DOWN IN A MINE. 119 



XXI. DOWN IN 



MODES OF OCCURRENCE OF THE METALS. 



WHO has not heard of the Comstock Lode ? Who has 

 not read something about the hundreds of millions of gold 

 and silver extracted from its deep depositories ? What names 

 in the annals of mining enterprise are more familiar than 

 those of Gould and Curry, William M. Stewart, Adolf Sutro, 

 James G. Fair, J. W. Mackey, John P. Jones, William 

 Sharon ? What is the Comstock Lode ? It is a body of gold- 

 and-silver-bearing mineral matter lying in the Virginia range, 

 a spur of the Sierra Nevada, about 25 or 30 miles east of the 

 Sierra crest. The range trends a little east of north, and the 

 lode appears to be the filling of an imperfect fissure four or 

 five miles long the principal part of which is about 10,000 

 feet. The fissure extends into the mountain with an eastward 

 dip varying from 33 to 45. At the north end it divides 

 into three or more diverging and somewhat irregular branches, 

 and at the south end it terminates in two branches. The 

 fissure has, at its broadest part, along the middle, a width 

 of about 600 feet, which becomes 1,400 feet when measured 

 along the sloping surface ; and it narrows toward each end. 

 The lode also narrows downward, and at about 1,800 feet ver- 

 tically, has a thickness of about 120 feet. The part above 

 this seems to be formed from two fissures and the wedge- 

 shaped mass of "country rock" included between them. 

 This wedge of country rock was cut off from the east side, 

 where the rock is diabase that is, composed of grains of 

 augite and a plagioclase feldspar. On the west side, the 

 country rock is diorite that is composed of hornblende and a 

 plagioclase feldspar. In miners' language a fragment of 

 country rock included in a lode is a " horse." The fissure 

 along each side of this enormous " horse" is filled chiefly with 

 quartz. The east wall in this case, is the " hanging wall," 

 and the west, is the "foot-wall." The hanging wall is much 

 decomposed, and the decomposition extends through the dia- 

 base for five thousand feet. 



