When fissures are formed in the midst of loose argillaceous materials, such as are frequently produced by the decomposition of volcanic rocks, the waters which issue through them are sometimes so charged with muddy matter that this accumulates to form cones having all the general characters of volcanic mountains, and which occasionally rise to the height of 250 feet. The gases and vapours which issue from these 'mud-volcanoes' are those which are known to be emitted from volcanic vents at which the action going on is not very intense. Daubeny and others have suggested that these mud-volcanoes may be the result of actions which have little or no analogy with those which take place at ordinary volcanic vents, and that the combustion of subterranean beds of sulphur and similar causes would be quite competent to their production. But inasmuch as these mud-volcanoes are almost always situated in regions in which the more powerful volcanic action has only recently died away, and the gases and vapours emitted by them are very similar in character to those which issue from volcanoes, there does not appear to be any good reason for doubting that they should be classed as truly volcanic phenomena.

Mud-volcanoes are found in Northern Italy near Modena, in Sicily near Girgenti, on the shores of the Sea of Azof and the Caspian, in Central America, and in other parts of the globe. The gas frequently escapes from them with such violence that mud is thrown into the air to the height of several hundreds of feet. Sometimes this gas is inflammable, consisting of sulphuretted hydrogen, hydrogen, or some hydrocarbons, and these gases occasionally take fire, so that true flames issue from these mud-volcanoes. In other cases the mud-volcanoes appear to be formed by either hot or cold springs containing large quantities of suspended materials, and the liquid mud issues from the vent without any violent eruptive action.

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.'

When springs charged with silica or carbonate of lime rise upon the slope of a hill composed of loose volcanic materials, they give rise to the remarkable structures known as sinter- and travertine-terraces (see fig. 80). The water flowing downwards from the vent forms a hard deposit upon the lower slope of the hill, while the continual deposition of solid materials within the vent tends to choke it up. As a new vent cannot be forced by the waters through the hard rock formed below, it is originated a little higher up. Thus the site of the spring is gradually shifted farther and farther back into the hill. As deposition takes place along the surfaces over which this water flows, terraces are built up enclosing basins. Of structures of this kind we have remarkable examples in the sinter-terraces of Rotomahana in New Zealand and the travertine-terraces of the Gardiner's River in the Yellowstone Park district of the Rocky Mountains.