340 SLIGLDIGES, SAE SI GIOIEIN IES 
In considering the force couples in normal and abnormal com- 
posite folds, each composite limb between trough and crest must 
be separately analyzed. (See Figs. 17and 18.) Innormal anti- 
clinoria and synclinoria gravity has been seen to be the efficient 
force which causes differential movement (pp. 328-330). Grav- 
ity works downthe slope. The axial planes of the secondary 
folds dip toward the force (see Figs. 8and 11), and the secondary 
crenulations are therefore overthrust folds. In abnormal com- 
posite folds it has been explained (pp. 330-333) that the differ- 
ential movement may be caused by (1) normal readjustment, (2) 
increased strength of the rocks with increasing depth, and (3) 
the possible decreasing lateral stress with increasing depth. The 
first of these is eliminated for the present purpose. The second 
-and third, either singly or together, must be sufficient to over- 
come gravity and to give a resultant force directed up the slope, 
in order that abnormal folds may be formed. (See Figs. 12 and 14.) 
The axial planes therefore dip toward the force, just as in normal 
composite folds, and the secondary crenulations are therefore 
overthrust folds. 
By the above it is not meant to imply that underthrust folds 
may not be produced in either normal or abnormal composite 
folds, for if the conditions given on previous pages favorable for 
underthrust folds locally occur, crenulations of this type may be 
formed. 
The formation of monoclinical folds is sometimes well illus- 
trated by the crenulations of a lavabed in which there was differen- 
tial flow down a slope, the upper layers moving faster than the 
lower. Monoclinal folds thus formed are usually not large. The 
directing force was gravity, and the axial planes dip toward the 
force. The crenulations are therefore overthrust folds. Since 
ordinary folds, which form without fracture, develop in the deep 
seated zone of flowage, the analogy with contorted lava currents 
is believed to be closer than might be at first thought, although 
it is not meant to imply by this that the folded rocks are really 
fluid, but merely that a plastic solid under sufficient differential 
stress is deformed in the same fashion as is a viscous liquid. 
