DEFORMATION OF ROCKS 345 
formed by simultaneous or successive thrusts in a uniform direc- 
tion. However, if there be sufficiently strong thrusts in two or 
more diagonal directions, or two thrusts at right angles, two sets 
of folds are produced which intersect each other. Such a dis- 
trict may be described as one of complex folding. The more 
important set of folds, corresponding to the major thrust, may be 
called the major or longitudinal folds, and the cross folds, cor- 
responding to the minor thrust may be called the minor or trans- 
verse folds. 
CHARACTER OF COMPLEX FOLDS. 
From analysis, as well as from observation, it is found that 
cross folds are usually nearly at right angles to each other ; for as 
already explained, if the diverse thrusts be inclined to one another, 
they will be resolved into two forces, one of which forms the 
major folds, and the other of which produces cross folds in a 
direction at right angles, or nearly so, to the first set of folds. 
Major and minor cross folds may be produced by continuous 
forces in both directions, or in each direction each force may be 
continuous or discontinuous. To ascertain these points the same 
criteria are available as in the case of thrust in a single direc- 
tion (see pp. 342-343). 
Longitudinal and transverse folds may each be classified into 
upright, inclined, or overturned. Each of these may be ordinary, 
isoclinal, or fan-shaped. Simple folds may unite to form com- 
posite folds. The composite folds may be normal or abnormal, 
and each synclinorium or anticlinorium under each class may be 
upright, inclined, or overturned. 
As complex folds actually occur in the field, usually the 
compression is not close in both directions. Cases are known, 
however in which a set of longitudinal overturned folds have trans- 
verse folds with vertical dips, and the dips of the transverse folds 
in intricately folded districts are in many cases as steep as 45° 
or 60°. 
Wheg the major folds are close, as compared with the minor 
folds, the complex folds have great length as compared with the 
breadth and are canoe-shaped. The Appalachians in the closely 
