Gold prospecting is pre-eminently a method location, location of gold veins and other values within them, and is therefore not confined to the surface all the time. The method of procedure in underground gold prospecting or exploration is usually more definite and reliable than surface works for similar deposits. In those sections of the deposit where erosion is rapid, and the rainfall is great, the location of outcrops is a comparatively easy task, but when these conditions do not obtain the outcrops are usually hidden or obscured by a mantle of soil or wash, thus rendering their location extremely difficult.
The general considerations affecting the work of prospecting and the methods employed are as follows: change in direction and grade of streams may be produced by their intersection with veins. Further, gold bearing veins have been located above holes and depressions, which occurrences are occasionally observed in some places such as the extinct river channels of California. In general, in the search for gold in streams, preparatory to search for gold veins, the grade of the stream must be first taken into consideration. If the grade is heavy, the search for values should be made at wide portions of the channels, on the sides and the inner rims of curves. With moderate or light grades the long straight channels are more favorable to the collection of gold.
Gold having been found in the stream bed or bank and traced to a point where no more can be found or there is a marked diminution in that found, the search extends to higher ground; the accumulations of gravel often found in the form of terraces, or benches, on the slopes and occasionally at considerable altitude should be carefully examines. Basically, thin hill-side deposits are usually auriferous to a marked degree, which deposits may extend even into the valleys. All portions of such deposits should be tested for gold which, being composed largely of wash and formed on fairly grades may be a heterogeneous mass of gravel, sand, soil and clay. The prospecting of wind-formed deposits and wash, especially when shallow is rendered comparatively simple due to the fact that the values usually lie close to the surface, while water-lain deposits more often carry the values on or close to bed rock, and the same may be partially true of wind-formed deposits if the country is rough.
It is important to indicate that a careful geological reconnaissance should be made of the surrounding country in order that the factors governing the present slopes may be ascertained. Underground work may be necessary to supplement such investigations. When shafts are employed, they should be given the minimum size consistent with the work to be done, a shaft two and one-half by four feet is large enough for considerable depth. Regarding the choice of location with respects to formation, the junction of various formations as the older rocks formations, as slates, schists, quartzites, shales, sandstones, and limestone with basalt, phonolites, andesites are favorable, while recent formations both stratified and igneous are usually considered of little importance. Where vegetation is poor or wanting there is evidently a thin covering of soil; nevertheless, more credit is given to the influence of mineral matter on vegetation. If considering depth of soil is found in the locality prospected an examination of cuts by streams and exposures made by landslides may be important.
Gold veins may be located above holes and depressions