s
difficult to tell how much movement there has been in the production of
a marble, because both kinds of processes seem to produce much the same
result. Commonly, however, the effect of dynamic metamorphism is to
produce a parallel arrangement of mineral particles and to segregate the
mineral particles of like kind into bands, giving a _foliated_ or
_schistose_ or _gneissic_ structure, and the rocks then become known as
slates, schists, or gneisses. Commonly they possess a capacity to part
along parallel surfaces, called _cleavage_. The development of the
schistose or gneissic structure is accompanied by the recrystallization
of the rock materials, producing new minerals of a platy or columnar
type adapted to this parallel arrangement. Even the composition of the
rock may be substantially changed, though this is perhaps not the most
common case. Whereas by weathering the rock is loosened up and
disintegrated, substances like carbon dioxide, oxygen, and water are
abundantly added, and light minerals of simple composition tend to
develop,--by dynamic metamorphism on the other hand, carbon dioxide,
oxygen, and water are usually expelled, the minerals are combined to
make heavier and more complex minerals, pore space is eliminated, and
altogether the rock becomes much more dense and crystalline. While
segregation of materials is characteristic of the surficial products of
weathering, the opposite tendency, of mixing and aggregation, is the
rule under dynamic metamorphism, notwithstanding the minor segregation
above noted.

Dynamic metamorphism is for the most part unfavorable to the development
of mineral products. Ore bodies brought into a zone where these
processes are active may be profoundly modified, but not ordinarily
enriched. One of the exceptions to this general rule is the development
of the cleavage of a slate, which enables it to be readily split and
thereby gives it value. Contact metamorphism, on the other hand, may
develop valuable mineral deposits (see pp. 20, 45-46).

THE METAMORPHIC CYCLE AS AN AID IN STUDYING MINERAL DEPOSITS

All of the chemical, mineralogical, and textural changes in rocks above
described may be collectively referred to as _metamorphism_. The phase
of metamorphism dealing with surficial weathering, similar changes below
the surface, and the formation of sediments, is called _katamorphism_ or
destructive change. The phase of metamorphism dealing with the
constructive changes in roc