Gold prospectors know that potential gold dredging deposits have special geological characteristics, which indicate the presence of gold. While dredge mining may be conducted, under conditions apparently satisfactory to those engaged in the industry, there is an interesting geological feature worthy of more than a passing thought, and that is that all of the dredging is being done on what may be considered a superficial layer of gravel, underlaid by a stratum of volcanic mud and sand, the deposition of which, of course, antedated the deposition of this gravel. The gold-bearing veins existed and were eroded for a great period of time, for example, antedating the volcanic activity which is such a prominent feature of the Sierra Nevadas, and the ancient rivers deposited their gold-bearing debris upon the original bedrock, both within the limits of their rims and upon the floor of the great interior valley the present valley of the Sacramento River—which at that time was a vast fresh-water lake.
It is important to mention that subsequently the volcanic activity resulted, in many instances, in filling the old gold-bearing rivers to the brim with a vast quantity of volcanic material, now recognized in the mud, tufa, and breccia overlying the ancient river channels, and, as a matter of course, this volcanic material was carried out into the estuaries, extending even into the lake itself to a considerable distance, and covering the previously deposited gold-bearing gravel. The subsequent uplift of the whole system has resulted in the exposure of these lakebeds along the eastern edge of the valley. The subsequent erosion of modern streams, cutting through the original ancient river channels, and often into the underlying bedrock, with its complex system of gold-bearing veins, resulted in the formation of a later bed of gravel, which rests upon the volcanic sands and mud of the lake.
Doubtless this process has been repeated a number of times, and this point could be determined by drilling or by sinking shafts. But it is clearly evident that the present dredging is not in deposits of gravel resting on the true bedrock—the slates and schists which form the adjacent foothills—but in later, and, quite likely, less valuable gravel. In many ancient river channels there are sometimes two or three gold-bearing strata, called by the gold miners leads, and the stratum of gravel on the bedrock is usually the richest, while the upper strata are relatively of less value in gold per cubic meter. There are, however, instances contrary to this, where one of the upper strata is of more value; but this Is due to local conditions, probably generally to the erosion of other preexisting beds of gravel. Why this principle should not be proved true in this lake is well worth considering. Similar conditions have been found to exist in different regions. It has been established that the discovery, however, of good value in these lower strata might not lead to satisfactory economic results, because the expense of handling large bodies of water that might be encountered would handicap any venture in that direction. It may seem a simple thing, to those who know, to say that there is a great diversity in methods of gravel mining caused by varying conditions of the deposits, and that the method of dredging in one case may mean failure in another.