Arborescent branching or dendritic gold.-Specimens of arborescent branching gold are frequently of great beauty. Small specimens were found in working a quartz vein at Mameluke Hill, near Georgetown, California, several years ago. In El Dorado County, at Spanish Dry Diggings, a mass of irregularly-grouped dendritic and arborescent crystals was found weighing 16 pounds, and was sent to Paris in 1867. The gold of the celebrated Loud deposit in Lumpkin County, Georgia, is also dendritic, and grouped in irregular crystalline masses.

Irish Creek, near Coloma, in Placer County, California, is another locality which has furnished some remarkably fine specimens. One taken out in 1853 is beautifully arborescent on one side, forming a curved plate about 1 inches wide, while the other side is covered with small but perfect octahedrons to the number of twenty or more, and from the one-sixteenth of an inch up to one-eighth or more in diameter.

The edges of all are slightly raised above the octahedral face, and one of the crystals is flattened parallel to a face, and to such a degree that it appears as a thin six-sided plate. The specimen seems to have grown from one point, and to have once been incased in peroxide of iron, as this substance fills some of the spaces. Length 23 inches; breadth, 1 inches.

Gold in moss-like aggregations is occasionally found. Some beauti ful examples of this form were taken out of the White Bull mine, in Oregon, a few years ago.

GOLD IN SHEETS AND PLATES.

The occurrence of gold in thin sheets or plates from half an inch to two or more inches in breadth is characteristic of the mines of Transylvania. Such plates are rarely thicker than a visiting card, and are generally covered with crystalline lines and markings revealing a pervading geometrical structure which becomes more apparent at the edges of the plates. Some examples of this obscure crystallization have been carefully studied by Von Rath, who finds the crystal to be complex twins or combinations of distorted tetrahexahedra.

Somewhat similar plates have been found occasionally at one of the mines at Grass Valley, Nevada County, California.

Zur Krystalisation des Geldes Zeitschrift fur Krystallographie und Mineralogie. Erster Band, 1877.

One of the best examples of sheet or plate gold is from the "French mine," California, 1853. It measures 3 inches by 13, and is about one millimeter thick. The whole surface is covered with triangular markings and projections like a tented field. These triangles are both faintly and strongly outlined. They are equilateral and appear to be the faces of incipient or flattened octahedra. Many of these measure three sixteenths of an inch across the bases, and have in the center, or toward one of the angles, a solid projecting pyramid of three planes, apparently the solid angles of the cube, rising above or through the octahe

dral face. At the extreme end of the plate the crystallization becomes more distinct, and a fringe-like series of flattened octahedrons is developed. On one side of the plate, however, there is a single salient crystal standing out above the general surface showing several planes of a pentagonal dodecahedron; one of the very few examples seen of a hemihedral crystal of gold from California. The faces of this crystal are flat and brilliant near the edges, but are cavernous and hoppershaped toward the center.

OCTAHEDRA WITH DIVERGENT PRISMS.

At Clancy, Clancy Creek, Jefferson County, Montana, minute crystals of gold were obtained by Dr. H. M. Hill, which present the novel appearance of an octahedral nucleus or head, with a long divergent brush-like attachment on one side or angle of the crystal, giving the whole the appearance of the drawings usually made of comets.

FIG. 43.

The total length of these crystals is about one eighth of an inch, and the long prisms are consequently very small, and it is not easy to determine their precise form. They appear, however, to be hexagonal prisms. They are brittle and are easily detached from the octahedral head. There are some indications that they are formed by the twinning of octahedra. Their appearance suggests their artificial origin, possibly from the evaporation of quicksilver charged with gold.

An interesting example of a wire-like rod of gold penetrating a leavage crystal of calcite, or pearl spar, with curved faces, is illus

trated by Fig. 44. The gold protrudes from each face nearly at right angles, and appears to have been originally enveloped in calcite, now removed by cleaving. From near Armagosa, on the Mojave River, California.

FIG. 44.

