DEFORMATION OF ROCKS Spi, 
Causes modifying the forms of folds —The foregoing discussion 
has been carried on as though the active forces of deformation 
are equal in opposite directions, and are acting in the same zone 
from opposite sides of the deformed area. If this were the case 
if the strata affected were of the same thickness and strength, 
if the initial dips were equal in opposite directions, and if the 
other conditions were the same, a strictly symmetrical arrange- 
ment of folds might be expected. But these conditions are 
never true. Inthe great majority of cases the facts do not depart 
so far from them but that the folds which form® fall within some 
of the classes given. However, there are a number of ways in 
which the forms of folds may be modified. 
Major faulting may interfere with their forms. Minor slip- 
faulting, as explained upon a subsequent page, may dominate an 
entire area. Igneous intrusions may disturb beds in many ways. 
Where these modifying causes are found the structure is the 
resultant of all the movements. 
Finally it often happens that there isa tendency for the axial 
planes upon one side of an anticlinorium or synclinorium to be 
steeper than those upon the other. In some cases the axial 
planes of all the folds throughout a mountain mass may be 
inclined in the same direction. Such folds may be called sono- 
clinal. In such cases the force, and consequently the movement 
of the strata, have usually been supposed to be more largely from 
one direction than from the other, and the axial planes of the 
folds have usually been regarded as dipping toward the force. 
Various explanations have been offered as to how the forces 
act upon the strata in the actual production of monoclinal folds. 
Of these explanations, that offered by Rogers appears most 
probable for piles of strata of like rigidity. Believing as he did 
that the folds of the Appalachians were analogous to the waves 
of the sea, he naturally concluded that the tendency to a south- 
eastward inclination of the folds of the Appalachians was due to 
the fact that the center of disturbance and resultant waves came 
from the southeast. While not following him in his explanation 
of folds as great waves suddenly formed, the idea seems reason- 
