NORTH MOSQUITO AMPHITHEATEE. 125 



On the south wall of this raised basin under Mount Buckskin the white 

 granite disappears and is replaced by gneiss and hornblende schists, which 

 show a remarkably contorted structure. Running nearly parallel to this 

 wall, and forming its face in certain parts, is a dike, thirty to forty feet 

 wide, of mica-diorite. It projects out into the valley in the direction of 

 the Red amphitheater, but could not be traced on the east side. 



North Mosquito amphitheater. The Arcliean exposures at the head of the 

 north branch of Mosquito gulch may properly be mentioned here. They 

 consist of the same general character of rocks gneiss, schist, and granite. 

 On its north wall the irregular shading of the dark mass produced by the 

 white pegmatite veins is particularly prominent. The coarse-grained red 

 porphyritic granites are more common lower down the canon, while towards 

 the crest of the range the fine-grained, eruptive-looking granite is found, 

 and apparently extends through to the west side at the head of Bird's Eye 

 gulch. 



In the neighborhood of the little lake in this basin many dikes, often 

 of the interrupted form, were observed, the more important of which have 

 been indicated on the map. East of the lake, in the center of the amphi- 

 theater, is a dike of Mosquito porphyry, similar to the dike No. 2 on the south 

 face of Mount Lincoln, though of somewhat lighter color, owing to a differ- 

 ence in the mica, and containing more ore in small specks. The oxidation 

 of this ore gives a brown color to the weathered feldspars, which when fresh 

 have a faint pink color. Under the microscope the groundmass is seen to be 

 fully microcrystalline. The apatites are dusty. The only mica seems to 

 be muscovite, which, judging from the associated yellowish grains and rarer 

 needles, has come from biotite. Part of the ore seems to be magnetite, and 

 part is entirely decomposed. The quartz grains contain fluid inclusions. 

 This porphyry is cut in one place by a mica-porphyrite, which is a light- 

 colored rock, containing numerous feldspars in a dark-gray groundmass. 

 with some hexagonal plates of dark-brown biotite. Quartz, in quite large 

 grains, can be distinguished by close examination. Single grains of pyrite 

 are scattered through the rock. Striations are distinctly visible on many 

 feldspars. Under the microscope plagioclase is seen to largely predominate 

 and the biotite to be quite fresh. The groundmass, which is fully crys- 



