Experiments in the cleavage-parallel compression of slate demonstrate important changes in the mode of deformation with increasing confining pressure. At pressures up to 3 kb shear fractures are dominant. In the range 3.5 to 7.24 kb the rock yields through the development of an intersecting network of narrow kink-bands. The transition is reflected in a marked change in the slope of the Mohr envelope at a confining pressure of 3 kb, indicating that different failure criteria are relevant to the two types of deformation, that also give stress-strain curves of different shape.
The geometrical relationship between shear fractures and kink-bands is described in detail. The shear fractures make a much smaller angle (mean value 30.7°) with the direction of maximum compression than do the kink-bands (mean value 55.4°).
At the highest confining pressures employed (about 7 kb), there is evidence of kink-band growth by lateral migration and separation of the bounding kink-planes. At all lower pressures the length of the rotating, kinked, segment of foliation is determined at inception.