340 A. C. LAWSON ISOSTATIC COMPENSATION CAUSE OF THRUSTING 



In nature the pressures are very commonly such that, when movement 

 occurs on a rupture plane, the limit of abrasion is exceeded, as is shown 

 by the prevalence of gouges and fault breccias. The frictional resistance 

 to movement is not the same when the opposed surfaces are dry as when 

 they are wet, nor is it the same when the surfaces are polished as when 

 they are rough. I have found experimentally that when polished granite 

 is pulled horizontally over polished granite with a bed of fine dry sand 

 between the two surfaces, the figure obtained for the coefficient of fric- 

 tion is twice as great as when the polished surfaces are in direct contact; 

 that when the sand is damp the value is two and a half times the value 

 found when no sand is used, and that when the sand is saturated with 

 water the value is three times as great. These results indicate that when 

 the pressure is sufficient to develop a fault breccia the frictional resist- 

 ance is thereby increased. When, however, a fault breccia is pulverized 

 to a gouge and then saturated with water, the gouge may, perhaps, act as 

 a lubricant. Gouge, moreover, may be developed directly by abrasion 

 without the intervention of the condition of coarse breccia; but move- 

 ment on an initial rupture is antecedent to the development of gouge, 

 and this early movement is, therefore, not promoted by lubrication. 



In the case presented I have postulated an existent rupture without 

 qualification as to the presence or absence of a gouge lubricant and with- 

 out consideration of its mode of origination. Such a rupture, however, 

 could only occur in the earth's crust as an expression of the relief of 

 strain, which had accumulated to the limit determined by the strength 

 of the rocks. The stress inducing this strain may have been applied to 

 the prism above the consequent rupture plane, in the direction toward 

 its emergence, or to the region below that plane in the opposite direction. 

 In the case before us consideration is limited to the first of these two 

 possibilities. In such a case it is presumable that the stress in the super- 

 ficial prism of the crust, having just exceeded the strength of the rocks 

 at the surface and initiated a rupture there, would immediately be rela- 

 tively intensified in the still unruptured region, so that the rupture would 

 be propagated rapidly back and down until it was partially relieved in 

 the Avhole of the prism affected. The residual stress would be measured 

 by the frictional resistance on the rupture plane thus established. It is 

 thus conceivable and probable that an intact prism might be detached 

 from the crust on its lower side ; but with the extension of the rupture 

 plane frictional resistance would increase till it balanced the motive stress. 

 In the development of such a rupture the amount of movement would be 

 slight, being limited to that necessary for the first culmination of strain. 

 This movement would be greatest at the surface and would diminish to 



