tars."

It is evident then that if this deposit had been subjected to excessive weathering and erosion it would have become decomposed and removed, ultimately to collect in some suitable place as a placer deposit of great value.

The product of the Red Point drift mine, Placer County, up to 1895 was $430,000 of which 85 per cent would pass a 10 mesh screen, but not a 40, while less than one-half of 1 per cent would pass a 40 mesh screen. Nuggets weighing from 1 to 2 ounces were occasionally found. At Bald Mountain Channel, Sierra County, 10 per cent of the product exceeded one ounce in weight and fully one-half weighed more than one-tenth of an ounce.2

The range in fineness of 800 placer and 200 quartz mines in Cali⚫fornia was: 650 to 988 and 550 to 980 respectively, while the average for the same was 899 and 820, showing a difference of 70 in favor of the placers. However, there are wide variations and only averages of a large number of cases bring out the difference. In this connection the following table is interesting:

The average fineness of the total product of the Ruby gravel mine, California, was 898; 3 per cent of the product, weighing above 10 ounces, was 860, which is a close approach to that cited above for quartz mines; while the largest nugget, weight 200 ounces, had a fineness of 849. The fine gold averaged close to 910. The difference between coarse and fine gold was about 5 per cent. The thickness of the enveloping layer of purer gold in nuggets is probably independent of the size, but constitutes the larger part of a small than a large piece, and as has been pointed out: "The degree of purification thus appears to depend upon the ratio of the super1 T. A. I. M. E., Vol. 25, p. 803, 1895. 1 Eng. and Min. Jour., Vol. 59, p. 101.

ficial area to the volume, being largest in the small pieces and scales than in the nuggets. Further, in the Ruby mine the purity of the gold from the center of the mine was found to vary largely with the presence or absence of water, the variation being from 933 to 935, or as much as one-half of 1 per cent.1

Mr. George Hewitt observed that in the Black Hills, South Dakota, little gold occurred in the vicinity of dikes cutting the Potsdam formation, and what did occur had lost its rounded and worn appearance and showed evidences of the action of powerful solvents. The placer gold of this district is seldom worth less than $18.50.2

Along the upper Burnt River, Oregon, the flour gold in the bench gravels has a fineness of 970, while the coarser gold of the stream gravels is 922. At Canyon the placer gold is 900 fine, but a few miles below the mouth of Canyon Creek, in John Day River, the gold has a fineness of 990. At Rye Valley the upper benches yield gold 750 fine, while in the lower benches it has a fineness of 800.3

At California Gulch, Leadville, Colorado, the gulch gold was worth $17 to $19, while that from the veins was valued at $15.1

In the Warren district, Idaho the bullion from the quartz-veins ranges from 300 to 550 fine, the placer gold from the small creeks 650, that from the larger creeks 725, while that from the Salmon River runs from 800 to 825 fine.5

It is evident then that there is a progressive refining action caused by gradual dissolution of silver, lead, copper, etc., from the surface of the grains."

For a brief and interesting summary of the possible influence chemical action might have upon the formation of gold in alluvial deposits, the reader is referred to the paper on the Geology of the Yukon Gold District, Alaska, by J. E. Spurr."

7

The following summary may be given regarding the occurrence of gold and its probable origin.

8

"First. Gold exists in the oldest known rocks, and has been thence distributed through all strata derived from them.

1 Eng. and Min. Jour., Vol. 59, p. 102.

T. A. I. M. E., Vol. 17, p. 573.

U. S. G. S. 22d Ann. Rept., Pt. 2, p. 637, 1901.

♦ U. S. G. S., Monograph No. 12, p. 516.

'U. S. G. S., 20th Ann. Rept., Pt. 3, p. 242.

U. S. G. S., 16th Ann. Rept., Pt. 3, pp. 292 and 293, 1894-1895.

' U. S. G. S., 18th Ann. Rept., Pt. 3, pp. 377-379, 1896-97.

8 Gold, Its Occurrence and Extraction, A. G. Lock, p. 803.

Second. In the metamorphosis of these derived rocks it has been concentrated into segregated quartz veins by some process not yet understood.

"Third. It is a constitutent of fissure-veins of all geological ages, where it has been deposited from hot chemical solutions, which have, leached deeply-buried rocks of various kinds, gathering from them gold with other metallic minerals.

"Fourth. By the erosion of strata containing auriferous veins, segregated or fissure, gold has been accumulated by mechanical agents in placer deposits, economically the most important of all the sources of gold."