The crystalline forms so far observed in California may be enumer ated as follows: Cubo octahedron; octahedron; dodecahedron; hexakisoctahedron; trapezohedron (in combination); and the pentagonal dodecahedron, to which may be added plates; arborescent and filiform masses; sponge gold, an aggregation of fine crystals; prismatic or elongated forms, distorted crystals, chiefly flattened and elongated octahedra.

ARTIFICIAL CRYSTALLIZATION OF GOLD.

Crystalline gold may be obtained artificially by slow cooling from fusion, by electrolysis, and by the treatment of gold amalgam with nitric acid, or the slow evaporation of quicksilver from amalgam.

Crystalline facets and markings are frequently found on the surfaces, especially in the depressed surfaces of gold bars which have cooled slowly from fusion. In the manipulation of a mass of pure gold at the British mint, London, fine crystals were produced, and appeared to be an aggregation of octahedra.* In an article in the American Journal of Science upon certain artificial crystals of gold Mr. Albert H. Chesterf states that in casting bars of pure gold for the manufacture of foil, traces of crystallization may may often be observed upon their upper surfaces. These are generally simple triangular faces slightly raised. Occasionally sev eral faces of the octahedron may be seen, the edge in some instances being half an inch in length and quite sharp and well defined. The purer the gold the more likely the crystals are to form. The presence of a very small amount of copper seems to prevent it entirely.

The same writer describes the process of preparing crystalline gold by electrolysis for dental purposes. This process was patented in 1560, and consists essentially in the decomposition of a solution of chloride of gold and ammonium by the battery current, both poles being made of pure gold.

The gold is gradually deposited in bright crystalline flakes having the appearance of feathers or fern leaves when examined under the microscope. Another process for obtaining what was known as "sponge gold" was patented in 1853. This consisted in dissolving gold in quicksilver and then digesting with nitric acid. The quicksilver is dissolved, and the residue is a spongiform mass of minute gold crystals. Professor Chester found such crystals to have a prismatic form, apparently regular hexagonal prisms, with pyramidal terminations, of which he gives figures and measurements. The figure of the hexagonal prism

Encyclopedia Britannica; article on gold.

American Journal of Science and Arts, third series, Vol. XVI, July, 1878, p. 29.

represents a crystal about 0.5mm in length and 0.06mm in diameter. The angle 01 has been approximately determined to be about 136°, which would give for I (the prismatic plane) and 1, 134°. Such crystals contain 6 per cent. of mercury, which may be driven off by heat, leaving the gold somewhat spongy and not brittle, as before.

It is interesting that Le Sage, a mineralogist of the last century, describes, in 1777, the formation of gold crystals in prisms by the evaporation of mercury from gold amalgam. He describes the prisms, however, as square prisms, with square pyramidal summits, and regarded them as elongated octahedra.

At Sonora, in Tuolumne County, California, the late Dr. Snell, of that place, gave the writer some minute prismatic crystals of gold which under the microscope, and even with a good pocket-lens, may be seen to be perfectly formed hexag onal prisms with brilliant sides and pyramidal terminations. There are no marks of twinning or compounding, but two or more prisms are sometimes attached side by side. The terminal planes are regular isosceles triangles. The angle between the prism I and the pyramid 1 is approximately 130°. These crystals separately appear to nearly agree in their form and angles with those described by Professor Chester, already referred to, page 586, and they may have been, and probably were, artificially formed.

GENERAL DISSEMINATION OF GOLD.

FIG. 45.

Gold is believed to be generally diffused in solution in the water of the ocean. This opinion was expressed by Wurtz in 1866, and Sonstadt in 1872 announced that the sea-water of the British coast contained gold, and by estimate to the extent of about one grain of gold to the ton of water. The metal is supposed to be kept in solution by the iodine liberated by the decay of organic matter.

