118 ELEMENTARY GEOLOGY 
sink into the earth. The decaying leaves furnish it 
with substances that impart more power. When it 
falls, it contains oxygen; from the decaying vegetation, 
it takes carbonic acid gas, and perhaps some of the 
humic acids, and possibly alkaline substances. So it pro- 
ceeds, armed with weapons of attack. 
Some minerals, like quartz or calcite, it is able to 
dissolve, and the loss of the material thus taken away 
weakens the rock a little. Or it may find other min- 
erals, like feldspar or hornblende, ready to change ; 
then it causes a reaction to begin. When these changes 
have gone far enough, the rock crumbles, either because 
some of its minerals are removed by actual solution, 
or else because some have been made weak and soft, 
like clay, leaving the others unsupported, so that they 
must fall apart (Fig. 53). 
This chemical process of oxidation, as it may be 
called, extends well down into the rocks, sometimes to 
a depth of several hundred feet. In the cuts made just 
outside the city of Washington, where they are grading 
some of the roads (and the same phenomenon may be 
observed in many other places), what appears to be a 
solid rock, with all the structure-lines present, has been 
so softened that it may be shovelled out like clay, which 
in reality, it is. 
In many mines, an ore of one composition changes 
to another, as the mine descends below the zone of oxi- 
i iat oe 
