Contents Page Table 9 1. Standard crushing strengths of rocks 116 2. Critical data of translation gliding 117 3. Critical data of twin-gliding 120 4. Tests of unconsolidated sand 122 5. Strength of rocks confined in steel jackets 122 6. Short-time compressive strength of unjacketed materials with confining pressure of kerosene 123 7. Resistance to shearing under high confining pressure 126 8. Creep of certain materials 129 9. Creep tests of wet alabaster at different stresses 129 Illustrations Page Figure 1. Tresca’s apparatus 110 2. Adams’ apparatus 111 3. Schematic diagram of Griggs’ high-pressure apparatus 112 4. Bridgman’s shearing apparatus 113 5. Karman’s stress-strain diagram of jacketed marble tested in compression under confining pressure 124 6. Böker’s stress-strain diagrams of jacketed marble tested in tension under confining pressure 125 7. Griggs’ stress-strain diagrams for unjacketed Solenhofen limestone 130 8. Creep curves of alabaster 130 In normal laboratory tests or in building applications most rocks are brittle rather than plastic and possess a well-defined strength which is independent of time or temperature within the usual limits of observations. This behavior has led to common acceptance of the concepts of “elastic limit” and “strength” of rocks, and these terms have been used frequently in reference to the behavior of rocks deep in the earth’s crust with the tacit assumption that such concepts apply under conditions of earth deformation without great change in form. On the other hand, geological observations provide abundant evidence that rocks have exhibited a high degree of plasticity in . . .