DEFORMATION OF ROCKS AR 
may be only one-half as thick as the troughs and crests. Where 
the folding is very close the troughs and crests must be several 
times as thick as the limbs in order that they may have similar 
forms. 
It is evident from the above that close folds can persist in 
depth with similar forms only by great differential movement of 
material, for the readjustment will only go so far as demanded 
by the differential stresses. It is therefore to be expected that 
the close folds of mountain districts die out rapidly with increas- 
ing depth. This would certainly be true if the lateral stresses 
diminish, as they must do if the theory of “level of no strain” 
at very moderate depth be correct. The foregoing doubtless 
partly explains the open folding often observed in the center of 
the mountain masses as contrasted with the closely folded flanks, 
although in many cases other causes undoubtedly enter. 
This rapid disappearance of folds with ‘increased depth is 
beautifully illustrated in a section along the Schuylkill River 
near Philadelphia, between Lafayette and Spring Mill, shown me 
by Miss Florence Bascom. In the center of the section is gently 
undulating gneiss. In passing toward the outer part of the 
gneiss it is closely folded. The gneiss is flanked on either side 
by still more closely folded mica-schist. Not only is the princi- 
ple illustrated by the entire section, but at various places close 
local folds may be seen, above and below which the folds rapidly 
become more open. The same is illustrated in the crumpled 
gneisses of the Ottawa River. The same principle is illustrated 
upon a much larger scale by the section along the Doe River 
west from Cranberry, N.C. The folding and the schistosity 
of the pre-Cambrian granitoid gneisses are gently composite, 
while the overlying Paleozoic quartzite shows many close minor 
folds. This case isa particularly good one, for it can hardly 
be supposed that the quartzite is much less rigid than the 
gneissoid granite. 
It follows from the above that where different beds are in 
folds of similar forms rearrangement within the beds, adjustment 
between the beds, and distortion of the beds all work together 
