DEFORMATION OF ROCKS 469 
along the sides of the lamine. As a result, the minerals receive 
an elongation in the direction of greatest movement, a less elon- 
gation in the direction at right angles to this and in the plane of 
movement, but are shortened at right angles to the plane of 
movement. Also new minerals which develop are controlled in 
the same manner. If the fissility be secondary to cleavage, the 
minerals were previously oriented with their two longer axes 
in the plane of differential movement, and the slipping of fissility 
but emphasizes an arrangement of the mineral particles which 
already existed. 
DEVELOPMENT OF CLEAVAGE AND FISSILITY IN HETEROGENEOUS 
ROCKS. 
When a set of layers, either sedimentary or not, of differ- 
ent lithological character are folded, and cleavage or fissility 
develops in them, the process is not simple. 
As the case of alternating sediments is the most important 
one, and somewhat different from any other, this will be first 
considered. 
The original series, instead of being homogeneous, is com- 
posed of beds of different characters; that is, it consists of 
alternations of mud, grit, sandstone, limestone, etc. Before 
these rocks are folded the forces of consolidation, cementation, 
and metasomatism may have been at work. As aresult of these 
prior alterations, combined with original deposition, the strata 
may have greatly varying strength. The sandstones may have 
been transformed to quartzites; the grits may have been changed 
to graywackes ; the muds may have been compacted into shales ; 
and the limestones may have become crystalline. 
When suchaseries is folded the accommodations occur mainly 
along the beds. The greatest readjustments and greatest com- 
pression are along the limbs of the folds (Geeupp a 207-210, 
and Figs. 2, 3, and 5 of my first article). Each layer shears 
over the one next below under enormous stress. The neces- 
sary movement at first is largely concentrated in the weak 
layers between the beds of stronger material (Fig. eu he 
