178 GEOLOGY OF TQE COMSTOCK LODE. 



tion of the trace of the fissure on the original surface concave toward the 

 lower country will be represented on the faulted surface by a ravine, and 

 each inflection convex toward the lower country will result on the faulted 

 surface in a ridge. 



Fig. 10 shows a contour map of the country shown in Fig. 8, the fissure 

 having reached the original flat surface of the earth on the undulating line 

 AB. 



It is evident that if the form of the trace were capable of exj^ression 

 by an algebraic equation, the equation of the faulted surface could be im- 

 mediately deduced, but such cases are not likel}' to occur, as deviations of 

 the trace from the right line are probably due to local variations in the 

 physical properties of the rock. Even when the original surface was irreg- 

 ular the same law holds, mutatis mutandis; for the locus of the point of 

 inflection of the double logarithmic curve will still be parallel to the trace. 

 The edges of the sheets on each side of the fault will be parallel to the 

 locus of the point of inflection, and where this is a contour they will also 

 be contours. 



It frequently happens that the dip-line of afissuie is straight and nearly 

 constant for long distances from the surface, while the strike varies. When 

 this is the case the intersection with the foot wall of a surface parallel to 

 the original surface at any depth below it will give the same line, and if the 

 locus of the point of inflection of the surface curve is a (Contour, the contour 

 of the foot wall of the fissure at any point will be identical with it and 

 with those of the altered surface, as far as the feulting action extends 

 unmodified. 



Fissures into the hanging wall. — Tlic diagrams showat a glaucc that when a 

 fault takes place under the conditions specified, the rock of the lower coun- 

 try near the fault, as seen in cross-section, assumes the form of a sharp 

 wedge, which is exposed to the same heavy pressure as the rock at greater 

 depths. In an actual case in nature, it is scarcely possible to suppose that 

 this wedge would remain intact. A very slight obstruction to the smooth 

 rise of the foot wall would produce a crack across this edge at some consid- 

 erable angle to the dip of the fissure, and such a crack might very probably 

 be held permanently open b)' fragments of rock. Fissures diverging into 



