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supply practically the entire amount of platinum and most of the tin of the world. In the case of the latter metal, however, instead of being in a pure or native state, it is found in the form of tin ore or the so-called "tin stone" (oxide of tin), which, like the pebbles of phosphate rock already described, is soluble with great difficulty, and is therefore left behind with the other insoluble matter, such as the ordinary quartz and other pebbles or sand with which it is associated.

Diagram showing Origin of Auriferous Gravel.

These stratified beds of gravel or so-called placer deposits are of such vast commercial importance that they deserve to be treated of in some detail, and classified by the name of the metal for which they are worked. The description above given is sufficient to give a general idea of what they are.

Gold Placers. As above stated, a very large portion of the gold of the world is derived from this source. In our western country gold to the value of hundreds of millions of dollars has been taken from various placer deposits which have been worked with varying success since the discovery of gold in this form in California. These placer gravels have been in some cases consolidated into hard rock, forming a bed of grit, or conglomerate, as it is called. But usually these gravel beds are as yet in an unconsolidated state, and are therefore easily washed down by the action of powerful jets of water. The accompanying illustration gives an idea of how such a gravel bank is attacked. They are often worked, however, in other and very much more primitive ways. In many cases the gravel is overlaid by a flow of lava, - which is of course now in the condition of hard rock, and has in itself prevented the washing away of many important gold bearing gravel beds. An illustration of this is also given to show two things: first, the great amount of material removed by erosion; and, second, that the present mountain top was once the bottom of a river, the valley of which was afterward followed by an outpouring of lava. No doubt these placers give rise to other

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placers; but it is certain that the minute seemingly water worn particles, or perhaps the larger nuggets of the gold, were originally in this form or another in veins, great or small, that is to say, in place in the solid rock which formed a part of the mountain range.

It has been suggested, however, and probably with some truth, that the minute nuggets of gold, or even those of considerable size, which are extracted from placer deposits have been formed in the gravel in their present position by precipitation from the creek or river water containing the gold in solution. If it is admitted that gold is even slightly soluble in the water which most creeks and rivers contain, then this theory would have much to recommend it. It seems remarkable that these nuggets of gold, sometimes of very considerable size, are very rarely met with in vein mining, though abundant in placer gravels. Sometimes there is found what is known to the placer

Section of Table Mountain, California, showing Old River Gravel covered and protected from Erosion by a bed of Lava (Whitney). Later River Gravel shown on either side.

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Flume and method of attacking Gravel Bank in Hydraulic Mining (9th Ann. Rep. State Mineralogist, California).

miner as "wire" gold, the edges of which are very sharp, and do not show any evidence of having been rolled or abraded by the action of water and sand. There seems to be every reason to believe that the so-called "wire" gold of the placer miner was deposited in its present position in the gravel or sand surrounding

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it, from waters which had derived the gold from a section of the country nearer the source of the stream traversed by gold bearing and usually pyritiferous veins. If this is admitted, it is easy to understand how what may possibly be small nodular segregations - which we know as grains or nuggets may have been formed.1 The fact that the richest portion of the deposit is near or upon bed rock, or near or upon a "false" bed rock, is explainable by this theory. This view of the disposition of gold in placer gravels finds favor among many scientific men; but it is not necessary to discuss it further.

It does not necessarily follow that the discoveries of rich vein deposits nearer the source of the stream (present or ancient) will follow upon the discovery of a rich placer ground below; for, as can be readily imagined, a great number of small veins, each in itself too small to be profitably worked, may yield when eroded a large amount of gold in the aggregate, and form a very much more valuable placer ground at varying distances below the position of the veins. It is simply nature's way of concentrating gold, - an accident which has been taken advantage of by man.

There are some rocks (conglomerates, grits, and sandstones) which now contain a great deal of gold, copper, etc., but which certainly did not contain these metals when they were laid down as sediments. These rocks are in the majority of cases tilted or turned on end, and some deep break has occurred either in them or in close proximity to them; so that the waters or vapors containing these metals in solution, and which have subsequently ascended through or near them, found in them a suitable repository for their solutions. The famous Calumet and Hecla copper conglomerate offers a good example, as well as certain copper mines in Bolivia in sandstone, and some of the gold in South Africa is said to occur in much the same way. That is to say, it is found filling some of the interstices between the pebbles in a way that makes it very probable that it got into the stratum subsequently to its consolidation and being tilted, — probably to its present position, and not as grains and nuggets when the deposit was accumulating. The writer has seen a deposit of somewhat similar character to this in South America, and there

1 It has been pointed out, on the other hand, that these grains and nuggets of gold do not present the round or peculiar

mammillated appearance of many nodular segregations.

is no doubt that it is of more common occurrence than is generally supposed.'

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Section showing Auriferous Gravel overlaid by Lava, also effect of Erosion since the outpouring of the Lava (9th Ann. Rep. State Mineralogist, California).

These latter deposits would very probably in the majority of cases (always providing that the ore of valuable metals in them is reasonably continuous) be regarded from a legal point of view in the same light as fissure veins or as lodes, and under our United States statutes should be so located. They are mentioned simply in order that no confusion may arise. It is usually very easy to determine whether they were originally placer gravels, or whether the metals which they contain found their way into them subsequently and after consolidation and upturning of the rock. In California it has been decided that an old bed of placer gravel, now covered by a capping of lava and tilted so that the gravel bed dips at a considerable angle, has nevertheless not lost its distinctive character as placer ground, and should be located as such, and that therefore a location upon it as a lode is improper.

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Platinum Placers. As above stated, practically all the platinum of the world is derived from deposits of gravel or placer ground. This country supplies only a small quantity of this valuable metal, the great majority of the supply coming from the Ural Mountains in Russia. It is found, however, in small quantities in Northern California, Oregon, Washington, throughout British Columbia, and in small quantities in South America, nearly always associated in these localities with gold in the gravel. Quite a good deal of iridium is found along with the platinum, but it possesses at present comparatively little commercial importance. As in the

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case of gold, platinum is found in small seemingly water worn and irregularly rounded nuggets or minute grains, which have certainly been derived from some vein or other deposit in the solid rocks further up the stream, and perhaps have been worn to their present shape by the action of water, or rather, to speak more accurately, by being rolled for ages by the force of mountain torrents along with the grains of sand and the gravel, admixed with which they are found.

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The same theory, however, which has been suggested with regard to the formation of gold placers (see discussion pages xciv and xcv) may apply equally to the formation of platinum placers.

Tin Placers. — A very considerable portion of the tin of the world is found in gravel deposits, and is known as "stream tin." It has always certainly been derived, like the foregoing, from some vein or other class of deposit carrying tin ore in the solid rock formation further up the stream,- in the present or in the old bed of which the tin stone is found. The other two metals, gold and platinum, are always found in their native state, simply because they usually occur in this state in nature. Tin, however, as has been pointed out, does not occur native, but as tin ore, usually as tin oxide, which is very insoluble. It is therefore retained in the gravel below the tin veins or other tin deposits. Like the former, it is always very much water-worn, but also like them it can be readily separated from the pebbles or sand with which it is associated, owing to its greater weight.

II.

UNSTRATIFIED MINERAL DEPOSITS.

We have now considered all of the principal kinds of deposits, which, for the sake of convenience, we will call stratified or sedimentary mineral deposits, and to which the common law maxim of cujus est solum ejus est usque ad cœlum always applies when the owner of the soil is the same as the owner of the mineral rights; that is, when there has been no severance of these estates. Although, as we have seen, they may not always be technically stratified, or in a geological sense sedimentary in character, they are such under the legal classification adopted in this book. In

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