336 SLIDERS, IHN (SIGINEIN ICS 
ing crestward, normal secondary folds pointing outward would 
have developed. 
When two groups of rocks of unequal strength, not deeply 
buried, are folded into a syncline, on account of the natural read- 
justment of the 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 of the rock mate- 
rial on either side away from the axis of the synclinorium over 
the lower, older rocks, thus producing secondary folds, the axial 
planes of which diverge upward and converge downward. Had 
the rocks been of equal strength, or had the weight of the super- 
incumbent strata been sufficient to more than overbalance the 
difference in strength and difference in stress tending to produce 
folds pointing troughward, normal secondary folds pointing 
- inward would have been developed. 
Crystalline or core rocks are apt to be more massive and 
stronger than the little altered sedimentary beds, and therefore the 
core rocks usually act to a certain degree as a unit when subjected 
to thrust. Secondary abnormal folds are frequently found at the 
contact of massifs and the overlying rocks. However, even in 
these cases the folds will be normal if only the thickness of the 
superincumbent beds be sufficient. At a considerable depth the 
different strength of rocks is not so potent as gravity in giving 
form to folds. From the above it does not follow that the mas- 
sifs or portions of them do not take part in the folding. That 
they do in many regions is certain, as is shown by infolding of 
core rocks with sedimentary beds, even when the massifs were 
originally granite. Also that massifs may take part in the fold- 
ing is shown by the minor and major folds of their parts, which 
in form are like those of the associated sedimentary rocks. 
It is recognized, however, as explained upon a subsequent 
page, that massifs, because of their homogeneous character, 
because they are underlain by no definite stratum of rock of a 
different character, and because they are often so deeply buried, 
may act in quite a different fashion, under the forces of folding, 
from ordinary sedimentary layers. 
