Hypothesis and inference play a considerable part in arriving at any conclusion as to the source of cementing materials,—with the result that there is often wide latitude for difference of opinion and of emphasis on the relative importance of the different sources of ore minerals.
Dynamic and contact metamorphism. Beneath the surface rocks are not only cemented, but may be deformed or mashed by dynamic movements caused by great earth stresses; the rocks may undergo rock flowage. The result is often a remarkable transformation of the character of the rocks, making it difficult to recognize their original nature. Also, igneous intrusions may crowd and mash the adjacent rocks, at the same time changing them by heat and contributions of new materials. This process may be called contact metamorphism, but in so far as it results in mashing of the rocks it is closely allied to dynamic metamorphism. The former term is also applied to less profound changes in connection with igneous intrusions, which result merely in cementation without mashing.
Dynamic and contact metamorphism may in some cases produce rocks identical in appearance with those produced by ordinary processes of cementation and recrystallization without movement. For instance, it is 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 rocks, 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.