368 ELEMENTARY GEOLOGY 
This change of the clay stratum may proceed with 
the development of other new minerals; and if the 
conditions are favorable, mica flakes may appear, and . 
then the rock be transformed to a mica schist (Fig. 45). 
Mica schist may be produced from various classes of 
rocks; but in each case it is the result of a change in 
mineral composition. Instead of mica, the new mineral 
may be hornblende or some other, thus producing horn- 
blende schist, etc. Here again, among these products, 
one of the most noticeable features is the banding of 
the minerals, producing a_ schistose structure, along 
which the rock easily breaks. Even further change 
may take place; and out of the fine-grained clay or 
other rock, there may develop a coarse, crystalline prod- 
uct, In many respects resembling a granite, except that 
the minerals are more or less distinctly banded. This 
is a gneiss (Fig. 46). 
Frequently the rock resulting from these variations, 
is of the same chemical composition as the original, the 
change having been merely in the relation of the min- 
erals; but in other cases there have been fresh ele- 
ments actually introduced from outside, so that there 
is an addition of new mineral matter. This transfer 
of materials is evidently the result of water action, and 
the substances brought have been carried in solution. 
In many cases the elements thus transported, and the 
changes produced in the minerals, indicate that the 
