DEFORMATION OF ROCKS 327 
part of the primary anticlinoria, each of which includes a cen- 
tral massif and both its flanks. By an examination of the pub- 
lished transverse sections of the Alps and Green Mountains (see 
Fig. 1g) it will be seen that they are usually complicated fan- 
shaped anticlinoria, which are composed of complex normal and 
sometimes abnormal anticlinoria and synclinoria. In each 
normal anticlinorium, of whatever order, the axial planes of the 
folds of the next higher order diverge upward and converge down- 
ward, while in each normal synclinorium the axial plane of the 
folds of the next higher order converge upward and diverge 
downward. In the abnormal anticlinoria and synclinoria the 
reverse is the case. 
Origin of normal folds.— As has been stated, the forces which 
act upon rocks when being folded are assumed to be tangential 
thrust and gravity. In the smaller folds, thrust may be thought 
to be the dominant force, the other being the modifying force of 
varying strength. In the great folds of the earth, gravity may 
be thought to be the dominant force, which by differential 
depression relatively raises a great anticline or depresses a great 
syncline, while thrust may play a subordinate part, being the 
dominant force in the production of folds of the second and 
higher orders. In folds of intermediate size, each of the forces 
may be about equally important. The relative value does not 
matter so far as the foregoing analysis is concerned, as rime all 
three cases the resultant forms fall within the classes given. As 
long as we are so far from agreeing upon the forces which pro- 
duce mountain ranges, and their manner of work, it seems best 
to classify the forms of folds as we find them, and to explain 
their origin so far as we are able. If thrust and gravity be con- 
ceived as acting uniformly upon horizontal homogeneous rocks, 
which are under such conditions as to bend without breaking, 
normal anticlinoria and synclinoria will be produced. Because of 
initial dips (as explained by Willis), or unequal superincumbent 
weight, or other causes, or one or more of these together, rocks 
when subjected to thrust and gravity rise into an anticline here 
or fall into a syncline there. But there is unequal weight upon 
