56 RESOURCES OF CALIFORNIA. 
sumetimes pretty large. No satisfactory explination has been 
given of the fact that placer-gold is usually in particles so much 
larger than that found in quartz. 
Gold is fine and coarse mechanically—that is, in the size of 
its particles—and chemically in its composition. Most metals 
are found in ores, combined chemically with non-metallic sub-' 
stances which hide them. The ores have usually neither the 
color, the specific gravity, the strength, nor the other peculiar 
features of the metals. Native gold is never found as an ore; 
it is always in a metallic form. The reason of this is, that it 
does not rust on exposure to the air, nor is it dissolved by any 
of the simple acids. And yet it isnever found pure, but always 
mixed with silver, in nearly all possible proportions. Fre- 
quently copper and Jead are also found in native gold. The 
amount of other metal in gold is designated by figures of fine- 
ness, estimated according to thousandths. Perfectly pure gold 
is 1,000 fine; gold containing one-tenth of its weight in silver 
is 900 fine; that is, 900 parts in 1,000 are gold, and 100 are 
silver. In gold 600 fine, 400 parts in 1,000 are of other 
metal. 
The native gold in California varies in fineness from 500 to 
990, averaging about 880. One large piece, found at Downie- 
ville, was 992 fine. In Mariposa, Fresno, and Buena Vista 
counties, and at Mono Lake and Walker’s River, east of the 
Sierra Nevada, the fineness is very low. The gold of the Col- 
orado is very fine. In other districts there are great variations 
in the fineness within small distances. One has gold 900 fine; 
another, one hundred yards distant, has gold only 800 fine. 
Ordinarily, all the gold in a gully or in a river-bar is of the 
same fineness; so also all the gold in a quartz-lode is of the 
same fineness. But there are the same differences of fineness 
between the gold taken from different quartz-lodes as in that 
taken from different gullies. For these differences there is no 
satisfactory explanation. 
Let us now run through the list of the principal mining dis- 
tricts of the state, giving the fineness of the placer-gold of each: 
