WATER AS A CHEMICAL AGENT. 691 



more or less slowly, whenever rocks containing these iron-bearing 

 minerals are accessible to air and moisture. The action of the car- 

 bonic acid cannot be perceived ; but the oxydation of the iron, the 

 secondary result, is very manifest in the brownish or reddish color* 

 which the exposed rock acquires, and also in its disaggregation. In 

 the case of a close-textured rock, like much doleryte (trap), the 

 change gradually extends from the surface inward, making a dis- 

 colored crust. This crust loses at surface at the same rate that it pro- 

 gresses inward ; and hence its thickness, for a given variety of rock, is 

 nearly uniform. ♦ 



B. Organic Acids. — The work here attributed to carbonic acid 

 is also performed, though to a less extent, by organic acids, made 

 from vegetable or animal decomposition. They contribute to the 

 solution and erosion of limestones, and also to the process of oxyda- 

 tion. 



C. Silica. — Silica is present, in minute traces, in most natural 

 waters. 7,500 parts of water will dissolve one part of silica in the 

 gelatinous or soluble state ; and the shells of Diatoms, which are pres- 

 ent over the bottoms of most waters, are silica in this soluble state. 

 If the waters are at all alkaline, the proportion of silica that may be 

 taken up is much larger. 



The geological effects of the silica of cold solutions appear to be 

 of only infinitesimal importance ; while the siliceous solutions made 

 by heated waters, like those of geyser and other hot-spring regions, 

 have great destroying power, though at the present time confined to 

 small areas. They act on limestones, expelling carbonic acid, and 

 making silicates containing lime ; and this is probably a prominent 

 source of the carbonic acid gas given out in some solfataras, and also 

 of that which has made the region of Yellowstone Park as remark- 

 able for its calcareous as for its siliceous waters. 



D. Sulphuric Acid and Soluble Sulphates. — Waters hold- 

 ing in solution sulphuric acid or soluble sulphates (alums, vitriols, etc., 

 made through the decomposition of sulphids), act erosively on most 

 rocks within reach, and especially on limestones. 



II. FORMATIVE WORK. 



The destructive work in geology is all preparatory to new forma- 

 tions. 



1. Through Calcareous Waters. — The carbonate of lime taken up 

 by carbonated waters, making them calcareous, is the means by 

 which limestones have been consolidated ; even sea-water contains 

 enough carbonic acid to take up some carbonate of lime. The cal- « 

 careous sands of a beach washed over by the tides, and thereby 



