PETROLOGICAL ABSTRACTS AND REVIEWS 27.2 
of crystalline schists are now settled, viz., the source of the crystalline 
schists, and their relation to time. They have developed from both 
sedimentary and igneous rocks in various periods, and are not the result 
of conditions peculiar to the Archean, as formerly supposed. Opinions 
are still divided, however, on the processes of schist development. These 
diverse views can be grouped under two heads: (I) those which ascribe 
the origin of schists to processes independent of the agency of igneous 
rocks; (II) those which regard the agency of igneous rocks as controlling. 
I. The origin of crystalline schists without the agency of igneous 
rocks. 
a) High temperature of the ecarth’s interior—Hutton and Lyell 
emphasized the alteration of normal sediments into schists through 
fusion and recrystallization effected by the high temperature of the 
earth’s interior. This view and the experiments of Daubrie on the 
alteration of minerals by superheated water, formed the basis of 
Giimbel’s diagenesis (1868), the alteration of mechanical sediments into 
crystalline schists by superheated waters. 
b) Hydrochemical metamorphism.—Bischof and others interpreted 
the gradation between clay, slate, mica schist, and gneisses as resulting 
from the chemical action of percolating waters. 
c) Dynamic metamorphism.—The fact that schists appear in areas 
of folded younger rocks led Lossen (1867) to assume that schists are 
sediments which have been recrystallized through moisture and pressure. 
Later Rosenbusch (1889, 1891) asserted that the crystalline schists in- 
clude both igneous and sedimentary rocks which have been altered by 
geodynamic processes. In 1892 F. Becke concluded that there are two 
types of dynamic metamorphism. One of deeper zones is effected by 
recrystallization and resembles contact metamorphism by granites; 
that of shallow depths involves granulation and is like propylitization 
in its chemical effect. 
In 1894 L. Milch argued that recrystallization could not be effected 
by pressure alone but must be traced to water heated or superheated 
by pressure, and that therefore pressure from load would have the same 
effect as pressure from crustal shortening, and thus differentiated. 
Belastungs and Dislokations—metamorphism. lL. Milch, (1899), T. J. 
Sederholm (1891), and Lepsius concluded, independently, that rocks are 
chemically altered by pressure and percolating solutions combined. 
Pressure produces secondary parallel structure (schistosity) by 
granulation of the original particles, by rotation of platy or columnar 
particles, so that their major dimensions lie in a plane normal to the 
