, toward the production of a few substances like
clay, quartz, iron oxide, and calcite, which are transported and
redeposited to form clay, sand, and limestone. Cycles of this kind may
be repeated indefinitely.

By weathering of sedimentary rocks are produced some soils, certain
commercial clays, iron ores, lead and zinc ores, and other valuable
mineral products.

CONSOLIDATION, CEMENTATION, AND OTHER SUBSURFACE ALTERATIONS OF ROCKS.

=Cementation.= No sooner are residual weathered mantles formed or
sedimentary rocks deposited, whether under air or water, than processes
of consolidation begin. Settling, infiltration of cementing materials,
and new growths, or recrystallization, of the original minerals of the
rock all play a part in the process. The mud or clay becomes a shale,
the sand becomes sandstone or quartzite, the marl becomes limestone or
marble. All the minute openings between the grains, as well as larger
openings such as fissures and joints, may thus be filled. At the same
time the cementing materials may replace some of the original minerals
of the rock, the new minerals either preserving or destroying the
original textures. This process is sometimes called _metasomatic
replacement_. Igneous rocks as a rule are compact, and hence are not so
much subject to the processes of cementation as sedimentary rocks; but
certain of the more porous phases of the surface lavas, as well as any
joints in igneous rocks, may become cemented. All of these changes may
be grouped under the general term _cementation_.

A special phase of consolidation and cementation is produced near
intrusive igneous rocks through the action of the heat and pressure and
the expelled substances of the igneous rock. This is called _contact
metamorphism_ or _thermal metamorphism_. The processes are even more
effective when acting in connection with the more intense metamorphism
described under the next heading.

By cementation some of the common rocks, especially the sediments,
become sufficiently compact and strong to be useful as commercial
products, such as building stones and road materials.

More important as mineral products are the cementing materials
themselves. These are commonly quartz, calcite, or iron oxide, of no
especial value, but locally they include commercially valuable minerals
containing gold, copper, silver, lead, zinc, and many other mineral
products.

It is a matter of simple and direct observation, about which th