an average of 6 ounces of gold to the ton.1 Other instances might

be cited to the same effect.

Although the information cited is far from conclusive even for a given locality, yet it is only through the collection and correlation of such data that anything definite as to theory can be arrived at regarding the occurrence of mineral, in paying quantities, with depth. The lack of extended and published records of observation is largely responsible for the lack of more detailed discussion in this connection. In conclusion the relation of gold values to depth may be summarized as follows: first, the contention that workable values are not co-extensive with depth is fairly well established; second, the change in values takes place comparatively slowly; third, the distribution of values seems to be more uniform in depth, at least masses and bonanzas are of less frequent occurrence; and fourth, the relation of value to depth depends largely upon economic mining and extraction. The reduction of cost of operations is then the working basis of permanency of values in depth which combined with any natural, local advantage of occurrence makes for higher profits, and larger production.

Occurrence of Gold in Gravels, by States and Territories.

Introductory Remarks. - Gold always occurs in gravels in the metallic state and when so found has various physical characteristics in the deep placers it usually has no luster, often having no appearance of gold whatever, and resembles the associated iron sands. In the shallow placers, however, the gold usually has more marked characteristics, although varying considerably in color. Dark-colored gold is found to become clearer on exposure to atmospheric agencies and is more amenable to amalgamation. This is shown to advantage in the case of the Red Gulch mine, El Dorado County, California, where it has been found both possible and profitable to rework the gravels at least six times, which fact has led to a belief among miners that the gold is constantly being renewed, i.e., that it "grows again."

113

There are three classes of gravel deposits, namely; 1st, bar, ravine and cañon placers; 2d, bench or hillside placers; and 3d, extinct river-channel deposits usually overlain with lava.

1 Eng. and Min. Jour., Vol. 67, p. 495.

2 Min. and Sci. Press, Vol. 92, p. 41.

Min. & Sci. Press, Vol. 60,
p. 297.

The grade of most streams traversing auriferous areas is usually considerable and it is therefore only the coarser pieces of rock which find their way into them that find lodgment, the finer and lighter particles being carried down-stream to a point where the current velocity is less, unless they are checked by some especially favorable condition. The tendency then is to sort the materials entering the streams, each particular stream varying that action with its varying conditions of grade, width, depth and character of channel. Such variations are constantly happening at any particular place owing to changes in shape of channel and seasons. It is evident then that there will be a considerable overlapping of the grades of sorted products, including the accumulations of gold and associated minerals. Disintegrated fragments of quartz-veins entering a stream may be comparatively large fragments, pebbles or sands, which continue to be reduced in size by abrasion, thus gradually approaching the normal specific gravity of the gold. The movement then of such a fragment under the action of running water becomes more and more retarded until freed of all adhering gangue it attains a maximum specific gravity, and a minimum volume, when it stops and is added to the accumulation in the bed of the stream, while the gangue passes on.

It has been observed that coarse gold tends to be concentrated on soft bed-rock in preference to hard. The reason for such ocurrence is that the soft bed-rock wears away much more rapidly than the hard and therefore the disintegration of the quartz-veins is more rapid at such points and consequently more gold is freed and in larger pieces than would be the case were the wear less rapid. Further, the baselevel of a stream in a soft country-rock is more quickly attained and the deposits of gravel are therefore more uniform.

"In the larger streams, when the current which transports gold along with other detrital material suffers a decrease of velocity, and hence of carrying power, some material may be dropped to the bottom, while the water flows on more slowly and gradually becomes comparatively clear. The rate of cutting and consequently the load carried by a stream has much to do with the deposition of the gold carried. A greatly over-loaded current will deposit too rapidly to admit of the concentration of the gold and especially the fine gold. Evidently then there is a certain relation between the velocity of current and the load of débris carried that is of considerable importance in determining the character and location of the pay streak. Further, this condition of affairs varies considerably with the position

of deposit with respect to the banks - there may be an unloading on the outer rim and an overloading on the inner.1

It thus happens that in long rivers which flow through auriferous formations or whose feeders cut auriferous rocks there are at intervals deposits of fine gold in gravelly accumulations. Such accumulations of detrital material are known among the miners as bars, and when sufficiently rich in gold to repay working, these are called 'bar diggings.' Bars are formed wherever the curves of the channel are such that the current flows off at a tangent to the bank, thus producing slack water and often back water or an eddy in the immediate vicinity of the bank. Thus the most common loci for bars are points immediately below where a pronounced curve changes suddenly for another. Slightly concave irregularities in an otherwise straight channel also produce the conditions necessary for these deposits, since such concavities will not ordinarily influence the course of a moderately swift current."2

The character of the bed-rock when not covered with gravel is largely responsible for the nature of the deposits and may contain within itself the larger part of the gold. In comparatively soft bed-rock the gold may be found in paying and often large quantities for a depth of 18 to 24 inches, while hard bed-rock, especially when fissured and cut by cleavage planes, may hold the largest values. Subsequent deposition of gravel may form a deposit of many feet depth throughout which much gold may occur, thus forming an exceedingly rich gravel bed. When a stream crosses a formation consisting of stratified rock more or less tilted so that the upturned edges of the strata form riffles on a large scale, or when under similar conditions hard and soft strata alternate one with the other, exceptionally favorable conditions exist for the collection of both coarse and fine gold.

