It has been indicated by geologists that country rocks play a special role in the formation and filling of gold bearing veins. A thorough examination of the line of the outcrop of the gold deposit before erecting reduction works, not only to avoid the necessity of change, but to determine the probable extent of the ore-bearing ground; for if the ore be confined to a certain class of rocks in any particular zone of the deposits, the extent to which such rocks are developed is an important element in the future development of the operation. Basically, long belts of country rocks produce similar ores, while parallel belts at no great distance may produce a totally different series over a like extent of the country.
The ores on one side of a mountain range should present a totally different appearance from those on the other, where both series have been subjected to the action of the same rocks or to no such action on either side. It is simply because they occur in parallel belts of rocks of different composition, the outcrops presented on the flanks of the mountain ranges in which they lie, more or less parallel to the general summit of the range or axis of elevation. Below the free gold of the mother vein in some old deposits in California, there lies in the foot hills a band of copper bearing rocks of equal extent, while higher up in the range there is a zone with which are associated ores of a more complex character. When gold ore is found in a certain rock, it is important to ascertain to what extent that rock is developed, and whether the associated rocks are of such character for hardness or softness that a gold vein. In some circumstances, extensive regions present the expenditure of large sums of veins that a very slight geological knowledge helps to determine their real value, because rich the surface ore, they were contained in the fragments of a bed of schists overlying a tough solid rock, in which the gold vein exposures were very narrow, and into which the veins found in the schists would inevitable pass in their lower portions.
It is important to indicate that under some circumstances, it may happen that there is only one well defined wall, the vein matter being firmly attached to the other, and gradually fading out into the low gold grade rock. The condition of the walls is a matter of much interest to gold prospector. Country rocks have different characteristics and the metallic content is also different. There must have been an original area of a special type of rocks to start the gold-bearing veins filling, for without this amount of folding or dislocation could have not produced a gold vein. There should be one strong mother vein as the main fissure would offer more unimpeded channels for the circulation of fluids. If the fissure is entirely in the same rock, it may maintain a tolerably uniform character for its entire length and depth, but if it cut rocks of various kinds it may pinch out almost entirely in those which are tough, expanding in those which fracture more easily, either from splitting readily, as slates, or from extra brittleness, as certain quartzites.