

ORE-FORMATIONS 233 
character, consisting, in some places, of coarse gravel and 
shingle, or of finer gravel, grit, and sand. Most placers are 
of Recent and Pleistocene age, and are usually more or less 
unconsolidated. Many, however, occur in the Tertiary 
system, while a few date back to Mesozoic, and some even to 
Palzozoic times. These older deposits are, as a rule, con- 
solidated, forming coarse grits and conglomerates. The 
metals and ores of highly porous placers are usually con- 
centrated in the bottom layers. In the case of finer grained 
alluvia, however, they may be sparingly scattered through the 
whole thickness of the deposits. Should the bed-rock under- 
lying a placer be more or less fissured and shattered, the 
metal or ore not infrequently finds its way down for a few 
inches into cracks and crevices. While the gold occurring in 
quartz-veins, etc., is often intimately associated with metallic 
= Se -_— a oo | ee ————— > — 
Saini > ae a = ——— SST ee — - SaaS ES SSS 


— 
Uf TEL 

FIG. 83.—SECTION OF AURIFEROUS LEAD (OR PLACER) ON THE LOWER 
MURRAY, NEAR Corowa. (After E. F. Pittman.) 
sulphides, such as iron-pyrite, the gold met with in placers is 
usually in the free state. During the processes of disintegra- 
tion and denudation, the sulphides containing the gold are 
gradually dissolved, and the process of solution is carried on 
in the placer itself, so that sooner or later the gold becomes 
freed from its baser associates. The crystalline surfaces 
occasionally presented by placer-gold, and the usually smooth 
and unscratched appearance of the nuggets, are suggestive of 
chemical deposition. Many mining men, indeed, believe that 
nuggets grow by slow accretion. 
Placers being of fluviatile origin, it will be readily under- 
stood that such formations can seldom be of great geological 
antiquity. Terrestrial accumulations are only exceptionally 
preserved—the Mesozoic and Palzozoic systems consist for 
the most part of marine formations. The further back we 
trace the geological record, therefore, the scantier become all 

