B. Silliman on the Grass Valley District. 243 
after the real average amount is reached, while in fact it is neither 
greater nor less; but the metal is no longer available by com- 
mon methods of treatment. 
¢ Eureka mine.—From the date of its location, February 7, 
1857, to the close of 1858, the mine proved only a source of ex- 
pense to its owners, and its history is instructive as suggesting 
that shallow surface explorations, in gold mining, may be as un- 
satisfactory as they are known to be in other mining enterprises. 
Progressive increase of gold with an increase of depth. _ - 
rom October, 1865, to December 31, 1865, the quantity 
quartz crushed was twenty-four hundred and forty-five em 
Yielding an average of $33.87 per ton, and costing to mine an 
reduce $13.51 
From January 1st to June Ist, 1866, the crushing was forty- 
seven hundred ‘and three tons, averaging $42.68 per ton, at a 
Ushed was forty-two fourths 
‘ons, giving an aveinie yield of $60.33 per ton, at @ cost of 
$15.78 per ton. 
