ORIGIN OF FOLIATED CRYSTALLINE ROCKS 309 
be expected that the SiO, content would tend to become lower or 
higher than that in the original rock, according as to whether 
the original SiO, lay above or below that range. This is illus- 
trated in the following figure. 
On the ordinate are plotted the percentages of silica, the com- 
position of the original rocks being represented by circular signs, 
the white symbols representing quartzite, the shaded shale, and the 
black limestone. The crosses immediately above or below each 
Se aes 
ae | 
Oe a 
il ool eee eee 
na oa 
eeegen qveaee 
Oe ee ae 
\é 
ay, 
40 60 80 100 120 140 160 180 200 
Si0:. % in fresh 
SiOz % in altered X 100 
FIG. 12 
of these signs represent the percentage of SiO, in the altered rock. 
Along the abscissa are plotted the ratios of the SiO, percentages 
of the fresh rocks to those of the altered rocks. For convenience 
the ratio is multiplied by roo in the figures. Rocks appearing to 
the right of the roo division show a decrease in silica percentage 
during alteration according to the distance which the symbol is 
from the roo division, and similarly those to the left of the 1oo 
division have gained in silica according to the distance of the 
symbol from the 100 division. The dotted horizontal lines include 
the approximate range for silica in the platy minerals. It is not 
to be supposed that the theoretical percentage is reached in any 
case though it is probable that those below the requirements have 
