Numerous as have been the deposits of gold found in various ages and countries, they all confirm the general geological conclusions above stated. The main and most abundant sources of gold which were known to the ancients, occurred among the sands of rivers, and amid the gravels and shingles which formed their banks. Such were the gold-washings in the beds of the Phasis, the Pactolus, the Po, the Douro, the Tagus, and the mountain streams which descended from the alpine heights of Greece, of Italy, of America, of Asia Minor, and of many other countries. These rivers all descend from, or, early on their way, pass through or among, ancient rocks, generally old and altered Silurian strata, such as those we have spoken of, in which the gold originally existed, and from which the existing rivers, since they assumed their present channels, have in some few cases, and to a small amount, separated and brought it down. And if in any region, as in Nubia, Hungary, Bohemia, and Macedonia,[[6]] the ancient or mediæval nations followed up their search to the sources of the rich rivers, and were successful in finding and extracting gold from the native rocks, later explorations, wherever made, have shown that these mines were situated among old and disturbed deposits of the primary and Silurian age.
The more modern discoveries in America, Siberia, and elsewhere, prove the same. So that, among geologists, it is at present received as an established fact, that the primary, the so called azoic and palæozoic rocks, are the only great repositories of native gold.
There are no known laws, either physical or chemical, by which the almost exclusive presence of gold in these ancient rocks can be accounted for or explained. A conjecture has been hazarded, however, to which we shall for a moment advert.
From the fissures and openings which abound in volcanic neighbourhoods, gases and vapours are now seen continually to arise. Whatever is capable of being volatilised—driven off in vapour, that is—by the existing heat, rises from beneath till it reaches the open air, or some comparatively cool spot below the surface, where it condenses and remains. Such was the case also in what we may call the primary days of geology.
Gold is one of the few metals which occur, for the most part, in the native or metallic and malleable state. But in this state it is not volatile, and could not have been driven up in vapour by ancient subterranean heat. But, as in the case of many other metals, the prevailing belief is, that it has been so volatilised—not in the metallic state, however, but in some form of chemical combination in which it is capable of being volatilised. No such combinations are yet known, though their existence is not inconsistent with—may in fact be inferred from—our actual knowledge.
It is further supposed that, at the period when the primary rocks were disturbed by intrusions of granites, porphyries, serpentines, greenstones, &c., which we have spoken of as volcanic-like phenomena, the elementary bodies, which, by their union with the gold, are capable of rendering it volatile, happened to exist more abundantly than at the period of any of those other disturbances by which the secondary and tertiary rocks were affected; and that this is the reason why signs of gold-bearing exhalations, and consequently gold-bearing veins, are rare in the rocks of the newer epochs.
According to this view of the introduction of gold into the fissures and veins of the earliest rocks, its presence is due to what we may call the fortuitous and concurrent presence in the under crust of other elementary substances along with the gold, which by uniting with it could make it volatile, rather than to the action or influence of any widely-operating chemical or physical law. The explanation itself, however, it will be remembered, is merely conjectural, and, we may add, neither satisfactory nor free from grave objections.
But from the geological facts we have above stated, several very interesting consequences follow, such as—
First, That wherever the rocks we have mentioned occur, and altered as we have described, the existence and discovery of gold are rendered probable. Physical conditions may not be equally propitious everywhere. Broad valleys and favourable river channels may not always coexist with primary rocks traversed by old volcanic disturbances; or the ancient sands and shingles with which the particles of abraded gold were originally mixed may, by equally ancient currents, have been scoured out of existing valleys, and swept far away. But these are matters of only secondary consideration, to be ascertained by that personal exploration which a previous knowledge of the geological structure will justify and encourage.
Whenever the geology of a new country becomes known, therefore, it becomes possible to predict the presence or absence of native gold, in available quantities, with such a degree of probability as to make public research a national, if not an individual duty. This led Sir Roderick Murchison to foretell the discovery of gold in Australia, as we have already explained; and similar knowledge places similar predictions within the power of other geologists.