710 IDAHO MINING DISTRICTS. 



the Easter mine, as well as in the bottom of the creek near the Silver 

 Spray claim. 



A still more basic facies occurs at Horseshoe Bend on both sides of 

 the river, and extends for a distance of 3 miles down along the can- 

 yon, bordering on the south against the porphyritic dike subsequently 

 described and on the north against the dioritic granite, with very indis- 

 tinct and ill-defined outline. This rock varies much in appearance, 

 from dark-gray, medium-grained, or slightly porphyritic to coarse- 

 granular, the latter consisting apparently of white feldspar and rather 

 abundant green. hornblende. The peculiarity of these dark, granular 

 rocks is that besides being poor in quartz they also carry augite, and 

 may be designated pyroxene-diorites. The rock at the Horseshoe 

 Bend bridge carries imperfectly idiomorphic augite and a little 

 hypersthene, the crystals being of a maximum length of l mm , but 

 usually less. There is also a little biotite. These three constituents 

 are embedded in a clear feldspar mass, which consists of short prisms 

 of labradorite, with zonal extinction, cemented by smaller anhedral 

 grains of probably the same feldspar. The structure in this, as in 

 other specimens, is hypidiomorphic granular. The coarse-granular 

 rock three-fourths mile west of Horseshoe Bend consists of white 

 feldspar, green hornblende, and a smaller amount of biotite in large 

 sheets. The microscope shows a few large, irregular grains of ortho- 

 clase, in which are embedded smaller prisms of labradorite. In this 

 specimen there is a little quartz between the large feldspars. Most 

 of the feldspar grains doubtless consist of labradorite. The augite 

 is largely converted into hornblende, and but little of the original 

 mineral remains. 



DIKE ROCKS. 



A series of dikes extends diagonally across the districts. Begin- 

 ning as narrow dikes near the Horseshoe Bend post-office, these rocks 

 extend in a widening belt over toward Rock Creek, there attaining 

 their maximum development at the crossing of Rock Creek. The 

 belt is here fully one-fourth mile wide, and practically forms one 

 dike, though with smaller included masses of diorite. From Rock 

 Creek this same dike extends up toward Crown Point Hill, but gradu- 

 ally contracts and ends before reaching the summit. Scattering dikes 

 are, however, found on that hill, and appear to continue from there 

 in a southwesterly direction, the last prominent dike appearing near 

 the Dynamite claim at Pearl. 



The dike rocks are of somewhat varying character. Pegmatitic dikes 

 hardly occur at all, and light-colored granite-porphyry, so common 

 elsewhere, is not abundant here. The usual dike rock is a coarse 

 diorite-porphyrite. The rock occurring on the river hill between 

 Horseshoe Bend and Rock Creek is prominently porphyritic by large 

 white feldspar crystals, up to l cm long, and by smaller crystals of 



