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University of California Publications. 



[Geology 



badly broken appearance. As a structural feature, Little Valley 

 lies between these two crests. Tbese two ridge lines, with the 

 valley between, persists north to the limit of the map, where 

 volcanic flows have obscured the structure. AVest of Slide Moun- 

 tain is a locality not yet examined. It is also without the limits 

 of the east range of the Sierra, and is therefore not of primal 

 importance to this paper. The northwest corner is glaciated, 

 and moraines are mapped by Mr. Lindgren on the Truckee 

 quadrangle just west of the map limits. 



The Genoa Area. — The Genoa topographic area, lying south 

 of the Carson area and west of Carson Valley, is the simplest 

 portion of the region under discussion. As opposed to the two 

 and three ridge lines of the areas to the north, with their com- 

 plex faulting, the east-west profile shows but one crest. From 

 Carson Valley a few miles north of Genoa the mountain rises 

 over an unbroken and magnificent fault-scarp for nearly 4,500 

 feet to the summit of Genoa Peak. The summit of the range is 

 here a well-marked though dissected remnant of the older plateau 

 surface, the most conspicuous of all the many isolated areas. 

 (See pi. 26a). From this tableland the slopes lead in general 

 gently down to Lake Tahoe. The details of structure, as far as 

 made out at present, are not many. The fault-scarp above Car- 

 son Valley is broken, not longitudinally, as to the northward, 

 but transversely. There are two well-marked east-west scarps 

 north and south of Genoa Peak, situated in such a position that 

 the peak forms the apex of a four-sided pyramid, whose western 

 side and part of the north and south sides are missing. The 

 general effect is as if a wedge with the edge lying in an east-west 

 direction, had been forced a short distance to the east, by a 

 small motion of rotation about an axis perpendicular to the edge 

 of the wedge. The topographic map shows the lowest contours 

 near the valley are not displaced, but merely notched at the 

 bases of the east-west scarps. But, as greater elevation is gained 

 the main eastward-facing scarp becomes more and more dis- 

 placed toward the east. Genoa Peak is therefore to the east of 

 the actual crest line. The sharply notched canons that follow 

 these cross fault-lines are occupied by small wet-weather streams. 

 The question naturally arises, how much of the canon is due to 



