ks, due to cementation, dynamic movements,
and igneous influences, is called _anamorphism_. Some geologists confine
the term metamorphism to the changes involved in contact and dynamic
metamorphism, and call the resulting products _metamorphic rocks_.

The zone in which katamorphism is most active, usually near the surface,
is called the _zone of katamorphism_. The deeper zone in which
anamorphism is preponderant is called the _zone of anamorphism_. There
are no definite limits of depth to these zones. A given rock may be
undergoing katamorphism while rocks on either side at the same depth are
suffering anamorphism.

By katamorphism rocks break down to produce the surficial rocks, and by
anamorphism the surficial rocks are again consolidated and altered to
produce highly crystalline rocks, which are not dissimilar in many of
their characteristics to the igneous rocks from which all rocks trace
their ultimate origin. In other words, anamorphism tends toward the
reproduction of igneous rocks, though it seldom fully accomplishes this
result. These two main groups of changes together constitute the
_metamorphic cycle_. Some rocks go through all phases of the cycle, but
others may pass directly from one phase to an advanced phase without
going through the intermediate stages. For instance, an igneous rock may
become a schist without going through the intermediate stage of
sedimentation.

Rocks are not permanent in their condition, but at practically all times
and places are undergoing some kind of metamorphism which tends to adapt
them to their environment. The conception of rocks as representing
phases or stages in a progressive series of changes called the
metamorphic cycle aids greatly in correlating and holding in mind many
details of rock nature and origin, and brings into some sort of
perspective the conditions which have produced rocks. A schistose
sediment comes to be regarded as an end product of a long series of
alterations, beginning with igneous rocks and passing through the stages
of weathering, sedimentation, cementation, etc., each of which stages
has been responsible for certain mineralogical, chemical, and textural
features now characterizing the rock. The alternation of constructive
and destructive changes of the metamorphic cycle, and the repetitions of
the cycle itself, periodically work over the earth materials into new
forms. Usually the cycles are not complete, in the sense that they
seldom bring th