220 GEOLOGY OF THE EUREKA DISTEICT. 



Although upon the siu-face this granite body is hmited in extent, there 

 is reason to suppose that it represents a much larger mass and has exerted a 

 considerable influence on the geological structure of the ridge. Prospect 

 Mountain quartzite, the oldest sedimentary beds in the District, surrounds 

 the granite xipon the northwest and northeast slopes and dips away from it 

 in u-regular broken masses. To the south the Prospect Mountain lune- 

 stones, which form the main ridge, occm- resting directly upon the granite. 

 That this exposure of granite along the steep slopes of the ravine is due to 

 the erosion of the crushed quartzite can not well be questioned, and the 

 ravine itself probably owes its existence to the fracturing of the quartzite 

 from some upward orographic movement of the granite. The farther away 

 the quartzite lies from the granitic center, the less distiu'bance is seen in 

 the beds. 



The age of the granite is vmknown. Quite possibly it formed a 

 portion of an Archean body around which the sands were deposited, 

 subsequent orographic movements disturbing and breaking up the sedi- 

 mentary beds. If the granite is later than the sedimentary beds it only 

 broke through the quartzite at the northern end of the range and failed to 

 cut the limestone. No intrusive dikes of granite are known penetrating 

 the overlying beds. At the time of the sinking of the Richmond shaft the 

 workmen found, at a depth of 1,200 feet from the surface, near the bottom 

 of the shaft, fragments of a decomposed crystalline rock. They were so 

 highly altered that the microscope failed to detect with certainty the nature 

 of the rock, though it carried quartz and mica and kaolinized feldspar. 

 This evidence, though slight, would indicate an off-shore deposit for the 

 quartzites. 



A peculiarity of this granite is the varied texture it offers over so 

 small an area, presenting, however, a true granitic habit. The essential 

 minerals are quartz, orthoclase, plagioclase, hornblende, and mica, the lat- 

 ter being very abundant. The quartz is grayish white in color, in irreg'ular, 

 pellucid grains. Hornblende is apparently more abundant in the fine than 

 in the coarse grained varieties. 



Quartz-porphyry.— Only two Small exposures of quartz-porphyry have been 

 observed, both occurring on Mineral Point, north of Adams Hill, and, 



