Limestones divide themselves into types produced by chemical precipitation and those due to the accumulation of the hard parts of organisms; but in many of the latter types chemical precipitation also plays a part. Organic action, moreover, frequently promotes the deposition of the chemical types. _Detrital limestones_, that is, limestones formed from the debris of older ones, are comparatively unimportant. They occur in certain zones of the Chalk and of the Carboniferous Limestone in our islands, and record the breaking up in shallow water of beds that had already become consolidated. The Miocene _Nagelfluh_ conglomerates of the north side of the Swiss Alps are often formed of pebbles of the far older Mesozoic limestones. Similar conglomerates, cemented by calcium carbonate, are now being formed in the river-beds of the limestone karstland of Hercegovina. Limestone, however, as a rule goes to pieces before the buffetings sustained by mixed rocks on a shore. Even if it survives for a time in gravels, percolating waters ultimately dissolve it, and only a porous skeleton, formed of its impurities, remains. LIMESTONES DEPOSITED FROM SOLUTION Though calcium carbonate is far less soluble than calcium sulphate, large quantities are carried invisibly, owing to the presence of carbon dioxide, in river waters, and thus accumulate in inland seas that have no outlet except by evaporation. Here _Calcareous Tufa_ may be deposited as a crust upon the shores and on the growing islets, as the water shrinks away, and before the more soluble gypsum and rock-salt can separate out. Hot springs of volcanic origin, like the Sprudel of Karlsbad in Bohemia, may deposit calcium carbonate as the water cools and is relieved from pressure. At Karlsbad, little grains of granite, or of the minerals of granite, serve as centres, and encrusting layers are formed round them, until pea-like bodies are produced. These become cemented together, giving rise to the well-known freshwater _pisolitic limestone_ or _roestone_. On the shores of the Great Salt Lake of Utah, calcareous tufa occurs also in the form of grains resembling little eggs. These are the oolitic grains that were first known as constituents of fossil limestones. The calcium carbonate of oolitic grains at Karlsbad, from the Great Salt Lake, and from the sea, is deposited in a form that gives the reaction of aragonite when boiled in cobalt nitrate. A. Lacroix, however, finds that the material at Karlsbad has a specific gravity lower even than that of calcite, and that its double refraction is also distinctly weaker. He has called this form of calcium carbonate "ktypeite."