Water has an effect on the filling process of gold veins. Water or fluids penetrates into the rocks everywhere, the cooler waters descend, the heated ones tending to rise, or being forced to the surface along the lines of least resistance, such as the fault fissures, by the pressure of others fluids. Essentially, the ability of water to decompose rocks depends on the presence of carbonic acid and alkaline material such as carbonates and its solvent powers are increased with a rise of temperature. Its first carbonic acid is derived by rain water from the atmosphere, and the moment it touches the earth, it begins its work of decomposition and rearrangement, picking up and dissolving one mineral and depositing another.
Water traversing an open fissure would thus leave a portion of its contents on the walls, both sides alike, as it gradually cooled in its ascent, just as sugar candy will crystallize out of the sugar solution as it is cooled or evaporated, the process of cooling producing the same results as that of evaporation, in both cases the liquid being unable to carry so great a load drops it at the first opportunity. In this manner has formed some veins.
The change in the deposited gold bearing minerals may have taken place either because of the exhaustion of the locality from which it had been collected by the fluid or from such divergence in the course of the underground flow by disturbances like earthquakes that the material was drawn from a new series of rocks. A similar incrustation of small cavities by waters abounding in silica has formed agates, the banding being due to the changing presence of the coloring material. Compact and close-grained are in reality porous. When the cavity has not been completely filled, it is either left with a smooth surface or lined with crystals of quartz or calcite. In such manner other minerals were introduced into the cavities of basaltic lavas. For this reasons, some gold bearing minerals were deposited on the top of each other by just such changes in the character of the circulating fluids. Where there has been much of this action, it is important to look for more or less numerous veins parallel to the main lode, which may carry more or less free gold or gold bearing minerals, but being only incidents accompanying the main disruption are less likely to afford the conditions which will develop them into gold veins of equal value.