344 SIQIDINES IMO SI OLOIGIN Hrs) 
may itself become the primary fold. On any fold a new second- 
ary fold may appear, and when followed longitudinally may 
become more and more important until it is the dominant fold. 
In short, in a composite set of folds each fold of any order is 
constantly changing in character and importance. . 
In a given fold all of the changes may occur, and thrust may 
have acted only ina single direction. The initial dip of the beds 
may have been different. The thickness and strength of the 
beds may have varied from placeto place. Thrust may not have 
been equal along the border of the entire area affected. It may 
not have continued to act as long in one place as in another. 
Therefore there is great variation in the character and size of 
folds at their different cross-sections. Gravity is always toward 
the center of the earth; therefore variation in its direction does 
not enter as a modifying force. 
Further, in the foldings of rocks thrust is rarely, if ever, in a 
single direction. Usually, when complex thrusts are decom- 
posed in two directions at right angles to each other, one is more 
powerful than the other. The greater force may be called the 
major thrust, and the lesser force may be called the minor thrust. 
Major and minor thrusts may unite in a resultant effect and 
produce a set of folds in a position intermediate between those 
that the two sets would have if each thrust had been alone. It 
is possible and even probable in many cases, that after a thrust 
in one direction has produced a set of folds, a new thrust at an 
angle less than a right angle to the first may be decomposed into 
two forces and result in further folding the first set of folds, and 
perhaps in the production of transverse folds, rather than in pro- 
ducing a new diagonal set; for when rocks are once bent ina given 
place they bend farther much easier at this place in the same 
direction than at a new place in a new direction. This principle 
is well illustrated by the folds of the Jurassic limestone of the 
Jura Mountains (Fig. 6). The transverse component of the lat- 
eral thrust may be too weak to produce any considerable effect. 
In such cases it would be difficult or impossible to discriminate 
aset of folds thus formed by two diverse thrusts from a set 