General Discussion. - Gold is not confined to veins of definite limits and well defined outlines, but is often found in massive rocks and zones of impregnation where there is no pronounced vein formation. However, when auriferous deposits in massive rocks are thoroughly explored they are usually found to have fairly well defined limits though extremely irregular, and in certain cases depending largely upon the question of economic working. Further, massive rocks when gold-bearing usually have suffered alteration by mechanical movement, although the resulting fracturing may be incipient in character and not noticeable except under the microscope. On either side of the line of faulting or movement the intensity of the fracturing force diminishes until even the incipient fracturing ceases which varies largely with both kind and character of formations present. Therefore the zone of fracture will be within definite limits, which may, however, be so wide as to eliminate all resemblance to that of a vein. A few cases will serve to illustrate this point: At Hedges, San Diego County, California, the deposits are really impregnated zones of hornblende schist, very siliceous, being blue, green and gray in color, the auriferous portion usually being colored. The zones are quite irregular and were probably influenced by the power of penetration of the mineralized currents, which brought in gold and silica. In the Black Hawk Mountains, San Bernardino County, California, a heavy stratum has been extensively broken up by step-faults and thrusts now existing as terraces on the mountain side. So severe has been the fracturing action that in many places the rock-mass is in granular form. The whole mass is stained with hematite, which lines the faces of the fractures and in which gold occurs in varying quantities. Often the rock is cemented into a solid mass of ore. At Gold Mountain, San Bernardino County, California, there are gold-bearing bedded

quartzites, the quartzites having suffered considerable fracturing. Elsewhere in California there are broad zones of amphibolite schists, often with little quartz. At the Blue Gouge Mine, El Dorado County, California, there is a zone of slaty rock upwards of 600 feet in width, which is occasionally cut by small quartz-veins. The whole zone is low-grade ore. The Alaska Treadwell mine, Alaska, is an altered mass of eruptive material and is gold-bearing throughout although low-grade. The Cripple Creek deposits of andesitebreccia and granite are auriferous, and often lack most of the characteristics of veins. The presence of silica is noticeable mainly for the reason that there is so little of it. The Homestake mines of South Dakota often attain a width of 500 feet or more, and is practically all gold-bearing though the values are largely localized in quartz. It consists of a hydro-mica schist cut by many veins and lenses of quartz. The granites of California are often gold-bearing, but owe their values to neighboring veins. In Calaveras County, California, gold has been found in beds of volcanic mud, gravels and tufa.1

Gold may be found in any formation and in fact in practically any geological age-it has been found in limestone, sandstone, andesite, diorite, rhyolite, granite, porphyry, schists, coal, etc.

The following interesting summary of the rocks in which gold and silver, especially the latter, occur has been given: "Gold, silver, copper, lead, etc., are found in all rocks. Silver ores occur in all kinds of rocks, as granites, porphyry, trap, limestone, sandstone and shales. The famous mine of Guanajuato, the most productive of Mexico, and which yields one-quarter of its product of silver, intersects shales and porphyry. In Zacatecas they occur in wacke, a sedimentary trap rock; while in Sombrete they are found in limestone, in which there are extensive deposits of antimonial sulphuret of silver, one of which yielded in the short space of six months 518,000 pounds troy of silver, over $6,000,000. The veins of the Real del Monte district pass through decomposed porphyry. The mines of Chili, lying on the western slope of the Cordilleras, are connected with stratified deposits of a shaly sandstone of conglomerate character. The mines of Buenos Ayres occur in a mountain of argillaceous shale. In Norway the silver ores of Kongsberg are found in gneiss and slate, in a gangue of calcspar. The deposits in the Hartz Mountains are usually intersected by argillaceous shale, a clayey slate, and the gangue is carbonate of lime, though it is some

1 Min. and Sci. Press, Vol. 80, p. 148.

times quartz. In the mountains of Siberia the veins of argentiferous galena occur in crystalline limestone. Other Russian mines, those of Altai, for instance, occur in a coarse clay-slate in the vicinity of porphyry, and contain besides silver ores, those of gold, copper and lead.""

Gold may occur in such quantities or in such a state in the rocks as to be indistinguishable. The tellurium minerals of Cripple Creek, Colorado, and Ragged Top district, Black Hills, cannot be seen nor separated. The latter ores are highly siliceous carrying no perceptible free-gold nor other heavy mineral which could act as a vehicle for the gold.

Gold is known to exist in certain ores so united with a siliceous matrix that the resulting compound is apparently no heavier and may be lighter than the accompanying gangue.2

As previously pointed out the surface ores are rich and easily treated, and often occur in a more or less bunchy manner, while at some depth, depending upon prevailing conditions, the values become disseminated and more uniformly intermingled with the sulphides of the baser metals, and are then spoken of as being refractory or rebellious. With this general relation of minerals with depth and their consequent distribution in mind, it is not difficult to see the application to the economical extraction of the useful and valuable elements. For where it would be possible to work a narrow vein with profit as is often the case with outcrops, it becomes an entirely different proposition when the vein has expanded to a width of 40 or 60 feet, and in certain cases to 150 feet. Added to this the change in character of the ore as from a free-milling to a refractory or sulphide ore and a mine which has been able to be worked with profit must of necessity prove worthless when the new conditions are encountered at a depth.

In certain localities tellurium is the disturbing ingredient, while in other localities a selenium alloy may prove to be the obnoxious element. The elements combined either together or singly with iron pyrites or even pyrite by itself may very materially augment the difficulties experienced in making an economical extraction of the precious metal. "Though it be conceded that in many or all cases, auriferous veins become unproductive of commercial profit in proportion as they proceed in depth, it would be very unphilosophical to base on such assumed fact, anything respecting 1 American Journal of Min. Vol. 2, p. 213.

2 T. A. I. M. E., Vol. 29, p. 228.