60 THE AURIFEROUS GRAVELS OF THE SIERRA NEVADA. 
by the aid of which the gold is separated from the gravel are, in general, 
quite simple, as will have been gathered from the very brief notice given of 
them above. The sluice seems to have been introduced about the year 
1851. 
We may now endeavor to trace the course of the discoveries and develop- 
ments which have taken place in California with reference to the way in 
which the auriferous gravels occur. Each one of the rivers flowing down the 
slope of the Sierra Nevada, within the limits of the auriferous belt, may be 
considered as a kind of natural sluice, the edges of the upturned slate rocks 
and the cavities resulting from their irregular decomposition or erosion havy- 
ing acted most efficiently the part of riffles. If we conceive of this region as 
once having been covered with auriferous débris, and notice that it is now 
traversed by streams running in deep gorges, with a very rapid descent, it 
will be easy to see that we have here all the requisities, furnished by natural 
causes, for concentrating a considerable portion of the gold within compara- 
tively narrow limits, so that it may be easily got at. Hence it will not be 
surprising to learn that during the first two years of the Californian gold 
excitement, when perhaps 50,000 miners were at work on the flanks of the 
Sierra, at least nine tenths of them were engaged in what was called “ river 
mining.” By this term is meant washing the accumulations of sand and 
gravel along the channels of the present rivers, or in their immediate vi- 
cinity. This was done either by attacking the “bars,” as the sand and 
gravel banks along the river are called, during the season of low water, or 
by “fluming” the streams, — that is, taking up the water in wooden flumes 
constructed along the banks for some distance so as to leave the bed of the 
river dry and accessible at all points. Such localities on the principal streams 
in the heart of the gold region were often found exceedingly rich, and they 
have all been washed over and over; in some cases a dozen times, until they 
have become almost entirely exhausted, so as to be no longer worth working, 
except as here and there a party of Chinese miners, content with very mod- 
erate gains, may find it worth while to attack them. Along the sides of the 
canons, in rich districts, the crevices between the projecting edges of the 
slates, in the ravines or so-called “ gulches” leading down to the rivers, were, 
during the early years of gold mining, scraped out with knives, and the detri- 
tus thus obtained panned out, often yielding large returns. From the spring 
of 1848 to 1851 nearly all the mining was of the character thus indicated, 
that in the river beds being called “wet diggings,” and that in the ravines 
