DEFORMATION OF ROCKS 479 
On account of the varying rigidity the two layers adjacent to 
the center will have a certain amount of differential movement, 
which may be called 1. The layers next to them toward the 
outside will have a movement which would be represented by 2; 
those next to them, 4; those next to them, 8; and the outside 
layers a movement of 16. Now, connecting similar points in the 
different layers, a curve is produced which corresponds very 
aS 
Fic. 11.— Diagram showing rotation of fissility which originally developed in 
—— 
——S— 
the shearing planes to a position nearly parallel to the bedding. 
nearly in form to those which have been observed in nature. 
The structure produced in the diagonal direction, in the centers 
of the layers, has been rotated to the position indicated. In 
different rocks the variation of the coefficient of rigidity would 
be different from that supposed, and it would undoubtedly vary 
irregularly instead of regularly. A more accurate discussion 
would consider each of the layers as indefinitely thin, and the 
coefficient of rigidity in passing toward the center of the bed as 
increasing by a minute increment. If different numerical sup- 
positions be made, curves would be produced differing from 
those represented by the figure, but the same in essential char- 
delete 
A third way in which the curved fissility above described 
may be produced is as a structure secondary to cleavage. That 
cleavage can be produced having the same curves and relations 
to bedding as just described for fissility has already been shown 
(pp. 471-474). Such previously developed cleavage would give 
parallel curved surfaces of weakness. When the rock passed 
into the zone of fracture fissility would develop along these 
shearing planes whether they were those of maximum tangential 
stress or not. 
*Compare Geology of the Comstock Lode and the Washoe District, by GEo. F. 
BECKER, Mon. U.S. Geol. Surv., Vol. III, pp. 156-178, 1882. 
