DEFORMATION OF ROCKS 465 
combination is ordinarily such as to give steeply inclined cleav- 
age cutting the bedding, and therefore a cross cleavage. There 
are, however, undoubted gradations between cross cleavage and 
parallel cleavage. In all cases in nature the final resultant or 
direction of movement depends upon a union of all the forces 
concerned. 
DEVELOPMENT OF FISSILITY IN HOMOGENEOUS ROCKS. 
If a rock be in such a position and under such conditions that 
it is deformed by regular fracture, it is probable that the second- 
ary structures form in the shearing planes. Just as in the case 
of cleavage, at any place ina rock-mass the three principal stresses 
are usually unequal. In the zone of fracture if the differential 
stress surpasses the ultimate strength of the rock, fracture occurs 
along the two sets of shearing planes which incline toward the 
greatest stresses and are parallel to the minimum stress. The 
planes of maximum shearing stress in the case of normal pres- 
sure are 45 from the greatest pressure and are at right angles to 
each other, but Hoskins shows that the fractures may incline at 
a smaller angle than this to the direction of greatest pressure, 
the latter bisecting the acute angle made by the intersecting 
ruptures. It is only in case the two lesser stresses are equal, 
or nearly so, that concoidal fractures are produced, such as occur 
in ordinary building-stone tests of cubes. In this experiment 
there is one direction of great stress and two directions at right 
angles to this of very subordinate equal stress. 
Hence, in the zone of fracture, where the differential stress 
surpasses the ultimate strength of the rock, there may be pro- 
duced a fissility in two sets of intersecting planes equally inclined 
to the greatest pressure. It is well known that a fissility in two 
directions occurs in many homogeneous rocks, and such structures 
have doubtless developed along the shearing planes under this 
law. 
In case the direction of greatest normal pressure is nearly 
horizontal, the planes of fissility would be at angles of about 
45° with the horizon. _ However, as no rocks are strictly homo- 
