In placer gold deposits, the metal is found imbedded in layers of earthy matter, such as clay, sand and gravel; in the latter it is encased in veins of rock. The methods of mining must be adapted to the size of the particles of gold, and the nature of the material in which they are found. In placer mining, the earthy matter containing the gold, called the "pay-dirt," is washed in water, which dissolves the clay and carries it off in solution, and the current sweeps away the sand, gravel and stones, while the gold, by reason of the higher specific gravity, remains in the channel or is caught. Placer deposits have been the earliest sources of natural gold throughout the world and since 1848, the date of the first important find in California, it is estimated that historically, about 60 percent of the total output of California has been produced by the different forms of alluvial mining in this State. Gold mining was at first only carried on in the beds of rivers and ravines or along their banks, where coarse gold was found, with comparatively little labor or expense in the extraction of it. However, as the easy gravels were depleted, the miners moved from one technology to the next in their quest to obtain placer gold. Placer mines are divided into many classifications. The first and most important is into deep and shallow. In the former the pay-dirt is found deep, twenty feet or more beneath the surface ; in the latter, near the surface.
Historic Placer Gold Mining Technology:
1.
The Gold Pan
2. The Rocker or Cradle
3.
The Sluice Box, Part I
4.
The Sluice Box, Part II
5. Dry and Underground Placer Mining Methods
6. River Mining and Ground Sluicing – Part I
7. River Mining and Ground Sluicing – Part II
8. Hydraulic Mining – Part I
9. Hydraulic Mining – Part II
10.
Bucket Line Dredging
The shallow or surface diggings are chiefly found in the beds of ravines and gullies, in the bars of rivers, and in shallow flats ; the deep diggings are in hills and deep flats. The pay-dirt is usually covered by layers of barren dirt, which is sometimes washed, and sometimes left undisturbed, while the pay-dirt is taken out from beneath it through tunnels or shafts. So far as our present information goes, we have reason to believe that no gold country ever possessed so large an extent of paying placer mines, with the pay-dirt so near the surface, and with so many facilities for working them as California. In Australia the diggings are very deep and spotted, that is, the gold is unevenly distributed, and the supply of water for mining is scanty. In Siberia the winter is terribly cold during six months of the year. In Brazil the diggings were not so extensive nor so rich as in this state. Here we have numerous large streams coming down through the mining districts, very large bodies of pay-dirt, and a mild climate. After dividing placers into deep and shallow, the next classification will be according to their topographical position, as into hill, flat, bench, bar, river-bed, ancient river-bed, and gulch mines. Hill diggings are those where the pay-dirt is in or under a hill. Flat diggings are in a flat. Bench diggings are in a " bench " or narrow table on the side of a hill above a river. Benches of this kind are not uncommon in California, and they often indicate the place where the stream ran in some very remote age. Bars are low collections of sand and gravel at the side of a river and above its surface at low water. River-bed claims are thos beneath the surface of the river at low water, and access is obtained to them only by removing the water from the bed by flumes or ditches.
Ancient river-bed claims are those of which the gold was deposited by streams in places where no streams now exist. Gulch claims are those in gullies which have no water, save during a small part of the year. A "claim" is the mining land owned or held by one man or a company. The placer mines are again classified according to the manner in which, or the instruments with which they are wrought. There are sluice claims, hydraulic claims, tunnel claims, dry washing, dry digging, and knife claims. In the California gold rush of 1849 and 1850, the main classification of the placers was into wet diggings and dry diggings, the former meaning mines in the bars and beds of rivers, and dry diggings were those in gullies and flats where water could be obtained only part of the year or not at all. That classification was made while nearly all the mining was done near the surface, before the great deposits of pay-dirt in the hills had been discovered, and before ditches, sluices, and the hydraulic process had been introduced. The class of mines then known as the "dry diggings," and which for several years furnished nearly half of the gold yield of the state, are now, with a few unimportant exceptions, exhausted, or were left to the attention of the Chinamen. The purpose of all placer miners is not to catch all the gold in the dirt which they wash, but to catch the greatest possible quantity within a given time. It is not supposed that any process used in gold mining catches all the metal. Part of it is lost; in some processes a considerable proportion. The general estimate in California is, that one-twentieth of the gold in the dirt which is washed is lost. Many of the particles are .so very small as to be invisible to the naked eye, and so light that their specific gravity does not avail to prevent them from being carried away by the water like sand.
The larger pieces will sink to the bottom and resist the force of the water; the smaller the particles, the greater the danger that it will be borne away. Many devices have been tried to catch all the gold, but none have succeeded perfectly, and some which have caught a portion of what escaped from the ordinary modes of mining, have been found to cost more than their yield. The miner does not grieve about that which he cannot catch. He is not careful to catch all that he could. His purpose is to draw the largest possible revenue per day from his claim. He does not intend to spend many years in mining, or if he does, he has become thriftless and improvident. In either case, he wishes to derive the utmost immediate profit from his mine. If his claim contain a dollar to the ton, and he can save five dollars by slowly washing only six tons in a day, while he might make ten dollars by rapidly washing fifteen tons in a day, he will prefer the latter result, though he will loose twice as much of the precious metal by the fast as by the slow mode of working. The object of the miner is the practical dispatch of work, and his success will depend to a great extent upon the amount of dirt which he can wash within a given space of time. He regrets that any of the gold should be wasted, but his regret is because it escapes from his sluice and his pocket, rather than because it is lost to industry and commerce.
The various methods for the recovery of placer gold even today depend largely on the high specific gravity of that metal. A particle of gold of a certain size is so much heavier than a particle of associated rock of the same or even larger size, that a current of water washes the particles of dirt and rock away and leaves the gold behind. At Sutter’s sawmill, water turned through the ditch to operate the water wheel washed away the dirt and gravel and left the bright flakes of gold. These were caught on the rough bottom of one of the ditches where James Marshall found gold. Over the years, many different methods have been used to capture the gold. The successive steps in placer mining were the miner's gold pan, the cradle or rocker, the long torn, the riffle-box or sluice box, the ground-sluice, booming or gouging, river mining, drift mining, hydraulic mining, the hydraulic elevator, and dredging. The pan, rocker, and long tom are very well known but a word of comment is still worthwhile to explain them.
