478 SAGLDIUES, LHONS SIM MOVEIN TOS 
the rigidity is less, perhaps originally developed diagonally, may 
be rotated to a nearly parallel position. This rotation may 
change the direction of the planes of fissility either in the com- 
pressed or the stretched diagonal. The cross fissility will grade 
into parallel fissility by a gentle curve. On opposite sides of a 
layer the curves are in opposite directions, just as in the case of 
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Fic. 10.— Parallel fissility and cross fissility in heterogeneous rock strata. 
cleavage, and by these curves it is easy to determine the rela- 
tive direction of movement of the layers (Fig. 10). 
This rotation is explained by Fig. 11, which is supposed to 
represent a bed of rock made up of thirteen layers differing in 
rigidity. In passing from the outside of the bed to the center 
the coefficient of rigidity of each layer is supposed to be twice 
as great as that of the one next adjacent. The greatest stress 
is supposed to be the same throughout the bed, and in an inclined 
direction, and, as shown (pp. 471-472), these conditions may be 
approximately complied with upon the limbs of folds. When 
the differential stress exceeds the ultimate strength of the rock, 
parallel fractures along shearing planes will be formed. The 
fracturing will continue until the stress falls below the ultimate 
strength of the rock. The differential stress may still surpass 
the elastic limit of the rock, or if not, it may again accumulate 
until the elastic limit is exceeded. Flowage will then begin. . 
