_Third_, That the drifted sands and gravels, in which gold-washing is profitable, occur only in the proximity, more or less near, of such ancient and altered (so called metamorphic) rocks. They are, in fact, the fragments of such rocks broken up, pounded, and borne to their present sites by natural causes, operating long ages ago, but similar in kind to those which now degrade and carry away to lower levels the crumbling particles still torn off from our hardest mountains by the ceaseless tooth of time. 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.