In compressive stress-strain tests on a coarse-grained marble at various confining pressures up to 1000 kg cm−2, special attention has been paid to the transition from brittle to ductile behavior. The application of low confining pressures suppresses the longitudinal fractures characteristic of failure at atmospheric pressure, and well-defined shear failures develop. The formation of the localized shear zones accompanies a gradual decrease in the stress needed for continued deformation but does not lead immediately to complete fracture. This behavior may be initiated by a type of plastic instability. Its relevance to geological faulting is discussed, since the shear zones developed in the marble specimens appear to be similar to geological faults.

At confining pressures above about 300 kg cm−2, the stress-strain curves rise continuously without reaching a maximum, and the deformation is distributed throughout the specimen instead of being localized in shear zones or faults. This suggests that there is a depth in the earth's crust below which faulting cannot be expected in this rock.

The stress-strain curves have been analyzed to show the effect of confining pressure on the yield stress and on the stress at the maximum load. A few measurements on a fine-grained limestone are also included in an appendix.

Coloration of the marble by X radiation occurs after deformation at low confining pressures, but the effect becomes more marked at higher confining pressures.

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