WEATHERING 119 
dation. At Leadville, Colorado, for instance, the su- 
perficial ore was carbonate of lead; but below the line 
of oxidation, this becomes the sulphide, which is the 
real, unoxidized ore in this vein. 
Action of Plants. Still another chemical action of 
rock-change is effected by plants. We have noted 
already that they give to percolating waters, certain 
substances needed to cause the changes which are 
going on in the rocks. They also do a direct chemical 
work. Every plant that grows in the soil is drawing 
up in its sap some mineral substances, which, when 
the plant is burned, enter into the ash. By this 
means, certain compounds are drawn in such abun- 
dance from the soil, that farmers are obliged to replace 
these forms of plant food by some kind of fertilizer. 
Evidence of Chemical Changes. Returning to our 
granite ledge, we are able to see that this chemical 
change is also in progress there. If we break off a 
piece of the rock, we find the interior to be much 
fresher and harder than the surface portions, which 
are exposed to the air (Fig. 52). All through its mass, 
the quartz is clear, fresh, and glassy ; but the feldspar is 
dulled and whitish, and there is a yellow stain of iron 
rust, caused by the leaching out of iron from the horn- 
blende. A bowlder of granite, or of almost any crystal- 
line rock, will show similar change; and upon almost 
any ledge these lessons in rock decay are illustrated. 
