334 SLODIES FOR STUDENTS 
in passing away from the central zone of plications either into 
the more rigid rocks below or into the softer rocks above the 
secondary folds become normal. Considering the two figures 
put together to represent a great flexed mountain mass, and sup- 
posing erosion to truncate the layers to the horizontal line drawn, 
there would be exposed normal folds at the center of the anti- 
cline, abnormal ones upon the flanks, and normal ones at the 
outer parts of the mountain mass. In nature we can never hope 
to see such a great composite fold in all its parts. It is only in 
the great mountain masses where such folds have been dissected 
that we can get at their character. In such cases the older and 
newer strata would be expected to show the normal forms, the 
intermediate strata the abnormal forms. The change from 
normal to abnormal and to normal again is apparently that 
which actually occurs in the Alps from the St. Gothard massif 
south to the great valley of Switzerland. 
The manner in which the more rigid rocks escape large plica- 
tions while the weaker beds are strongly plicated, producing 
abnormal folds, is well shown by Fig. 13, given by Heim as a 
general section showing roof structure in folded sediments and a 
central massif. In the production of actual abnormal anti- 
clinoria and synclinoria it is probable that accommodations as 
illustrated by Figs. 15 and 16 are largely concentrated as illus- 
trated by Figs. 17 and 18. Therefore the production of abnormal 
anticlinoria and synclinoria may be summarized as follows: 
When two groups of rocksof unequal strength, not deeply 
buried, are folded into an anticline, on account of the natural 
readjustment of strata, of the relative weakness of the upper, 
newer group of rocks, and probably of decreasing differential 
stress with increasing depth, there may be differential flow on 
either side toward the axis of anticlinorium over the lower, older 
rocks, thus producing secondary folds, the axial planes of which 
converge upward and diverge downward. Had the rocks been of 
equal strength, or had the weight of the superincumbent strata 
been sufficient to more than overbalance the difference in 
strength and difference in stress tending to produce folds point- 
