THE CHEMISTRY OF UNDERGROUND WATERS. 815 



column of salt water has been spouting for forty years. As a counter- 

 part 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 dif- 

 ferent from those which ruled in the depths, and are consequently sub- 

 jected 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 condi- 

 tions, 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 ba- 

 sins with a stony precipitate. Industry has profited by the property, 

 and has obtained in petrifactions bas-reliefs and images of a very deli- 

 cate 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 asso- 

 ciated with it by the aid of intermediate agencies, and form combina- 

 tions' which are even the predominant elements of some springs, as at 

 Plombieres, Bagn^res-de-Luchon, Ax, Saint-Sauveur, and Amelie-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 quan- 

 tities that the beds can be worked. It is known, according to the con- 

 ditions 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 sur- 

 face, 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. 



