444 CIO CLE J, IGITCOISITIE 
rotational strains in which scission forms a component. These 
strains are due to external pressures, always inclined to the nor- 
mal to the plane of cleavage, and the reason why only one set 
of cleavages makes its appearance in slate is that viscous resist- 
ance in the conceivable second set has prevented flow. 
It appears to me possible to avoid this conclusion only by 
denying either that rocks undergo solid flow or that flow produces 
cleavage. No geologist will think of denying that rocks flow. 
The evidences of it are too numerous to be worth mentioning. 
The mechanical conditions are also well understood. Solid flow 
without rupture will hardly take place unless there is some lateral 
obstruction to deformation as well as external pressure. The 
effect of the lateral confinement is to convert part of the pressure 
into more cubical compression; consequently the forces produc- 
ing shears and scissions rise very slowly with increase of external 
pressure. Under such conditions the deforming stresses may 
for a long time be kept close to the elastic limit and an infinite 
amount of flow may be produced in any substance not ideally 
brittle. The conditions appropriate to flow must be more preva- 
lent at great depths then at small ones, but they cannot be 
confined to great depths. 
That flow really produces cleavage seems to me demonstrated 
by experiments on solids such as iron. There is evidence that 
red-hot bar iron or steel is a true solid, and it is known that 
manufactured bar ironis fibrous and cleavable. This is especially 
well brought out in experiments on iron plates with high explo- 
sives. Evenif hot iron were no true solid, the way in which 
conjugate systems of cleavages or joints may appear in each, and any one of these 
four systems may be suppressed by the action of viscosity." One of the shears may 
fail to act on account of lateral resistance, thus a rock may show 4, 3, 2 or I sets of 
structures due to the same force. 
The spacing of fissures is an interesting topic very important in mining districts. 
My theory is that fissures will so form as to afford the greatest relief by pressure per 
square foot of rupture. “This leads to a definite distribution of faults in homogeneously 
strained rock. 
Tension will not produce joints or cleavages. The theory of the distribution of 
tension cracks is the explanation of columnar structure. 
