DEFORMATION OF ROCKS 205 
mineral particles of different kinds and of different sizes. In a 
larger way it is composed of rock masses of different characters. 
Frequently these masses of different characters and strength are 
divided by vertical or steeply inclined planes of weakness, 
rather than horizontal ones, as in the sedimentary rocks; hence, 
major movements take place along the major inclined planes 
and minor movements along minor planes and between the 
mineral particles. Complex minor folding and shearing there- 
fore occur in the readjustment of a massif to its new position. 
The major planes of shearing may be called fault planes, but they 
differ from ordinary faults in that the parts moved over one 
another are always in close contact, and probably are also always 
welded. 
When a set of rock beds are bent even slightly, readjustment 
and rearrangment of the material must occur to some extent, and 
the amount increases in proportion as the rocks are closely folded. 
If a steel bar be bent the molecules are separated to some 
extent upon the convex side and compressed upon the concave 
side. When released from stress the bar, by virtue of elasticity, 
springs back to its original position. While to some slight 
degree rocks are elastic when subjected to forces continuing for 
a short time, it may be doubted whether this property is of 
importance in considering the slowly acting and long-continued 
forces of rock folding, except perhaps in the slight flexures of 
great extent. Single rock beds, when much deformed, are rather 
to be compared to a wrought-iron bar, which when bent takes a 
permanent set. In this case there is an actual flowage of material 
or rearrangement of the particles to meet the new conditions. The 
half of the bar on the convex side subjected to tension is length- 
ened, and, to compensate, it is of less cross-section then originally. 
The half of the bar on the concave side subjected to compression 
is shortened, and, to compensate, it is of greater cross-section than 
originally. Each homogeneous rock stratum when bent acts like 
the iron bar toa certainextent. There is rearrangement of its mate- 
rial to new positions, and when the bending occurs without fracture 
the movements of the rock particles may be like those of the par- 
