SCAISTOSITY AND SLATY CLEAVAGE 443 
a scission this load is at each instant applied to a new set of par- 
ticles, which offer not only elastic resistance but viscous resist- 
ance as well; they are practically and actually stronger for this 
coéperation. Now suppose that the load has just reached the 
limit at which flow can take place in the horizontal planes; then 
this load must necessarily be insufficient to produce flow on the 
inclined planes. Hence. if flow produces structure at all, such 
a mass will show structure in one direction and in one direction 
only. 
In the case of combined scission and shear illustrated in 
Fig. 5, the same principles apply. The fibers torming the 
wedge y are subjected to maximum tangential strain ten times as 
long as are the corresponding fibers in the wedge Rk. Hence 
those in the larger wedge offer greater effective resistance to flow 
than those in the smaller wedge and the pressure might be so 
adjusted as to render the mass cleavable only in the direction of r. 
The difference between X and vy exists whenever scission forms 
an element in the strain or whenever there is a couple acting at 
the point considered. 
At first sight this result seems almost too sweeping. It might 
seem to imply that double structure should be very rare which it 
certainly is not. This small difficulty is readily explained; for 
in any substance with a moderately large difference between the 
elastic limit and the ultimate strength, shear and scission may 
be so combined that flow without rupture will take place on both 
sets of planes though faster on one set than on the other. The 
very rare instances are those in which the two structures are 
equally well developed indicating pure shear." 
In my opinion then true slate, cleavable in directions so 
nearly parallel that no considerable divergence appears, is due to 
«The limits of this paper preclude the explanation of a variety of structures 
arising from minor modifications of dynamic conditions. Flow on one set of planes 
may be accompanied by sharp joints on the conjugate surfaces. Such are the master 
joints in slate. When the force is rapidly applied, or when the mass is very brittle or 
when the lateral support is insufficient, two sets of joints (each with its own spacing) 
may result. One such set of joints may be suppressed by the action of viscosity. 
When deformation in two planes at right angles to one another is considered, two 
