338 STUDIES FOR STUDENTS 
able that the ‘forward thrust operating upon the flexures 
would steepen the advanced side . . . _ precisely as the wind 
acting upon the billows of the ocean forces forward their crests 
For we now 
Yr 
and imparts a steeper slope to their leeward sides. 
regard folded rocks as plastic when bent. The compressive 
stresses do not extend toan indefinite depth, but are limited by 
the level ot no lateral stresss)) hey theretore) atect the vouter 
skin of the earth, just as does the wind the superficial water of 
the ocean. Asa result there is a differential movement due to 
friction, the amount of movement upon the average gradually 
decreasing below the zone of maximum movement. Of course 
the sums of the forces, including friction are always equal in 
opposite directions, but they constitute a vertical couple, z. e., 
“Two equal and parallel forces opposed in direction, but not in 
the same straight line.’ Asa result there is differential move- 
ment of the material of the upper zone as compared with the 
lower, the former being thrust over the latter. 
Other things being equal, where the differential thrust is 
greatest the first inclined fold is formed. The folding piles up, 
the strata. After a time the increased thickness of material is 
sufficient to present a larger total resistance to deformation than 
the thinner strata in advance. This stress will then be transmit- 
ted forward. On account of the greater stress per unit of area, 
a second fold, similar to the first, will then be formed, but this 
-results in again thickening the mass subject to the force couple, 
and again the stress is transmitted forward. A new inclined fold 
is then produced, and so on. 
It is not necessary that one inclined fold shall be completely 
formed before others begin to develop. Indeed, this is not to 
be expected, for as soonas any thickening of the deformed mass 
occurs the conditions are favorable for the forward transmission 
of the effective stress. Thus many folds may be in process of 
formation at the same time and so far as I can understand, 
*On the Physical Structure of the Appalachian Chain, as exemplifying the laws 
which have regulated-the elevation of great mountain chains generally, by W. B. 
ROGERT, Proc. Assn. Am. Geol. and Nat. for 1840-2, p. 512, Boston, 1843. 
