130 KANSAS ACADEMY OF SCIENCE. 
Heat, water, and pressure are great agents of metamorphism, and they pro- 
duce three kinds of alterations in rocks — mineral, microstructural, and macro- 
structural changes. Under mineral changes, we have among the alkaline silicates 
the alteration termed sericitization, forming an intercalating network of hydro- 
micas. Also saussuritization, embracing the changes whereby plagioclase 
feldspar is converted into alkaline earth silicates. In albitization the feldspar 
is changed into an interlocking albite mosaic. Among the iron-magnesian sili- 
cates occurs: uralitization, where pyroxene is changed into fibrous hornblende ; 
viridization, or formation of green epidote chlorite mass, analogous to saussuri- 
tization ; chloritization, and epidotization, analogous to albitization. 
Under microstructural changes are observed the strain phenomena in crystals, 
recognized by polarized light in a wavy extinction of light as the section is ro- 
tated. If the strain has been carried farther, gliding or twin lamelle may be 
observed, as in the metamorphic marbles. Progressing to greater extent, the 
minerals are bent, twisted, and finally broken into an irregular mosaic, composed 
of interlocking mineral grains. Sometimes there is a stretching of the rock along 
certain lines, pulling the grains apart. 
Under macrostructural changes, the most prominent is the formation of sec- 
ondary foliation, or an arrangement of the minerals along parallel lines, which were 
so long taken as evidence of stratification. Though this distinction between foli- 
ation and sedimentary lines was noted early in the century by Voigt, Mohs, and 
Schmidt, it attracted little attention. Later it was observed that the lines were 
parallel over extensive tracts, even when the rocks were crumpled. This was 
explained as the result of crystalline force or the result of electric currents pass- 
ing around the earth. In 1846 it was shown to be due to pressure normal to that 
which developed the foldings. 
Such rocks, which possess this secondary foliation, are called crystalline 
schists. This is purely a structural term and has no connection with age. 
While most of these rocks are pre-Cambrian, there are numerous exceptions. 
The schists are divided into two main groups, those with feldspar and those 
without this mineral; and the former are called gneisses. This usage makes 
gneiss a mineralogical and structural term; mineralogical in that it contains 
feldspar, structural in that it is foliated. If the origin of the gneiss is determin- 
able it has the original rock name added as a prefix. Thus, a secondary foliated 
conglomerate is called a conglomerate gneiss. A foliated granite is a granite 
gneiss. When the word gneiss is used alone it represents a foliated feldspathic 
schist of unknown origin. 
Down to the end of the last century geology was a collection of hypotheses 
and sacred theories of the earth. Its students then began to observe and record 
facts, and later to form theories based on such observed facts. The study of the 
igneous rocks passed through a similar course of development. The study of the 
metamorphic rocks is now passing through such a course and it has entered the 
descriptive stage. It is now at the point reached by general geology in the time 
of Lyell, and reached by the study of the igneous rocks in the year 1870. 
The study of the crystalline schists, both of Archzan and post-Archean 
time, now becomes the great field for work, and all over the world students are 
trying to trace their origin and formation. 
