DEFORMATION OF ROCKS 455 
stress, depending upon whether the latter is nearer the maximum 
stress or the minimum stress. The phenomena subsequently 
described appear to show that there is more frequently elonga- 
tion than shortening along the direction of mean stress. It 
follows that as a result of flowage there is as one direction 
mo 
POODOONOORR CL 
At iy Hh 
ransvens KOON 
DORON ba 
ae 
CCEA HE 
COOOOOOONNIGING 
BrGaen iG, 2 
Figs. I and 2, Showing theoretical change in arrangement and form of particles 
when uniformly shortened by hydrostatic viscous flow. 
of shortening and two of unequal elongation. The plane of the 
minimum and mean stresses is believed to be the plane of cleavage. 
A coherent slate, which has the unmodified property of 
cleavage as here defined, shows but a single marked structure. 
This structure in the dense slates is often nearly normal to the 
direction of the greatest shortening of the strata. In the many 
instances in which cleavage is everywhere nearly parallel to 
intrusive igneous masses, there seems to be no escape from this 
conclusion. In such cases a zone of slate or schist surrounds a 
granite or other mass, the secondary structure varying in direction 
through 360°, thus making a circle about the intrusive core. 
The cleavage is therefore at each point nearly at right angles to 
the greatest pressure. Such zonal cleavage about batholites has 
been described in this country at various localities. The cleav- 
age in the mica-schist adjacent to the granite core of the Black 
Hills is everywhere parallel to the igneous mass. Lawson has 
described like phenomena at many places in the mica-schists of 
Ontario. The secondary structure is everywhere parallel to the 
adjacent igneous masses. Emerson finds that the same relations 
obtain between the secondary structure and the intrusive granites 
