17B GEOLOGY OF THE COM STOCK LODE. 



landslips are common during wet weather, often involving the movement 

 of only a few square rods of ground for a few feet. The material in this 

 case is far from rigid, but on the other hand it possesses a minimum of elas- 

 ticity. I have examined hundreds of such slips in the Contra Costa Hills 

 of California, and noted with surprise the fact that they are almost invaria- 

 bly accompained by a separation of the moving mass into sheets far more 

 regular than might have been expected, and parallel to the initial surface of 

 motion. 



It does not appear to me that the character of the curve assumed by 

 the edges of the sheets will be affected by the consumption of energy in- 

 volved in shearing them from the mass of country rock, for the work done 

 at each fi-acture will be the same and the effect will appear in the constants 

 of the equation, not in the form of the function. 



Frequency of compressive strains in faulting. DisloCatioUS of the Carth's SUrfaCC may 



no doubt occur under the most various dynamical conditions, and no gen- 

 eral law can be laid down as to the presence or absence of tangential press- 

 ure. It is evident, however, that the lateral extension of a faulted area is 

 increased by faulting whenever the hanging wall sinks or the foot wall rises. 

 If A is one-half of the total slip measured on the dip of the fissure, the in- 

 crease of horizontal distance between an}' two points on the logarithmic 

 surfaces of the rising and sinking countries respectively, so far removed 

 from the fault plane as to occupy positions which are sensibly on the asymp- 

 totes of the curves, will be 



2 A cos /?. 



It is evident that this increase in lateral extension will be accompanied 

 by lateral pressure and consequent friction, unless the fault is the result of 

 a tangential tensile strain. The general theoi-ies of dynamical geology, and 

 the study of sedimentar}' rocks, however, show that strains in the earth's 

 crust are commonly compressive. 



Surface produced when the fissure is a plane.— It luiS beeU sllOWn that Uudcr CCl'tain 



conditions the surface line of the cross-section at any point of a fatdted 

 country will be a logarithmic curve, or a combination of two logarithmic 

 curves. If therefore the fault fissure intersects the earth's plane surface on 



