Alluvial Gold Drifts . 
129 
as rest immediately upon and are made up of detritus derived 
from Silurian rocks, are certain to contain gold in the vicinity of 
where the latter are traversed by auriferous quartz-veins. 
It so happens that in the neighbourhood of most of our 
gold-fields conglomerates of Upper Palmozoic or Mesozoic age do 
not occur, or have not been tested, but there are places where it 
is reasonable to expect that the lowest beds of such conglomerates, 
where resting immediately on the Silurian bed-rock , might be found 
auriferous. Those localities will be mentioned hereafter, though to 
test them would bo a matter involving considerable outlay and only 
to be entered upon after ascertaining beyond a doubt the auriferous 
character of exposed Silurian rocks within reasonable distance. 
Among the tertiary gold drifts we can now identify those of at 
least four distinct periods. (1.) That of Miocene or Middle Tertiary 
age formed prior to the outpouring of tho Older Volcanic lava- 
flows. (2.) That classed as Older Pliocene, which in some places 
appears to be of marine or littoral deposit, and in others the result 
of fhiviatile action. (3.) The deep lead gravels occupying gutters 
or channels cut through, and therefore newer than the Older 
Pliocene gravels, but older than the Newer Volcanic lava-flows. 
(4.) Post Tertiary drifts and gravels, more recent than the newer 
basalt, and made up of materials derived from the denudation of 
some of the foregoing or from recently effected abrasion of the 
Silurian rocks. 
The different modes of occurrence of alluvial gold are so familiar 
to our miners that it seems almost superfluous to describe them, 
but it is as well to mention the principal classes of alluvial gold 
workings in order to elucidate the remarks on the geological action 
by which the gold was conveyed into tho different positions iu 
which it is found. 
1. Surfacing. — Earth or thin layers of clay, rubble, and decom- 
posed rock on the slopes or summits of hills composed of Silurian 
rocks. The gold is found free or associated with fragmentary 
quartz, from tho surface-earth down into the chinks and crevices 
of tho bed-rock. In some cases tho gold thus fouud has been de- 
rived directly from some line of quartz reef traversing the Silurian 
rocks, and is found to bo u ragged ” and unaltered in form; in 
other instances a gravel drift has at one time existed, and has 
been wholly or partially removed, and the water-worn gold once 
contained in it — primarily derived from the disintegration of quartz 
reefs — has been left behind, and has simply dropped deoper as the 
rock surface became abraded. Combinations of the two cases are 
frequent, and some of our reef--washes, or other auriferous alluvial 
deposits on slopes of Silurian rocks underneath basalt, represent 
the precisely similar conditions which existed in former ages and 
have been preserved from further change by the covering of 
volcanic rock. 
