such depths it deposits these minerals in the pores of rocks,
cementing their grains together, and in crevices and fissures,
forming mineral veins. Thus below the zone of solution where the
work of water is to dissolve, lies the zone of cementation where
its work is chemical deposit. A part of the invisible load of
waste is thus transferred from rocks near the surface to those at
greater depths.

As the land surface is gradually lowered by weathering and the
work of rain and streams, rocks which have lain deep within the
zone of cementation are brought within the zone of solution. Thus
there are exposed to view limestones, whose cracks were filled
with calcite (crystallized carbonate of lime), with quartz or
other minerals, and sandstones whose grains were well cemented
many feet below the surface.

CAVITY FILLING. Small cavities in the rocks are often found more
or less completely filled with minerals deposited from solution by
water in its constant circulation underground. The process may be
illustrated by the deposit of salt crystals in a cup of
evaporating brine, but in the latter instance the solution is not
renewed as in the case of cavities in the rocks. A cavity thus
lined with inward-pointing crystals is called a GEODE.

CONCRETIONS. Ground water seeping through the pores of rocks may
gather minerals disseminated throughout them into nodular masses
called concretions. Thus silica disseminated through limestone is
gathered into nodules of flint. While geodes grow from the outside
inwards, concretions grow outwards from the center. Nor are they
formed in already existing cavities as are geodes. In soft clays