TERTIARY ROCKS 511 



crystals of the augite and feldspar. In mineral composition this rock is 

 closely allied to samples I, III, and VII. 



East of the Bonanza mine there is an area of "greenstone" of irregular 

 outline, but apparently about 2.5 miles long and nine-tenths of a mile in 

 maximum width; that is to say, within this area all the exposures of 

 partly decomposed bedrock along the North Fork of the Tunkey River, 

 on Eibabaones Creek, and near the Morning Star mine, in the valley of 

 Tunkey Ben Creek, all the residual boulders seen on two high ridges 

 and all the soil seen throughout it indicate that it is composed exclu- 

 sively of a single mass of the hard dark green crystalline. It is bordered 

 on the northwest by an acid intrusive and on all other sides by the ordi- 

 nary extrusive andesite. As it has a range of altitude of 1,000 or 1,100 

 feet and its contacts are apparently very steep, if not vertical, it has the 

 field relations of a small batholith, though the appearance of the rock 

 under the microscope suggests that it is not plutonic. The Venus vein 

 is partly in this area. A specimen from a boulder at the forks of the 

 creek on the Venus trail and which is identical in character with many 

 residuals in the area was identified by Lawson as augite andesite and 

 described as follows: 



V. This rock is very similar to II, except that the rock has a coarser grain, 

 and the augite occurs in larger crystals. The rock is holo-crystalline, and is 

 composed chiefly of feldspars. The augite occurs as fairly large phenocrysts 

 up to one-eighth of an inch long, and also as small flakes in the ground-mass. 

 Alteration has affected the augite, as on the edge and in the center chlorite 

 occurs. Secondary fibrous green hornblende occurs rather abnndantly in the 

 ground-mass. The feldspar is labradorite, and occurs in large tabular crys- 

 tals. As the rock shows an attempt at a porphyritic structure, it is probably 

 not plutonic. 



The Lone Star vein is situated in a "greenstone" area whose main body 

 is about 1.3 miles long and 1 mile wide. The rock, in a partially de- 

 composed condition, is extensively exposed in the Lone Star mine work- 

 ings, and elsewhere is indicated by residual boulders, stream gravel, and 

 the character of the soil. Topographically, it stands up well above the 

 neighboring rocks and includes Lone Star Hill, about 700 feet high. A 

 narrow arm of the "greenstone" forms a low ridge across the valley of 

 the !N"orth Fork of the Tunkey Eiver and has produced the Bonanza 

 Falls, 230 feet high. At the main fall the rock is well exposed and ap- 

 pears massive except for a little jointing. A specimen from near the 

 foot of the fall was identified by Lawson as augite andesite and described 

 as follows: 



