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GEOLOGY OK TONOPAH MINING DISTRICT, NEVADA. 



rectilinear boundaries of the Siebert tuffs. The position of one such fault, marked 

 A on the accompanying diagram (PI. XII), was determined by stratigraphic 

 evidence for a part of its course, as will be noted by consulting the geologic 

 map (PI. XI). Eastward of this part, however, it is bordered apparently on both 

 sides by the tuff, yet along the continuation of the line established by strati- 

 graphic evidence there is on its north side a slight scarp about 10 feet high. 

 Just north of this scarp a similar scarp, of about the same height, and, like 

 the former one, facing to the south, runs in a straight line, but in a direction 

 more nearly east and west than the one first mentioned. Toward the east the 

 foot of this scarp is in the bottom of a narrow depression; toward the west, 

 where the depression broadens, the scarp lies on the north side. In this broader 

 portion, however, the other side of the depression has little or no scarp, is at a 

 maximum of 3 or 4 feet in height, varying from that to nothing, and has no 

 straight or rectilinear course (fig. 11). This first-mentioned scarp is continued 



FIG. 11. Cross section of water runway, usually dry (c-d of PI. XII), showing bold, straight scarp on left, believed to be 

 consequent on faulting, and low, curved bank on right, believed to be due to occasional drainage. 



farther west, but is set off en echelon, although the corners are slightly rounded; 

 the set-offs are always in a northerly direction and the main trend corresponds to 

 that of the straight scarp farther east. With a slight interruption, caused by the 

 incoming of a depression which is probably due to an unusually soft fault block, 

 this scarp continues northwestward beyond the area mapped, and can be followed 

 with the eye a considerable distance farther, toward the little eminence called 

 Table Mountain. A sighted line along the scarp near the western limit of the 

 map has a general direction of N. 65 W. On examination, however, the front 

 of the scarp, which has a uniform height of 10 or 15 feet, and which always 

 faces the south, is found to be continuously set off en Echelon in the same sense 

 and fashion as the portion farther east. The conditions are indicated in PI. XII. 

 The two chief alternating directions of the scarp faces are, (1) chief, N. 85 E., 

 (2) minor (set-offs), N. 45 W. Along the whole of its course the relative depres- 

 sion to the south of the scarp is used as a runway for the occasional surface 

 waters, and can easily be mistaken for a depression due simply to erosion. 

 However, the south side of this depression does not partake at any point of the 

 peculiarities of the north side, being low and irregular in course, and without 



