DEFORMATION OF ROCKS 477 
general directions. In simple folding, as there is differential 
movement between the layers, the deformation of a portion of a 
given layer is that of a rectangle (Fig. 8), abcd, into a parallelo- 
gram (Fig. 9), efgh. If the layer were exactly homogeneous 
and the pressure normal, the secondary structures would be 
nearly at right angles to each other and at an angle of about 45° 
to the greatest pressure. According to Becker, in the case of 
inclined pressure the structures would have different positions, 
but they still would be planes. These conditions are most 
nearly approached in the center of a bed which at this place is 
massive. However, in passing from the center to the weaker, 
outer part, the original bedding may largely control the direction 
of parting, the partings occurring near the planes of bedding 
rather than in those of greatest tangential stress (Fig. 1O)) ee dilive 
result is that the planes of fissility may change from their 
diagonal position in the center of the layer, where it is most 
rigid, to nearly parallel to the bedding on the outer parts, where 
it is least rigid. 
The diagonal ad (Fig. 8) is shortened to eh (Fig. 9); there- 
fore the fissility along the diagonal gf 1s formed under condi- 
‘tions of compression. This results in producing many approxi- 
mately parallel planes of fissility. No sooner does a parting 
form than the lamine are sheared over one another, thus pro- 
ducing slickensided surfaces. Across this structure along the 
longer diagonal, in the plane of the shorter diagonal, there is 
actual stretching of the material. The length of the original 
diagonal cé is increased to gf. Parallel cracks are therefore pro- 
duced in this direction. As a crack once formed easily widens, 
the result is the production of a few cracks of considerable size. 
The broken parts do not rub over one another, and hence do not 
produce slickensided surfaces. These peculiarities frequently 
lead to oversight of the shearing along the planes of the shorter 
diagonal. The development of the cracks along the shorter 
diagonal are strictly analogous of the upward-pointing crevasses 
of a glacier. 
The planes of fissility near the border of the beds, where 
