Fissures are formed by movements of the earth’s crust or by local contraction of the rocks, and a fissure in a gold deposit is not necessarily one with well defined walls at considerable distances apart, filled after the formation of the fissure, but the common cracks or joint extending regularly to great lengths or depths. This condition illustrates the original fissures which were changed by percolating waters carrying minerals solutions into veins and deposits of gold ore. In some rock formations, these cracks run parallel to each other at various distances apart, often plentiful and close together. In situations where percolating waters were charged with base metals, gold and some vein compounds, and the necessary chemical and physical conditions occurred, the rocks lying between those cracks were modified and altered.
As one element was dissolved, another took its place. In this way a fissure vein may be only a sort of replacement of rock by mineral. For this reason is accepted that a wall of a vein is not necessarily one, cross-cutting, in order to determine the lateral boundaries of the gold ore, is safer than to rely on supposed walls. Slips are followed by prospectors as a supposed wall, until by accident they are broken and good ore is found on the other side. It is important to indicate that cross gold veins sometimes cut or fault and older vein. The point of intersection is usually rich in gold bearing minerals. Cross veins must not be confounded with leaders, which are the filling of minor cracks extending off from the vein and are sometime sufficiently attractive to extract valuable minerals.
It is interesting to mention that cross veins may lead a gold prospector to the main vein and also lead a miner underground away from the true vein. The splitting of a gold vein by large fragments of rocks lying in the vein, may be mistaken for a cross vein or the original fracture of the fissure may have been in the form of star or like spokes of a wheel radiating to the center. In these cases there are no true cross veins, but when there are two well defined sets of veins, striking in opposite directions, they cut each other diagonally, and the cut vein is the older. These opposite sets of veins were formed at different times and many contain a characteristically different class of variety of minerals.