SUMMAEY. 377 



The question of the character of the contact surface, whether it is 

 a faulted 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-sec- 

 tion to scale shows immediately that while the dip of the lode is 40° or 

 more, the maximum slope of Mount Davidson is about 30°. This fact, 

 taken in connection with the character of the west wall where exposed, indi- 

 cates that the surface is the result of faulting. A natural surface sloping 

 for a long distance at an angle of above 40°, too, is very unusual. On the 

 other hand, the coincidence between the contours of the west wall and those 

 of the exposed surface has been notorious from the earliest days of mining 

 on the Lode, and it seems a less violent supposition that the steep flank of 

 the mountain passes over into the still steeper wall of the vein than that 

 the range has experienced an erosion modifying its angle from 10° to 20° 

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



Discussion of faulting under certain conditions. The mOSt Striking and wldc-SprCad 



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

 faces 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 as com- 

 pared with the sheet adjoining it on the west. The consideration of a 

 sheet or plate of rock under the influence of friction of a relatively oppo- 

 site character on its two faces, therefore, forms the natural starting point for 

 an examination of the observed conditions. It is shown in Chapter IV. that 

 if a country divided like the Comstock area into parallel sheets experiences 

 a dislocation on one of the partings under a compressive strain equal at 

 each parting, a vertical cross-section will show a surface line represented 

 by two logarithmic equations. The discussion is also extended to the case 

 in which the compressive strain is not uniform, but varies proportionally to 

 the distance from the fault-plane. This case also results in a logarithmic 

 equation of a more complex character. 