The position of such riffles with respect to the stream has much to do with the collection of gold on the bed-rock and in the overlying gravels. To illustrate this point the bend of a stream cutting upturned strata may be taken. If the case is chosen where the course of the stream directly at the bends is at right angles with the stratification, there will be no other point in the stream's course except at the bends where the same conditions exist. Now, the effect of riffles placed normal to the current of a sluice (the same being applicable to streams) causes a uniform settlement of the transported gold, but 1 Min. and Sci. Press, Vol. 77, p. 108.

2 U. S. G. S., 18 Ann. Rept., Pt. 3, pp. 360, 361, 1896.

on changing the direction of the riffles with respect to the current it is found that there is a deflection of the sorted material, which occurs on the side of the larger or obtuse angle. It is evident then that on approaching such a bend, with the current, the pay streak of gravel beginning on the left hand side approaches the middle of the stream at the bend, after which it shifts to the right side and ultimately hugs the bank only to be again thrown across the stream by a reverse bend.' With streams of variable cross-section the above-described method of deposition of gold may be somewhat modified.

The bed-rock may be soft and decomposed, or hard, as granite and slate, but in nearly all cases it is usually uneven and full of crevices which form the riffles that have caught and held the gold. In most of the largest and most prominent camps the richest spots are generally those on hard rough bottoms, where the gold has been picked out by hand from the crevices and with small tools, as knives and spoons. Montana Bar, Confederate Gulch, near Helena, Montana, is said to have yielded four men $1,200,000 from threequarters of an acre in 100 days, the bed-rock being tough, hard and ragged. 2

The extensive distribution of gravels in benches on hill and mountain sides may be partially due to river action but not entirely so in some cases at least. Where there are evidences of recent submergence, as in Alaska, it is probable that the origin of the deposits of gravel is marine, or that they have been redistributed by marine action. However, the lower terraces or benches are probably remnants of older river-beds left by the down-cutting of the present streams.

The Summit Diggings of the Mojave desert have derived their gold from the flat mesa lands, the concentration having been effected by winds, and as might be expected the surface sands are richer than those at a depth.

Placers have also been formed by slides which, however, come from older gravel beds and must, therefore, be considered to have a water origin. An excellent example of such an occurrence was observed at Morris Ravine, where a mass of auriferous material several miles in extent was found to be gradually moving forward into the workings as hydraulicing proceeded from below, clearing

1 Min. and Sci. Press, Vol. 68, p. 165.

Min. and Sci. Press, Vol. 79, p. 60, and U. S. G. S., 18 Ann. Rept., Pt. 3, pp. 375 and 376, 1896-97.

away the deposit to bed-rock. There is no doubt but that landslides contribute in many instances to the accumulation of auriferous materials and possibly to a much greater extent than is usually suspected.

As a rule gold occurs on or close to bed-rock. However, many large bodies of gravel have been found in which the gold has been uniformly distributed from surface to bed-rock. The coarser gold. is usually found in the lower portion of the deposit unless the stream has built up from a lower base-level with comparatively long intervals of activity followed by others of inactivity when no change in level occurred. Under such circumstances different levels or zones of gravels can readily be traced out, in which considerable variation. in size of gold grains is noted and there may be several successive layers of coarse gold with quite a distance, vertically, intervening.

It is claimed that the concentration of gold, iron sands and other heavy minerals is also brought about by gravity" thus effecting a concentration variously perfect in different places, and that in this process the passage of the gold particles is mechanical." However, without some movement, although comparatively slight, such action could not take place. In ordinary placers the only cause for such movement would be the circulation of drainage water which in itself would be slight and probably accounts for the imperfect concentration noted, but in regions, as Alaska and Siberia, where alternating freezing and thawing is of daily occurrence the resulting expansion and contraction would suffice to produce a decided downward movement of the gold grains. The retention and concentration of the gold at the bottom of the gravel or on the bed-rock would then depend upon the existing conditions if rough, it would tend to remain at or near the point where it came in contact with the bed-rock, if smooth it might travel for some distance before coming to rest. It is evident then that in the former case the tendency would be to form a fairly uniform deposit on the bed-rock, while in the latter case the distribution would be very irregular, but exceedingly rich.

The occurrence of gold in gravel deposits may be still further complicated by the crossing of rivers, the gravel deposits formerly built up being washed out and redistributed by the subsequent action of another river running in a somewhat different direction, which action may be repeated a number of times, with added confusion as to occurrence of gold-content and thickness and character

1 U. S. G. S., 18 Ann. Rept., Pt. 3, pp. 375–377, 1896–97.