In searching for placers it is necessary to bear in mind that the watercourses of the country have not always flowed in the channels they now occupy. During the long periods of geological time many and vast changes have taken place in the contour of the earth's surface. Hence it is not an uncommon circumstance to find beds of auriferous gravel occupying the summits of hills, which must, at the time the deposit was made, have represented the course of a stream. In the same way the remains of riverine accumulations are found forming 'terraces' or 'benches' on the flanks of hills. Lacustrine beds may similarly occur at altitudes far above the reach of any existing stream, having been the work of rivers long since passed away. Another form of alluvial digging occurs in Western America and New Zealand, where the sea washes up auriferous sands. These are known as 'ocean placers' or 'beach diggings,' and are of minor importance. Whilst most placers have been formed by flowing water, some owe their origin to the action of ice, and are really glacial moraines. Others are attributed to the effects of repeated frost and thaw in decomposing the rocks and causing rearrangement of the component parts. Yet another class of deposits is supposed to have been accumulated by an outpouring of volcanic mud. And, finally, experts declare that some of the rich _banket_ beds of the Transvaal became auriferous by the infiltration of water containing a minute proportion of gold in solution. In all cases the recovery of alluvial gold is in principle remarkably simple. It depends on the fact that the gold is about seven times as heavy, bulk for bulk, as the material forming the mass of the deposit. The medium for effecting the separation is water in motion. The apparatus in which it is applied may be a 'pan,' a 'cradle,' or a 'tom,' for operations on a very small scale, or a 'sluice,' which may be a paved ditch or a wooden 'flume' of great length, for large operations. The method is the same in all: flowing water removes the earthy matters, while obstructions of various kinds arrest the metal. As a rule, it is more advantageous to conduct the water to the material than to carry the material to water. In many cases a stream of water, conveyed by means of pipes, and acting under the influence of considerable pressure, is utilised for removing as well as washing the deposit. This method is known as 'piping' or 'hydraulicing' in America, where it has been chiefly developed, but is now forbidden in many localities, because the enormous masses of earth washed through the sluices have silted up rivers and harbours, and caused immense loss to the agricultural interest by burying the rich riverside lands under a deposit that will be sterile for many years to come. The plan permits of very economical working in large quantities, but is extremely wasteful of gold. The water-supply is of paramount importance, and has led to the construction