GEMINI PEAKS. 149 



and the lines of outcrop can be traced with the greatest distinctness. In 

 tho distant view of the whole range from Mount Silverheels, as shown in 

 Plate III, these structure lines, as well as the curves of Sacramento arch 

 and of Sheep Mountain fold, can be readily recognized. Immediately west 

 of the fault the beds are perpendicular, and even bend over so that they 

 have a slight inclination to the westward. The change from this steep dip 

 to the average inclination of the whole hill seems to be less sudden than on 

 Pennsylvania Hill; but, as there, it is somewhat obscured. 



On the south side of the ridge facing Little Sacramento Valley is a 

 slight synclinal fold, no evidence of which is found on the north face of the 

 ridge. An explanation of this occurrence may be found in the fact that 

 the line of fault from the Sacramento arch southward apparently diverges 

 to the eastward, as compared with the strike of the beds, so that more space 

 is left between the fault plane on the east and the unyielding masses of 

 porphyry which form the crest of the ridge to the west. 



Gemini Peaks. In the long series of outcrops on the eastern slopes of 

 the Gemini Peaks, which comprise almost the entire thickness of the Weber 

 Grits formation, are some minor sheets of porphyry which have not been 

 indicated on the map. The two peaks themselves form the crest of an 

 immense body of Sacramento Porphyry which is exposed under the Weber 

 Grits, both on the north and south walls, in apparent conformity with the 

 overlying sandstones. The thickness shown, as derived from the angle of 

 the slope, must be about 1,200 feet. The north branch of Little Sacra- 

 mento Creek has cut to a great depth into this immense body of porphyry, 

 leaving on either side walls nearly 1,000 feet in height, in which the same 

 columnar structure in large masses or prevalence of vertical cleavage planes 

 is found that has been already noticed in the porphyiy mass on the summit 

 of Mount Lincoln. It evidently represents what was originally a huge lac- 

 colite, and it is probable that it stands above the original vent from which 

 the main flows of Sacramento Porphyry spread out into the adjoining rocks. 

 Immediately to the south and west of this is the main body of White Por- 

 phyiy, which forms the mass of White Ridge and of Mount Sherman. The 

 junction of these two great bodies is extremely interesting, and was ex- 

 pected to afford definite evidence of the relative age of the two rocks. The 



