METAMORPHISM 373 
carry it to another, and there build a mineral, as is 
done when quartzites are cemented by a deposit of 
silica. Or possibly the form of the mineral will change 
from the amorphous condition to the crystalline, as 
in limestone. Or certain minerals, like the kaolin of 
clay, combining with other materials, and altering 
in various ways by the complex chemical action of 
water, will build up new minerals, such as mica, feld- 
spar, hornblende, etc. 
Pressure will aid the work; and if the conditions 
are favorable, cause the particles to slip, either pro- 
ducing crushing, or else a movement much like the 
flowing of a viscous body such as wax. 
Pressure is also important in determining the arrangement of 
the newly growing minerals. As they grow, they develop in the 
direction of least resistance, which is at right angles to the direc- 
tion in which pressure is applied. So among mountains, with the 
pressure coming from the sides, minerals, and consequently the 
resulting schistose structure or slaty cleavage, will develop at 
right angles to this; that is to say, in the direction of the axis of 
the folding. This is often beautifully shown among mountains, 
where the slaty cleavage crosses the strata indiscriminately, but 
always extends in a single direction (Fig. 227). 
Among mountains where metamorphism has been in 
progress, we often find some layers very much changed, 
while others, less easily altered, but subjected to the 
same conditions, are hardly metamorphosed at all. 
Or the same layer, traced from one place of marked 
