Auriferous lead deposits seem to present closely similar features whether exposed or covered by basalt or sedimentary deposits, and its formation is related to alluvions. It is important to mention that even the hardest rocks are worn away by the combined action of air, rain, wind or sun, dew and frost, and are rendered capable of transport by streams. These latter acts in two ways: they cut into the rocks through which they flow and at the same time they deposit in their course the heavier particles of material brought down from the neighboring elevations. In this way, alluvions are formed.
If the stream is powerful, the fall not very great and the rocks soft, a comparatively wide deposit of drifts and clays will be arranged, according to the specific gravity of the materials, modified, however by the size and shape of the particles; and these strata will gradually increase in thickness till the lower part of the stream begins to acquire greater force from the steeper fall there produced and scoops out a deeper channel for itself. In time, it will wear back towards the source, and new and deeper course will be cut through the alluvions and bed rock.
It is supposed that the reef wash must necessarily be younger than the drift in the gutter, but it is quite possible that it may even be older in some instances. Evidently many successive layers of different ages may be formed in the same way and where it can be proved that the stream has always been cutting a deeper channel, it may be assumed with certainty that the upper drifts or reef washes are older than those lying deeper down. A stream will seldom continue to make a deposit in the direction of its course for a lengthened period and until its channel is changed; but this does occasionally occur where the bounding ridges are low. In such case, the bed will gradually be silted up and it will be almost impossible to ascertain the relative ages of the deposits, especially when the lithological features remain the same throughout.
The rate at which alterations are effected by streams of water depends in a great measure upon the nature of the rocks exposed to their action. When the strata are inclined, and composed of soft sandstones and argillaceous rocks, materials will be transported and new channels excavated in a manner which is not well known when rocks are hard or lie almost horizontally. Some streams would rapidly erode the rock against which it impinges and would be assisted by every shower of rain, the upper portion falling down as fast as the lower parts were worn away. The bed would constantly move towards the right, and the earthy materials of all kinds would be disintegrated and transported to form a new deposit in some portion of the bed of the stream.