138 ANNALS NEW YORK ACADEMY OF SCIENCES 



called the Superior fault. A second one of great size cuts across the- 

 ridge from the head of Silver Fork into the Alta basin. It is seen most 

 clearly on the ridge northeast of the Emma mine, where the fault breccia, 

 has weathered into relief, standing up like a great wall. This fault will 

 be described as the Silver Fork fault. In all of these movements, the 

 displacements are more in the vertical direction, lateral shifting beings 

 not so frequently met with. 



Superior fault. — The Superior fault as shown upon the map (Plate 

 VI) can be traced from the mouth of Superior Gulch in Little Cotton- 

 wood Canyon northward into South Fork. On the top of the ridge, it 

 is clearly marked by a wall of breccia which stands up ten feet above the 

 general level of the surface. The crushed zone marked by the breccia 

 may be followed northward for nearly a mile. In the upper tunnel of 

 the Cardiff mine, it is well shown for a distance of a thousand feet along 

 which the hanging wall is quartzite and the foot wall very hard limestone. 



From all indications in South Fork, where it was first encountered, it 

 may be explained as a normal fault with a throw of about a thousand 

 feet, but observations from the Alta side of the divide clearly show it to- 

 be a reverse fault of less magnitude, the displacement being about 600 

 feet. The limestones on the west are lifted. They belong to the lower 

 series exposed on the east wall of Superior Gulch and not to the lime- 

 stones of the Reade and Benson ridge as at first supposed. This was not 

 understood until the overthrusting which duplicated part of the series 

 was discovered at Alta. The limestones are all of the same age but they 

 occur in two series separated by nearly a thousand feet of older quartzite 

 belonging to the overthrust member. The faulting is clearly of later date 

 than the overthrusting. The under^anding of this relationship is of the 

 utmost importance to the mining people of South Fork, who have never 

 suspected the presence of a limestone series below the quartzites of the-. 

 Reade and Benson ridge. The cherty limestones forming the ridge south 

 of the Cardiff office and boarding house are the lifted, westward extension 

 of that lower series upon which the overthrust block rests. The relation 

 is clearly brought out in Section A- A, Plate VI (see also Plate III). 



The direction of this movement is more nearly vertical than horizontal 

 though the oblique flutings on the walls in the Cardiff tunnel indicate 

 an important horizontal component toward the north on the west side. 

 Surface evidence of faulting cannot be traced farther than the Cardiff 

 mine to the northward, though it is safe to assume that a movement so 

 pronounced at this last observation point must have continued for some 

 distance beyond. At a point about a mile and a quarter north of the 

 Cardiff, the bottom of South Fork is composed of limestone, and no- 



