DEFORMATION OF ROCKS 339 
there is no reason why differential movement should not be initi- 
ated at the same time wherever the conditions are favorable 
throughout the area in which monoclinal folds are observed. 
It is to be noted that, under the assumption that the effective 
stress moves the upper strata over the lower, the vertical compo- 
nent of deformation is upward rather than downward. In other 
words, it is in the direction of easiest relief, and this is the kind 
of deformation one would expect, and which doubtless prevails 
in the majority of movements of the first order, in which thrust 
is the dominant force, for it has been seen (p. 382) that, 
upon the average, rocks are stronger with increasing age, and 
hence, there is greater resistance centerward than surfaceward, 
Folds thus produced by upper differential movement may be 
called ‘‘overthrust folds.” The axial planes dip toward the effec- 
tive stress, hence overthrust folds are those in which the axial planes 
dip toward the force producing them. 
While the development of overthrust folds is the general law, 
it may not infrequently happen that under favorable conditions 
beds or formations may be thrust forward and downward. Folds 
thus produced by downward differential movement may be called 
‘“underthrust folds.” The axial planes dip away from the effec- 
tive stress, hence, wnderthrust folds are those in which the axial 
planes dip away from the force producing them. 
In the rocks of a system or group strong formations may be 
above weak formations. In this case the strong formations are © 
able to transmit the forces to a greater degree than the beds 
above or below. As pointed out by Willis, a forward downward 
movement may be directed by initial dip, and thus underthrust 
folds be produced. In asecond case the strata pile up as a result 
of the folding. The relatively raised masses may then to a cer- 
tain extent escape active thrust. (See Fig. 6 and p. 318.) 
The strata largely transmitting the thrust in front of the folded 
material may under these conditions be pushed under the higher 
mass. Underthrust folds are most likely to occur if the two con- 
ditions above given favorable to their formation are combined, 
?. e., weak formations below and piling up of strata. 
