THE WEATHERING OF ROCKS. 315 
impurities from decomposing organic substances in the soil 
amongst which are ulmic and humic (organic) acids. By 
means of these — Oxygen, Carbon-dioxide, and organic 
acids—it is able to work many chemical changes it would 
be unable to accomplish if it were pure water. 
All rocks are more or less soluble in pure water, some, 
é.g., Rock-Salt and Gypsum, being removed with relative 
rapidity. When, however, the water is charged with the 
impurities just mentioned, its solvent power is much 
increased. The presence of dissolved Carbon-dioxide 
enables it to attack Limestone and carry it away in solution. 
In many Sandstones and Clay-rocks the cement binding 
the particles together is Carbonate of Lime; this is acted on 
by the carbonated water and gradually removed, so that 
the whole mass in time becomes loose and friable. 
Then, again, about one-fifth of the air consists of Oxygen 
gas, a body very active in promoting the decay of rocks. 
Oxygen has a great aftinity for most substances, readily 
uniting with them and forming oxides ; its combination 
with Iron is known as rust. The Iron and other con- 
stituents which occur in many rocks are attacked by the 
Oxygen, and the oxides thus formed carried away in 
solution. The result of this is not only the removal of a 
portion of the rock, but the exposure of fresh surfaces to 
the action of weathering agents. Most of, if not the whole 
of the Oxygen concerned in these changes, is carried by 
water. 
Most igneous rocks are formed largely of a mineral 
ealled Felspar, which is a double silicate of varying 
composition—according to the kind of Felspar ; it may be 
a silicate of Potash and Alumina, or Lime and Alumina, 
or Soda and Alumina, or all of these substances—Potash, 
Lime, Soda, Alumina, and Silica—may appear in the Felspars 
