WATER AS A CHEMICAL AGENT. 703 



7.500 for one of silica in its gelatinous condition ; 460 for one of sul- 

 phate of lime, or gypsum. With heated water, the amount for sul- 

 phate of lime is the same. 



With the exception of gypseous rocks, there is consequently no ap- 

 preciable erosion, through the action of 'pure water ; but these are 

 rapidly worn away. 



Many minerals tend to combine with water, and thus become altered 

 in constitution. 



Anhydrite, or anhydrous sulphate of lime, changes to gypsum, or hydrous sulphate 

 of lime ; and great beds of the latter mineral have been made out of the former. Mica 

 and many other minerals often take in two or three per cent, of water, through incipi- 

 ent change. Feldspar, according to Hunt, may owe its decomposition and change to 

 porcelain clay, or kaolin, to a tendency to combine with water. In most of these cases 

 of hydration, carbonic acid has accompanied the action of the infiltrating waters, and 

 has been essential to the process. 



2. At an elevated Temperature. — Water at high temperatures, espe- 

 cially above the boiling point, as superheated vapor, has great dissolv- 

 ing and destroying power. No silicate will withstand it. The feld- 

 spars, the most universal of silicates, yield before it with great facility. 

 It takes the alkalies, and also the silica, making the siliceous waters 

 of most hot-spring regions. At the present time, the disaggregation 

 of rocks going on by this means is small ; but in all regions of meta- 

 morphism, this has been a prominent source of the changes. 



2. Through the Elements op Water directly. 



Water consists of oxygen and hydrogen, in the proportion, by 

 weight, of 8 O to 1 H. The oxygen is the element of chief import- 

 ance. But water has acted conjointly with atmospheric air in these 

 changes, and the latter is often the source of the oxygen ; the car- 

 bonic acid of the air (p. 51) gives aid, as explained on page 70 G. 

 Water alone usually protects the rocks it covers. 



A. Oxydation at the Ordinary Temperature. — The ten- 

 dency of iron to oxydize is a prominent source of rock-destruction, 

 and thereby of soil-making and gravel-making. In the change, the 

 iron passes to the sesquioxyd state ; and it either remains anhy- 

 drous, in which case the powder is red, and the material is hematite 

 (p. 59) ; or it becomes hydrous, has then a brownish-yellow powder, 

 and is the mineral limonite. 



The most common minerals undergoing the change are: (1) The 

 sulphids, pyrite and pyrrhotite (p. 59) ; (2) carbonate of iron, and 

 any calcareous carbonate containing iron ; (3) silicates in which iron 

 is present in the protoxyd state, especially pyroxene (augite), horn- 

 blende, black mica, and chrysolite. 



