WATER AS A CHEMICAL AGENT. 695 



those formed in place, because a marsh gather's much dead animal matter, 

 and therefore the ore usually contains phosphates (p. 59). Even much 

 of the Archaean ore contains phosphate of lime (apatite) in visible grains. 



The oxydation of iron has also taken place without any attending 

 destruction of rocks. In the Marquette iron region, and others, there 

 are imbedded octahedrons of iron ore, which are now hematite Fe 2 3 , 

 or, what is the same, FeOt, but which were originally magnetite, FeCK, 

 as is proved by their having the crystalline form of magnetite, instead 

 of that of hematite. They show that the great bed of ore, of which 

 they are a part, has been in seme way oxydized (receiving in it a sixth 

 more of oxygen). This was probably done through the aid of the 

 moisture penetrating the whole, when at a high temperature. Igneous 

 rocks usually, contain magnetite rather than hematite. 



Consolidation of rocks is another effect, in some cases, of the pro- 

 duction of iron ore. Limonite becomes distributed among pebbles, and 

 thereby makes an ironstone conglomerate. 



The waters, filtering through soil and gravel, often take up enough oxyd of iron to 

 cement a bed of pebbles lying, at a lower level, on another layer sufficiently close in tex- 

 ture to hold the water and give the iron a chance to deposit; and this is one way in 

 ■which what is called hard-pan is sometimes made. The underlying impervious bed is 

 not absolutely necessary to the result, although promoting it. The pebbles wet with the 

 ferruginous waters, when they dry, in times of drought, take a deposit of iron; and 

 this process may end in complete consolidation. 



When a low degree of heat is concerned in the consolidation of beds of sand, contain- 

 ing iron-bearing minerals in grains, the red oxyd of iron is usually produced, reddening 

 the rock, and acting also in some degree as a cement for the sand; the same heat, how- 

 ever, often leads to the production of a solution of silica, which aids in the consolidation. 



The fumes of chlorid of iron from a volcanic fumarole, in contact with water in vapor, 

 give up the chlorine to the hydrogen of the vapor (making hydrochloric acid), and the 

 iron to the oxygen of the same, making oxyd of iron, or hematite. In this way, crystal- 

 lized hematite is sometimes formed in scorias about a fumarole. But according to Pal- 

 mieri, this is not the only or common way. Iron exists in the liquid lava, in the state 

 of magnetite ; and the oxydation of magnetite may be the more common method. 



4. Through Decomposition of Feldspars. — Feldspars change to 

 kaolin (the clay of which porcelain is made), on decomposition, losing 

 the alkalies and part of the silica, and taking in water ; so that feld- 

 spar, consisting of one part atomically of alkali, one part of alumina, 

 and three to six parts of silica, becomes reduced to one of alumina, two 

 of silica, and two of water (or kaolin). Thus the large beds of kaolin 

 have been made, and larger beds of clay slate free from alkalies. 



5. Through the Action of Sulphuric Acid. — Limestone (carbonate 

 of lime) is changed to sulphate of lime by sulphuric acid ; and thus 

 beds of gypsum and anhydrite have been formed. The sulphuric acid 

 may come directly from the decomposition of sulphids ; or from the 

 oxydation of sulphid of hydrogen or of sulphurous acid, in volcanic 

 regions. Alumstone (sulphate of alumina) and alum efflorescences 

 (sulphates of alumina and the alkalies, or magnesia, or iron) are often 



