WATER AS A CHEMICAL AGENT. 711 



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 oxj'gen 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. Tlirough 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 (carbonato 

 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 

 produced when alumina is present. Different sulphales of iron, or 

 vitriols, and some related products are other results. 



6. By Deoxydation. — Organic matters, owing to their tendency to 

 oxydation, may take oxygen from sesquioxyds, and make protoxyds of 

 them ; so that carbonic acid, if at hand, can combine with the iron, and 

 form carbonate or bicarbonate ; and organic acids, as Hunt has urged, 

 may form soluble organic compounds. In the decomposition of a 

 rock containing feldspars, in which iron is present, the clay, when first 

 made, is usually colored ; but after the bed of clay has thickened 

 to a few inches or feet, it is often found that the oxyd of iron has all 

 been washed out, leaving it nearly or quite white. This is accom- 

 plished by the process of deoxydation just mentioned. By means of 

 it also, large beds of carbonate of iron have sometimes been formed. 



In a similar way, sulphates have been reduced to sulphids. In tho 

 black marsh-mud deposit of the Quaternary of Louisiana, there is 

 some pyrite, derived through the deoxydizing process of organic 

 matters. (Hilgard.) 



7. Through the Evaporation of Sea-water, and attendant Chemical 

 Changes. — The ocean is a mineral spring that dates from the period 



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