Alluvial gold deposits are characterized by the greatest gold accumulations in the gravels or earthy deposits of long disused river beds and streams, and in places which, from their extent and other indications, appear to have been the beds of seas or lakes. The sizes and forms in which gold is found are very variable. It is found as dust, nuggets, thin plates, or flakes. Gold prospectors apply the word nugget in a very general sense; any piece of gold weighing above three or five grams being termed a nugget, while the same name is given to all large masses found. The individual weight of many of them has exceeded 45 kg and in some instances over 60 kg in weight has been reached by single pieces.
Gold dust covers the infinite variety of sizes from particles so finely divided as to be held in suspension by water for considerable periods of time, up to the size known as nuggets. There is, however, a variation in name sometimes used, such as shotty gold for granular pieces like the smaller sizes of shot. Often a rudimentary form of crystal may be traced in some derivative from the cube; but though pieces are generally water-worn, many perfect crystals have been found. It may be assumed that crystalline forms are much more common amongst the finer sizes, than in large pieces; microscopic examination frequently reveals this to be the case. The precise depths from the surface at which the metal is found, varies, by reason of denudation and other causes, in an almost indefinite manner.
In a general sense the richer deposits are obtained in the last few centimeters of material covering the bedrock, but some of the most remarkable finds yet made have been on or within a few centimeters from the surface. In the beds of rivers and gorges gold deposits have been found in successive layers or bottoms, one above the other, and when so occurring, though generally divided by alluvium, cases have been known where conglomerate rocks, and even beds of basalt or other igneous rocks, have intervened between the deposits of gold; such occurrences have given rise to great argument amongst gold prospectors as to the difference between true and false bottoms. When deep sinkings are made to reach alluvial deposits, they are known as deep leads. Sometimes when these are put down or tunnels driven, it is found that the physical features of the country have been entirely altered, and that the ancient river bed, having in it a stratum of gold, has been covered by accumulations so as to become a hill or watershed, consequently the modern river finds quite another channel, frequently far removed.
The positions in which large nuggets are found vary in the most extraordinary manner. Some have been picked up amongst the grass roots, others in the roots of uprooted trees, many have been found surrounded by soft soil far above the bedrock; in fact, frequently, the first indication a miner has of one is either striking it with his pick or seeing it shining amongst the mass of surrounding earth or debris. The best example is the largest nugget discovered by two miners in Near Dunnolly, Victoria, a mass of gold weighing 73 kg, bearing the mark of the pick.
Gold was also found as dust in the sea sands on the coast of California, New Zealand, and other places, and in the detritus of many rivers. Some of these deposits have been of great value, but many present little or no permanence; for that which one day is visible and workable may the next be covered deep with sand or carried by currents to other localities. Alluvial gold has generally a smaller proportion of silver and other metals alloyed with it than gold obtained in rocks and veins. The minds of chemical geologists are still exercised in endeavoring to account for this peculiarity.