There is a much greater dissemination of gold in a ragged granular condition in situ in fine particles in the midst of rock formations and without any obvious connection with veins than is generally supposed. Prominent examples are found in the belts or zones of layers of soft slate in Georgia and in North Carolina, one in Georgia being well-known as the "Pigeon-Roost streak," where the slates are rusty and decayed to such an extent that they can be washed down by a stream of water. Quartz veins or "ledges" are not visible, but after washing away the soft ash-like earth and iron-rust a residue is left which consists chiefly of fragments of thin sheet-like veins or inclusions of quartz which were enfilmed with the micaceous slate. These thin sheets of quartz are generally not much thicker or larger than a knife-blade, and this has suggested the name of "knife blade veins," by which these thin seams are generally known. They are often very rich in gold, which ramifies through the substance of the quartz in a subcrystalline form. The gold at the Mameluke mine, Georgetown, Cal., occurs in a similar softened and decomposed slate, and is crystalline in its structure, occurring in moss-like dendritic and arborescent forms.

The Boly Fields gold vein, on the banks of the Chestatee River, Lumpkin County, Georgia, is another example of the occurrence of coarse, ragged gold in the midst of a mass of slate without any defined quartz vein. In this locality the slate, being swept clean by the river, does not show any decomposition, and the gold can be seen protruding

from the cleanly-fractured masses of hornblende slate, and closely associated with bornite, pyrites, and dolomite. Such rock in decomposing would yield up a large amount of coarse, ragged, or "vein gold" without any other evidence of the existence of a vein.

The dissemination of gold in the schistose rocks of North Carolina has been noted by Professor Kerr,* who describes the Rhodes mine, in Gaston County, as a body 9 to 12 feet thick of decomposed light gray gneiss with strings of quartz. And at King's Mountain, in the same county, gold is obtained not only from seams of quartz in a bluish-colored hydromicaceous schist, "but a 60-foot ledge of grayish-blue finegrained schistose limestone is quarried out bodily and sent to the stamps." Dr. Emmons has also noted that the quartzite schist is goldbearing over wide tracts of country. It is easy to understand that in the erosion and surface-washing of vast areas of gold-bearing rock, espe cially where such rocks have become softened and decayed to a depth of from 10 to 50 feet or more, there would be a great accumulation of gold without much evidence of quartz veins.

There are other facts which show that there is a wide distribution of gold in the mass of rocks irrespective of regularly formed quartzveins, and of the geologic age of the formations. Prof. Charles A. Schaeffer has described a district in Williamson County, Texas, of horizontal strata of porous ferruginous rock, underlaid by finely granular limestone of the age of the Cretaceous, where gold occurs in small quantity entangled in the pores and crevices with oxide of iron derived from the decomposition of pyrites. The existence of a minute quantity of gold in the clays underlying the city of Philadelphia is well known.

GOLD IN SULPHURETS.

The usual association of gold in quartz veins with sulphurets is well known. In gold regions the sulphid of iron (iron pyrites) is generally gold-bearing, as is also the arsenid of iron (arsenical pyrites or mispickel). The gold in these sulphides and arsenides is probably dif fused mechanically and not in chemical combination as some have claimed. The experience of many years in the examination of gold veins and gold ores sustains the conclusion that the gold is mechanically diffused. Although the metal is generally invisible in the undecomposed and brilliant crystals of pyrites it becomes apparent when such crystals are decomposed by oxidation either through the slow natural agencies of moisture and air or by artificial means. Decomposition by nitric acid is frequently resorted to as a test of pyrites for gold. As a result of such decomposition we frequently find particles of gold ragged in form, but of comparatively large size. So also when pyrite is changed to limonite by oxidation and without losing its form, gold which probably was before the change scarcely visible becomes distinctly visible in contrast with the brownish red color of the iron oxide. Deflagration of pyrites with nitrate of soda ("Holland's process"), which is simply another method of oxidation of pyrites, also liberates gold from auriferous pyrites, and permits it to be seen when the oxide of iron is washed away. Roasting produces the same result.

In amalgamating gold ores it is common to find that fragments of either raw or roasted pyrites, but more especially the roasted, adhere Transactions Am.

* Some peculiarities in the occurrence of gold in North Carolina. Inst. Min. Engrs., X, 475.

Transactions Amer. Institute of Mining Engineers, XI, 319.