FAULTING AND THRUSTING 297 



An open cut at the gypsum quarry exposes a very complicated fold. 

 It passes from the center of the top of the cut to the right (about 25 

 feet), then swings back in a low synclinal curve (some 35 feet), and 

 finally passes down vertically (10 or 12 feet) and disappears beneath 

 the floor of the cut. Furthermore, in this S-like fold the strata do 

 not follow straight lines or simply flowing curves, but are highly crenu- 

 lated and contorted. The details of the major fold show a great number 

 of different t} T pes of folds, including closed, overturned, and carinate 

 forms. Sheared forms are very rare and the amount of relative dis- 

 placement in such cases is very slight. This is probably due to the flexi- 

 bility of the gypsum, which makes up the greater part of the cut. There 

 are, however, a few thin layers of limestone interstratified with the gyp- 

 sum. 



Faulting. — Faulting is rather common in the bedrock complex. The 

 faults may be longitudinal or transverse to the range, normal or over- 

 thrust. The most common are the normal longitudinal faults. Such a 

 one is represented in the detailed section (plate 21) on the west side of 

 the Humboldt mountains, and will be more fully described later. 



A striking example was found in the gypsum area already referred 

 to. Looked at from the south, there appear to be two distinct beds of 

 gypsum strongly marked by their white color against the setting of dark 

 limestones and dull slates. There is, however, but one bed, which has 

 been duplicated in exposure by a north-south fault along the strike. 



This same gypsum bed is terminated at the south by a transverse 

 fault which strikes about north 60 degrees east. This fault determines 

 a canyon, the northwest side of which is made up largely of gypsum 

 capped by black limestone dipping north, while the opposite slope con- 

 tains no gypsum or black limestone, but is chiefly of slate, which dips 

 south. The southeast country has dropped relatively to the northwest 

 country not less than 200 feet, possibly much more. 



Thrusting. — Thrusting has already been referred to in connection with 

 folding, but one example will here be described a little more fully. 

 North of the main gypsum anticline and separated from it by about 

 three- tenths of a mile occurs a syncline of the same material, forming 

 with its covering of black limestone a canoe-shaped structure, the east 

 side of which is much steeper than the west. The south end has been 

 lifted, so that it is about 200 feet higher than where the anticline passes 

 down below the surface. The whole canoe is tilted along its axis, so 

 that from its southmost point it pitches north, the axial inclination be- 

 coming less from south to north. In other words, the synclinal fold has 

 been broken across the snout and thrust up over those rocks lying south 

 and east of its original position, at least several hundred yards horizon- 

 tally. 



XXXIX— Bull. Geol. Soc. Am., Vol. 15, 1903 



