SEDIMENTARY DEPOSITS AND BEDS.
In the early periods of creation the metallic ores seem to have been less abundant than during subsequent periods. They were either too minutely distributed through the materials of the globe, or too dense to be the first material vented from the con densing earth. We find but little mineral actually existing cotemporaneously with the older rocks, or, if existing, so minutely distributed as to be valueless for all practical pur poses. This may be inexplicable to many old miners, who have so often dug the tin and the copper from the heart of the granite; hut, if they will think a moment, they will recollect that all such veins are in volcanic districts, and that they exist in fissures formed not only through the granite, but through the gneiss and often through still newer strata, existing on, or over, the gneiss, which proves conclusively that such veins were formed at a period long subsequent to the granite. This seems to be the general rule among the metallic ores. All investigation proves our great metallic regions to be the creation of a comparatively late period, either as the indirect results of volcanic action or of sublimation. Most of our great metalliferous regions, however, exist in the gneissic or Azoic belt, and in the upper portions of the metamorphic. But, as we before demonstrated in the early pages of this work, the Azoic is the great region of volcanic action, for the simple reason that this belt exists along the weakest lines of the earth's crust, and, being the result of early volcanoes, is in the vicinity of those volcanic vents which filled the Appalachian basin : consequently, it would be natural this should be the great region of volcanic ores.
These facts would not only indicate that the metallic ores are of late production, but that, being more ponderable than the material formerly vented, they are always or generally the last to be vented, on the principle developed in the blast furnace. It might thus be expected that volcanic eruptions of to-day would be more productive of metals than those of former eras ; but, as the conditions then existing do not now exist, we could not expect the same natural results.
We are aware that all our sedimentary rocks were formed in water, and that the materials forming them are the results of volcanic action. The logical sequence is that
those volcanoes either existed in water or vented their lava into it. Metals are always heavier than their matrix, or the earthy strata in which they are found : thus, if the lava contained a large amount of metal it would be the first to be precipitated to the bottom of the water into which the lava was vented. The lava would not run in a solid stream from the crater and solidify as a stratum in the water, but the moment it touched the adverse element it would be shivered to atoms and thrown back into the atmo sphere with the steam it would create, and the lighter portions would naturally arise in dust and ashes, and be carried, by winds and waves and tides, to remote localities, while the heavier material would be precipitated in the vicinity in the order of their density.
This can scarcely be called a theory, since it is a natural process, and one that we know must have occurred. But we do not advance this as the only mode in which our metalliferous beds were formed ; it is only one of the many ; yet so far it appears satis factory, and we may refer most of our great Azoic beds of magnetic and specular ores and red oxides of iron to this cause, and their formation to these agencies. We may also refer the alluvial or drift gold in the "placers" of California and the " diggings" of Aus tralia to the same causes.
Gold is washed from the sands of many rivers and the beds of many plains, not only remote from the quartz matrix of gold, but where the streams cross no gold formations. But all such gold washings are in the vicinity of extinct volcanoes or trappean form ations. It is scarcely possible that the gold beneath Table Mountain, near San Fran cisco, in the beds of ancient streams, or the gold that is found beneath the " placers," was derived from the ledges of the Nevada or the quartz veins of the Coast Range, but resulted, in all probability, from the ejection of volcanic matter into the ancient waters. The lava contained gold instead of iron, and the gold was the first pre cipitated in the manner above set forth, but since the formation of the magnetic ores.