by the cementation of fragments of corals. The binding material, as in most stones, is lime; and this sort of rock is associated with coral reefs of either the past or the present. One of the best illustrations of this being the “Dolomite Mountains” in Tyrol. Coral rock, like coquina, has been cut into blocks and used as building stone, as in Bermuda. Encrinal Limestone (Plate 60) is a rock made by the cementation of fragments of the skeleton of crinoids. These animals belong to the group, echinoderms, and are now extinct except for a few so called “sea-lilies.” They were animals with a central mouth surrounded by long, jointed, flexible arms in multiples of five, and below this a small body inclosed in calcareous plates, all at the top of a long jointed stem. They lived in the sea and in the earlier geological times must have been very abundant; for their remains are so common in places as to make whole layers of limestone. Hydraulic Limestone is a fine-grained, compact, yellowish limestone with from 13 to 17% of sand, and some clay; which, when it is burned at a temperature a little higher than that used in burning lime, makes a product, that, while not as strong as Portland cement, still like it sets under water. Lithographic Limestone is a very fine-grained, compact limestone with clay impurities, the finest of the grain making it usable for making the stone plates used in lithographic printing. On slabs of this limestone figures are drawn in reverse with a special crayon. Then the slab is treated with acid, those parts which are not protected by the drawing being etched away, while the points protected by the drawing remain in low relief. From this slab figures can then be printed. Travertine is a general name, applied to calcareous deposits from fresh water lakes or streams, and has been precipitated either as a result of cooling or evaporation. Some travertines are porous, while others are dense; some are white, while others are colored, often beautifully, by impurities in the water. Porous deposits of travertine, when made on grass or other like substances, are known as tufa or _calc sinter_. Such masses are common around Caledonia, N. Y., Mammoth Hot Springs in the Yellowstone Park, etc. Onyx marble is a dense travertine, usually formed as a result of the deposition of lime from the water of springs. It is often banded, due to the presence of impurities in the water at one time, and their absence at other times. Till Till is an unconsolidated mass of boulders, pebbles, sand and fine clay, the unsorted material left behind by glaciers when they melted. The boulders and pebbles, while they show some wear, are not rounded like those that have been transported by streams, but have a more or less angular shape; and some of them are polished or striated on one side, where, while frozen in the ice, they were rubbed along the bottom.