THE HORSESHOE. 161 



the microscope there seems to be no fresh feldspar substance left in the 

 rock, although outlines of former crystals can often be plainly distinguished, 

 the interior being replaced by a mixture of calcite and a cryptocrystalline 

 substance, colorless in ordinary light, showing the alternations of light and 

 dark points characteristic of a homogeneous aggregation of minute particles, 

 probably quartz. The biotite leaves, both large and small, seem perfectly 

 fresh and in remarkable contrast to the condition of the feldspar. From the 

 great quantity of calcite present and the absence of muscovite or kaolin, it 

 seems evident that the feldspar was a plagioclase rich in lime, and the rock 

 a quartz-biotite-porphj'rite, although in external appearance it is quite unlike 

 any porphyrite observed elsewhere in the region. 



The larger amphitheater at the head of the south foi'k of Horseshoe 

 Creek has a less striking and regular form tlian the Horseshoe itself, but 

 presents the same geological structure. From the crest of the range at its 

 head, however, the Blue Limestone has been eroded off, and Silurian beds 

 form the surface. These are succeeded, as one goes south along the crest 

 to the head of Twelve-Mile amphitheater, by the Cambrian and Archean 

 successively^ 



South wall of Horseshoe gulch. — Oil the ridge ruiiuing from the crest of the 

 range to Sheep Mountain, along the south side of Horseshoe gulch, an excel- 

 lent continuous series of beds from the Archean up to near the top of the 

 Weber Grits are shown. The north side of this ridge is most admirably 

 delineated by a line sketch from the skillful hand of Mr. W. H. Holmes in 

 the Hayden report for 1873.' 



The same series of beds are here represented as were shown on the 

 ridge north of the gulch, but they occupy nearly double tlie space in lineal 

 extent along the side of the gulch; their angle of dip is consequently 

 shallower, and midway in the series is a small synclinal fold which enables 

 the same beds to cover a greater surface. The direct connection between 

 the two sides is obscured by ihe detrital material in the gulch. It is evident, 

 however, that the existence of a cross-fault is necessary to explain this dis- 

 crepancy, since there is no evidence that the beds of the south ridge curve 

 round to the east to join those on tlie north, their strike being the normal 



'Page 230, Geological and Geograi>Uical Survey of the Territories. Washington, 1874. 

 MON XII 11 



