20 The American Geologist. January, 185.7 
ment in rocks is not proof of sedimentation, a view which be- 
fore this time was not recognized. 
Heat, water and pressure are the great agents of metanior- 
phism, and they produce three kinds of alterations in rocks, 
mineral, niicrostructural and macrostructural changes. Un- 
der mineral changes, we have among the alkaline silicates the 
alteration termed sericitlzation, forming an interlacing net- 
work of hydromicas. Also saussuritization., embracing the 
clianges whereby the plagioclase feldspar is converted into 
alkaline earth silicates. In alhitization the feldspar is changed 
into an interlocking albite mosaic. Among the iron-magne- 
sian silicates occurs : uralitizatiort, where pyroxene is changed 
into fibrous hornblende, viridizat.ion or formation of green 
epidote chlorite mass, analogous to saussuritization. chlorifi- 
zatioii and epidotization, analogous to alhitization. 
Under microstructural changes are observed the strain phe- 
nomena in crystals, recognized by polarized light in a wavy 
extinction of the light as the section is rotated. If the strain 
has been carried farther, gliding or twin lamelhe may be ob- 
served, as in the metamorphic marbles. Progressing to 
greater extent the minerals are bent, twisted and finally bro- 
ken into an irregular mosaic, composed of interlocking min- 
eral 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 secondary foliation or an arrangement of the 
minerals along parallel lines, which were so long taken as 
evidence of stratification. Though this distinction between 
foliation and sedimentary lines was noted early in the centurj- 
by Voigt, Mohs, and Schmidt, it attracted little attention. 
Later it was observed that the lines were parallel over exten- 
sive tracts, even when the rocks were crumpled. This was 
explained as the result of crj'stalline force or result of elec- 
trical currents passing 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 a purely structural term and has 
no connection with age. While most of these rocks are pre- 
Cambrian, there are numerous exceptions. The schists are 
