WATER AS A CHEMICAL AGENT. 707 



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 

 alternately wet and dry, become coated with a deposit of carbonate 

 of lime from the waters ; and finally all are united into a solid mass. 

 Sands and pebbles of other kinds are treated in the same way ; and, 

 on shores bordered by coral reefs, the pebbles of basalt, and other 

 kinds, often have a milky exterior, from a film of carbonate of lime. 



The calcareous mud and sand of the reef under water become 

 solidified apparently without other means than the carbonated sea- 

 waters. 



Beds of limestone are sometimes made by depositions from calca- 

 reous waters, though small beds, compared with those of organic origin. 

 The travertine of Tivoli, near Rome, is a large deposit along the Anio 

 (p. 69), whose waters are there strongly calcareous. On the banks 

 of Gardiner's River, in the region of the Yellowstone Park, in the 

 Rocky Mountains, thick limestone deposits have been made, from 

 the waters of numerous and large hot springs and geysers, as well 

 illustrated and described in the Reports of Dr. Ilayden. The cal- 

 careous waters, in descending the slopes of the hills, have made a 

 series of parapets at different levels, inclosing basins, over which the 

 water drips or plunges on its way to the bottom, as illustrated in the 



