PLATINUM AND ALLIED METALS.	453
	In the Urals there are two kinds of alluvia containing platinum—the alluvium of old river beds passing over the original rock, or the all avium of more or less large rivers into which these old river beds lead. In the channels at the original sources of the platiniferous rivers, there are frequently thick beds of very ancient alluvium over the surface of which the present streams flow. Very frequently this alluvium is covered by eluvium. Platinum Is usually found at the base of this alluvial bed, and the productive sands are of varying thicknesses, rarely exceeding 1.2 to 1.5 meters, aud frequently not reaching the smaller limit. From this it does not follow that there is no platinum under these beds. It is found there In small—in fact, very small—quantities. The thickness of the barren ground varies; it frequently reaches to 10 meters, and even more, so that In many places the platiniferous beds are approached by underground workings. When we go past these old river beds, frequently It is not suspected that the whole lower stratum has been opened and worked out by drifts. Rarely only do we find here and there abandoned shafts by which the alluvium was brought to the surface. The position above referred to is not general. In many old river beds open workings are exploited, and the barren ground does not go deeper than 2 to 3 meters. In some cases the scarcely observed hollow of the river bed is taken up by eluvial products which have scarcely been subjected to scouring, rather than by alluvium. On the Nizhni-Tagil placers, for example, they are operating at a very small, almost un-noticeable depth; there would be scarcely water enough to wash the material produced, and they frequently work on not particularly steep inclines, on which the eluvial products, although not channeled, are distributed in known series of river systems. In river beds lying entirely in dunite the shingle of the alluvium has been formed exclusively of it, and of the vein rocks which traverse it. Among them we find large bowlders of chrome iron ore, and the sand re­maining at the sluice box after washing consists entirely of chrome iron ore octahedrons. The alluvium of these river beds is sometimes fabulously rich in platinum, as for example, in Nizhni-Tagil and on the River Iss. The palti-num in these parts is usually large, slightly rounded and often still in in-closures of chrome iron ore. The very largest nuggets, as a rule, are found in these river beds. At present, in nearly all the original platinum deposits, the alluvium of these river beds has already been washed, and is now being washed a second, and even a third time. I have often heard that these sands under the action of the atmosphere become weathered, and that they consist in great part of dunite rock, and that the process discloses fresh platinum. This view is erro­neous * * * Pieces of platiniferous clay which were not washed the first time. * * * being subjected to the action of the atmosphere, became a friable product which might contain platinum separable at a second washing. In the platiniferous rivers not far from the original deposits, alluvium is found ar­ranged as follows: (1) In vegetive ground and peat beds of greater or less thick­ness. But the turf is absent in parts where the river bed is on a steep incline, and is almost always there where the river flows along a flat and wide valley, which in this case is usually, to a large extent, marshy. (2) Under the peat where there is barren gravel, separated from it by a thin layer of bluish clay. The thickness of this gravel varies for the different rivers between 0.8 and 2.5 meters. In exceptional circumstances it reaches 5 to 6 meters. The material referred to which constitutes it is formed of various rocks, which may differ greatly in different deposits. (3) Under gravel, the platiniferous alluvium being distinguished both in form and in color from the gravel that overlies it. The depth of the platiniferous alluvium varies between 0.8 and 2.5 meters, and seldom exceeds this limit. Under the platiniferous alluvium there lies a bed that may consist of various kinds of rock and is always friable. The platinum often sinks deep into it, so that it has to be extracted thence from a depth of 0.3 to O.S meters. When the bed consists of limestone, always more or less fissured, then the platinum falls into the chinks and accumulates there; It may go very deep—several meters below the bed of the river—and accumulate there, as in the present known pockets, which are sometimes spoken of as fabulously rich in this metal. The shingle of platiniferous alluvium varies, and, we will add, that not far from the original platiniferous center there is no sign of dunitic shingle in the alluvium. This rule may be proved over the whole of the Ural platinum deposits. It is the result of the very rapid destruction of this dunite, both during the period of the flow of water and as a consequence of the resulting decomposition. With serpentine it is different; thus in the River Visin, save for several hundred meters from the junction with the Rivers Rublevik and Zacharovka, there is no dunite in the alluvium, while the shingle