being turned conical 12 degrees, will have a depth of 1% inches from centre to surface. The thickness may be 5% of an inch. The outer edge, perpendicular to axis, will require wood 21⁄2 inches thick for its construction. The best wood is Honduras mahogany."*

THE ROCKER.

The rocker is a box 40 inches long, 16 inches wide on the bottom, I foot high, with sides sloped like a cradle, and with rockers at the middle and back end.

The upper end is a hopper, 20 inches square, 4 inches deep, with a perforated iron bottom with half-inch-diameter holes. This top hopper is removable. Under the perforated plate there is a light frame, placed on an incline, upon which a canvas apron is stretched, forming a riffle.

In washing with the rocker the material is thrown into the hopper and water is poured on with a dipper held in

FIG. 57. THE ROCKER.

one hand, while with the other hand the cradle is kept rocking. The water washes the sand and dirt through the bottom of the hopper, and the gold or amalgam is either caught in the apron or picked up in the bottom of the rocker, while the sand and lighter material are discharged at the end, and the coarse material in the hopper is thrown aside. In California rockers were extensively used before the introduction of ditches, but now they

*See paper by Melville Attwood, "Transactions Cal. State Geological Soc."

are employed only when cleaning up placer claims and quartz mills, for the collection of finely subdivided particles of amalgam and quicksilver.

THE TOM.

The tom, said to have been an importation from Georgia, was first used in Nevada County in the latter part of 1849. It is a rough trough about 12 feet long, from 15 inches to 20 inches wide at the top, 30 inches wide at the lower end, and 8 inches deep. It is supported on timbers or stones, and set on an incline of, say, 12 inches

PLAN

SECTION OF THE TOM

FIG. 58. THE TOM.

(or I inch per foot). A sheet-iron plate, perforated with holes half an inch in diameter, forms the bottom of the lower end of the trough, which is bevelled on the lower side, so as to have the plate on a level.

The material, when fed in from sluices, on striking the riddle (or perforated plate) is at once sorted, the fine dirt with the water passing through it, while the coarser stuff is shovelled off.

Under the perforated plate there is a flat box set on an incline, into which the finer gravel passes. By the continual discharge of the water through the plate, and with the occasional aid of the shovel, the sand is kept loose, allowing the gold to settle. Since the introduction of sluices the tom has disappeared.

THE PUDDLING BOX.

The puddling box is a wooden box, usually 6 feet square and 18 inches deep, arranged with plugs for discharging the contents. The box is filled with water and clayey dirt containing gold. By continuous stirring with a rake the clay is dissolved in the water and run off. The concentrated material collected in the bottom is washed subsequently in a pan or rocker. The puddling box has been used to a very limited extent in California, but in Australia, according to Forbes, no less than 3,950 of them, worked by horse-power, were in use in Victoria alone in 1860.*

AMALGAM KETTLES.

In

The amalgam and quicksilver kettles are ordinary sheet-iron buckets or porcelain-lined iron kettles. cleaning up they are especially used as receptacles for floating the gold amalgam. The amalgam, previous to straining and retorting, is floated in quicksilver in order to free it of all foreign substances.

* J. R. Forbes," Mining and Metallurgy of Gold and Silver."

CHAPTER XIII.

BLASTING GRAVEL BANKS.

WHERE the deposits are very strongly cemented blasting is necessary..

The ordinary method of blasting gravel banks is as follows: A drift is run in from the face on the bottom of the deposit a distance proportionate to the height of the bank (as a general rule not over three-quarters of this for high banks) and the character of the ground to be moved. From the end of this drift a cross drift is driven each way (forming a T). The cross drift is charged with kegs of powder, the main drift is securely tamped by filling it up solid with the material which has been extracted, and the powder is exploded by means of a time fuse or an electric battery. In some instances when the ground is “heavy and bound" several cross, drifts are used The amount of powder used is determined by the position, character, and height of the bank, a quantity sufficient only to shatter the ground being employed.

Blast at Smartsville.-The following details of several large blasts are given as illustrating the general facts. A blast of 450 kegs of black powder was made at Smartsville in hard cement with an 80-foot bank, the ground being ordinarily bound (i.e., with two sides free). The main powder drift was run in from the face of the bank 85 feet, cross drifts being opened each side 40 feet and 85 feet from the mouth. Each cross drift was 45 feet long, and from its ends and centres two "lifters" were driven at right angles to it, extending respectively half way to the next cross drifts and to the face of the bank. After charging the cross drifts the main drift was tamped and the powder exploded by means of an electric battery.

The arrangement of the powder chambers for a 1,201keg blast made by the Smartsville Hydraulic Mining Company in December, 1868, is shown in the following diagram.

15' 20%

A

50'

40 B

X was a shaft 74 feet deep, from the bottom of which the main drift, A, was driven 185 feet. The cross drifts, B, three in number, were driven at distances respectively of 70 feet, 120 feet, and 170 feet from the shaft, X. They extended 15 each 20 feet on one side of the main drift and 40 feet on the other side. The several drifts marked C are called "lifters." Each "lifter" was 15 feet long. The total length of the drifts. aggregated 570 feet. They were 21⁄2 feet wide and 31⁄2 feet high The cross drifts were charged with 1,201 kegs (25 pounds each) of black powder. The main drift was securely tamped from the shaft to the first cross drift, a distance of 70 feet. The powder was simultaneously ignited by electricity at 12 different points.

BX

50'

50

70'

A

X

B

SHAFT

FIG. 59.

The ground moved was 270 feet long, 180 feet wide, with an average depth of 100 feet. The cost of the blast was about $6,000.

Blue Point Blast.-A large blast of 2,000 kegs (25 pounds each) was exploded December 29, 1870, at the Blue

Point Mine, Sucker Flat, Nevada County. The main drif