1. Basin of the Great Geyser, Iceland. 2. Hot spring cone. 3. Old Faithful. 4. The Great Geyser. 5. Liberty Cap. (2, 3, 4 and 5 are in the Yellowstone Park district of the Rocky Mountains.)] [Sidenote: FORMATION OF SINTER-CONES.] The soluble materials which waters issuing from volcanic vents deposit on their sides are chiefly silica and carbonate of lime. Hot springs, whether intermittent or constant, often contain large quantities of silica in solution. The solution of this silica is effected, at the moment of its separation from combination with the alkali or alkaline earths, during the decomposition of volcanic rocks, and is favoured by the presence of alkaline carbonates in the water, and the high temperature and the pressure under which it exists in the subterranean regions. When the water reaches the surface and, being relieved from pressure, begins to cool down the silica is deposited. By this deposited silica the basins around the geysers of Iceland are formed. Sometimes conical structures are built up around the vents of hot springs by the deposition of silica from their waters. Examples of this kind abound in the National Park of Colorado, where they have received fanciful names, such as the Beehive, Liberty Gap, &c. This deposited silica is known to geologists as sinter. The forms of some of the structures which surround the orifices of geysers is shown in fig. 79. The 'Liberty Cap' is an extinct geyser-cone fifty feet high and twenty feet in diameter. Hot and cold springs rising in volcanic regions are often highly charged with carbonic acid, and in passing through calcareous rocks dissolve large quantities of carbonate of lime. Upon exposure to the atmosphere, the free carbonic acid escapes and the carbonate of lime is deposited in the form known as 'travertine.' Such springs occur in great numbers in many volcanic regions. In the Auvergne great rock-masses occur formed of carbonate of lime deposited from a state of solution and taking the form of natural aqueducts and bridges. In Carlsbad the numerous hot springs have deposited masses of pisolitic rock (Strudelstein) which have filled up the whole bottom of the valley, and upon these deposits the town itself is mainly built. In Central Italy the deposits of travertine formed by calcareous springs are of enormous extent and thickness: St. Peter's and all the principal buildings of Rome being constructed of this travertine or 'Tibur-stone.' [Sidenote: FORMATION OF SINTER-TERRACES.]