THE METAMORPHIC CYCLE 309 
with liberal allowances for mechanical consolidation, the muds may 
still use more cementing material than other sediments combined. 
Cementation also affects igneous rocks to the extent of filling 
fissures and other openings, but the process is insignificant in terms 
of the effect on the igneous rock itself, as compared with the effect on 
sediments. 
Anamor phism.—When the cemented sedimentary rocks reach con- 
ditions of sufficient heat and differential pressure—which usually, 
though not always, means, when they reach the depth of rock- 
flowage—the reascensional, or constructive, development of the rocks 
continues under a set of processes which Van Hise has collectively 
designated as anamorphic. ‘The minerals are recrystallized. Some 
of the simple mineralogical and chemical units become combined 
into relatively complex ones. The kaolin of shales becomes dehy- 
drated, and this and other alumina silicates present are combined 
with the minor amounts of bases present to produce anhydrous 
silicates, which usually develop with a dimensional parallelism to 
meet the differential stress conditions, giving the rock a flow cleavage 
or schistosity. The quartz of the sandstones and quartzites combines 
with the minor amounts of clay and bases present to develop mica 
or other silicates—again usually, though not always, with parallel 
structure. The calcite of the limestone recrystallizes coarsely, usually 
without parallel structure, though sometimes possessing it; and there 
is a combination of the calcium with the small amounts of alumina 
silicates and bases which may be present, to develop pyroxenes, 
amphiboles, garnet, chloritoid, and other heavy anhydrous minerals. 
Chemically, the dominant processes are silication (and decarbona- 
tion) and dehydration, with a little deoxidation. The volume is 
reduced by closing pore-spaces, by the development of minerals with 
a higher average density (when taken in proportion of abundance), 
and, most important, by loss of materials. There is a tendency 
toward local mixing of substances, rather than segregation. Ores 
are modified, but not simplified and segregated. Adaptation to 
environment means, in short, the development of hard, dense crys- 
talline slates, schists, and gneisses, some of the minerals of which 
approach more nearly in complexity those of the igneous rocks 
than do the minerals of the sediments. 
