106 GEOLOGICAL RECONNOISSANCE 
The depth at which this junction of chert and limestone takes place, is 
from 30 to 80 feet. The average depth of the shafts, before reaching the 
main deposits of ore, may be put down at 65 feet. After the shaft has 
reached the level of the “ Sheet mineral,” a barrier is often found, which 
has to be penetrated before reaching the heaviest beds of ore, known, 
technically, by the name of the “bar rocks;” this is usually from 15 to 30 
feet through; it seems to be composed of porous calcareous matter, in 
which some sulphuret of lead, sulphuret of zine, (black-jack), and bitter 
spar, (magnesian limestone), is disseminated. After this barrier is broken 
through, the miner reaches the “sheet mineral,” lying, not perfectly level, 
but waving somewhat with the irregular, corroded surface of the rock on 
which it has been deposited, and mixed, more or less, with a “ tallow 
clay,” either red or white, which is a tenaceous, unctuous clay, sometimes 
ferruginous, in certain states of dryness cuts like tallow or soap. There 
are, also, various minerals, either amorphous (i.e. without any regular 
geometrical forms), or crystallized; such as pearlspar, bitter spar, carbonate 
of lead, carbonate and silicate of zinc, sulphuret of zinc, with occasionally 
crystals of sulphate and phosphate of lead, disseminated with the calc-spar, 
the principal vein-stone accompanying the galena. The so-called “ black- 
jack rock,” (i.e. a rock in which sulphuret of zinc is largely disseminated), 
is considered a good indication of lead ore. 
The material passed through in sinking the shaft, is mostly white chert 
in displaced and confused masses. This chert is often light and porous— 
almost possessing the structure of pumice. 
Three tiers of sheet-ore have been successively passed through; that at 
an average depth of 65 feet from the “grass,” has proved, as yet, the 
most productive. Ore has been reached, however, within 10 and 20 feet 
of the surface. 
The horizontal sheets of lead ore vary in thickness from a fraction of 
an inch to 2 feet, and even, in some extraordinary instances, to 3 feet. 
The average thickness may be put down at 6 to 10 inches. They are 
often so rich that it is not uncommon, after a shaft has been fairly sunk to 
the level of the ore, for two men to raise from 1,000 to 2,000 pounds, and 
even sometimes 3,000 to 4,000 pounds, in six or eight hours. 30,000 to 
50,000 pounds have been raised out of the Hopkins mine by 20 men, each 
man averaging from 400 to 300 pounds a day. 
At the Frazer shaft, from a quarter acre lot, 10,000 pounds were taken 
out; and, from all the Frazer claims, up to the present time, comprising 
10 acres, 400,000 pounds of lead ore have been raised. The total amount 
of ore raised in the last two years is about 800,000 pounds. 
The ore, as has been said, lies mostly in horizontal spaces, conformable 
