column of salt water has been spouting for forty years. As a counterpart to these phenomena of solution, waters form deposits which are interesting for more than one reason. Reaching the open, thermal and gaseous springs meet conditions of pressure and temperature different from those which ruled in the depths, and are consequently subjected to reactions to which the oxygen of the atmosphere frequently contributes. The deposits which they make are observed chiefly on the surface of the ground.
Lime, which is very abundant in the condition of a carbonate, is always in solution, in small quantities at least, and in larger quantities when free carbonic acid is present to assist the process. The conditions which provoke the disengagement of the gas at the same time determine the precipitation of the calcareous salt; and this is why calcic carbonated waters often give rise to important deposits. The ancients were impressed by the stalactites of caverns, and by petrifying springs which covered plants and other bodies immersed in their basins with a stony precipitate. Industry has profited by the property, and has obtained in petrifactions bas-reliefs and images of a very delicate molding. In many places the deposit has become extensive enough to constitute a rock formation like the travertine at Tivoli, which furnished building-stones for Rome.
Silica, although regarded as insoluble in water, may become associated with it by the aid of intermediate agencies, and form combinations which are even the predominant elements of some springs, as at Plombières, Bagnères-de-Luchon, Ax, Saint-Sauveur, and Amélie-les-Bains. Sometimes silica is so abundant that it is isolated as opal on coming in contact with the air. The basins of many geysers are thus carpeted with it.
Iron-ore, or limonite, is also constantly in formation in such quantities that the beds can be worked. It is known, according to the conditions under which it is deposited, under the names of bog-ore, field-ore, or lake-ore. It is generally buried at slight depths below the surface, forming thin beds. Its modern origin is demonstrated by the presence of products of human industry, such as fragments of pottery and utensils, which are met in the massive blocks; and it is, moreover, sometimes renewed in places where it has recently been worked. More than a thousand lakes in Sweden, Norway, and Finland supply this mineral in rounded and separated globules. Although its formation is in constant continuation, the cause of it has been for a long time misapprehended. It is a result of slow dissolutions which have been frequently observed in arenaceous clays. Rain-waters traversing them, having seeped along roots undergoing decomposition, take from them an acid principle, and thus acquire the power of dissolving oxide of iron as they go. Reappearing in the air, they abandon the hydrated peroxide of iron, leaving it as a brown, gelatinous precipitate. Organic substances in this way contribute to the formation of mineral matters.