ere is no
controversy, that many minerals are deposited as cements in the openings
in rocks or replacing rocks. As to the source of the solutions bringing
in these minerals, on the other hand, there has been much disagreement.
In general, the common cementing materials such as quartz and calcite,
as well as some of the commercial minerals, are clearly formed as
by-products of weathering, and are transported and redeposited by the
waters penetrating downward from the surface. The so-called _secondary
enrichment_ of many valuable veins is merely one of the special phases
of cementation from a superficial source. In other cases it is believed
that deep circulation of ordinary ground-waters may pick up dispersed
mineral substances through a considerable zone, and redeposit them in
concentrated form in veins and other trunk channels. For still other
cementing materials, it is suspected that the ultimate source is in
igneous intrusions; in fact, deposits of this general character show all
gradations from those clearly formed by surface waters, independently of
igneous activity, to those of a contact-metamorphic nature and others
belonging under the head of "igneous after-effects."

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 i