WATER AS A CHEMICAL AGENT. 687 



ward on the other, — which would give great irregularity there to the 

 scratches of grounded bergs. An iceberg " rocked by the swell of the 

 sea, and sometimes turning over," could not be good at scoring sub- 

 merged rocks. Moreover, these rocks, in the seas in which icebergs 

 melt and drop their freight of stones, would seldom be uncovered. 



4. WATER AS A CHEMICAL AGENT. 



"Water does its chemical work among the rocks, either — - 

 (1.) Through its capacities as water. 

 (2.) Through the affinities of its elements, directly. 

 (3.) By means of the substances it takes into solution. 

 This work is either destructive or formative. The air aids largely 

 in the results ; and hence its chemical effects are here in part included. 



I. DESTRUCTIVE WORK. 



1. Through its Capacities as Water. 



1. At the ordinary Temperature. — It takes 50,000 parts of pure 

 water, at the ordinary temperature, to dissolve one part of calcite or 

 carbonate of lime : over 200,000 for one of a silicate of alumina ; 

 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- 



