STRUCTURAL RESULTS OF FAULTING. 157 



Evidence of faulting. — Tlie evitleiice of faulting is manifold. The irreoular 

 openings of the vein, the presence of horses, the crushed condition of the 

 quartz in many parts, the presence of slickensides and of rolled pcdjl)les in 

 the clays, are all conclusive on this point. Both to the east and west of the 

 vein, too, the country rock shows a rude division into sheets, and along the 

 partings between the plates evidences of movement are perceptible, decreas- 

 ing in amount as the distance from the vein increases, according to some 

 law not directly inferable. All the evidence points to a relative downward 

 movement of the hanging wall. 



The ([ue.stion of the character of the west wall, whether it is a faidted 

 surface or a continuation of a former exposure of the east front of Mount 

 Davidson, is not to be settled by mere inspection. A cross-section, to scale, 

 taken from Mr. King's maps, shows immediately thai while the dip of the 

 lode is 45° or more, the maximum slope of Mount Davidson is about ;jO^. 

 This fact, taken in connection with the character of the west wall where 

 exposed, indicates that the surface is a result of faulting. A natural surface, 

 too, sloping for a long distance, at an angle of about 45°, is very unusual. 

 On the other hand the coincidence between the contours of the west wall 

 and those of the exposed surface has been recognized from the earliest days 

 of mining on the Lode, and it seems a less violent supposition that the steei) 

 face of the mountain passes over into the still steeper wall of the vein, than 

 that the range has experienced an erosion modifying its angle 1.0° and more, 

 and has still retained the details of its topography otherwise unaltered. 



It is plain that the elucidation of the faulting action on the Comstock 

 is a very important structural problem, and that it is most desirable to 

 account quantitatively for the results as well as to prove the existence of a 

 notable dislocation, and no apology is therefore required for presenting to 

 geologists a somewhat detailed discussion of the principles involved. 



Action offriction on the surfaces of a single plate. Thc UlOSt StHking and wldeSprCad 



evidence of the faulting is the apparent relative movement on the contact 

 surfaces between more or less regular sheets of the east and west country 

 rocks for a long distance in both directions from the Lode. Each sheet 

 appears to have risen relatively to its eastern neighbor, and to have sunk 



