20 KEMPS ORE DEPOSITS. 



narrow above a certain stratum as practically to fail. Quartzite 

 is a favorable rock for such effect. Despite all rules, faults are 

 often causes of great uncertainty, annoyance, and expensive ex- 

 ploration. 



1.02.10. If a number of faults succeed one another in a short 

 distance they are called " step faults." An older and completed 

 vein may also be faulted by one formed and filled later. In such 

 a case the continuous one is the younger. The figure below 

 will illustrate each case. At the intersection of the two, the later 

 vein is often richer than in other parts. 



1.02.11. If a faulted series of rocks is afterward tilted and 

 eroded, so as to expose a horizontal section across the strike of the 

 faulting plane, an apparent horizontal fault may result ; or if the 

 erosion succeeds normal faulting and lays bare two unconformable 

 beds each side of the fissure, a lack of correspondence in plan as, 



Cross V^fe= *^ VeTn C 



FIG. 5. Illustration of one vein faulting another at Newman Hill, near 

 Rico, Colo. After J. B. Parish, Proc. Colo. Sci. Soc., April 4, 1892. 



well as in section may be seen. Faulting fractures are seldom 

 straight ; on the contrary, they bend and corrugate. When the 

 walls slip past each other, they often stop with projection opposite 

 projection, and depression opposite depression. These irregularities 

 cause pinches and swells in the resulting cavity, and constitute one 

 of the commonest phenomena of veins. Fissures also gradually 

 pinch out at their extremities, or break up into various ramifica- 

 tions that finally entirely cease. They also pass into folds, as 

 stated above. 



1.02.12. /Secondary Modifications of Cavities. Fractures and 

 cavities of all sorts speedily become lines of subterranean drainage. 

 The dissolving power of water, and to a much smaller degree its 

 eroding power, serve to modify the walls very greatly. An en- 

 largement may result, and what was perhaps a small joint or fis- 

 sure may become a waterway of considerable size. This is 

 especially true in limestones, in which great caverns (like the 



