1038 A TREATISE ON METAMORPHISM. 



ORES FORMED BY MECHANICAL CONCENTRATION. 



Mechanical concentrates may be divided into residual deposits, stream 

 deposits, and beach deposits. The term placer, originally used for stream 

 deposits, has been, in some cases, extended to seabeach deposits." The 

 principles which result in the production of residual deposits, stream 

 deposits, and seabeach deposits are the same. 



The valuable materials segregated have a high specific gravity and 

 are relatively indestructible. In consequence of their capacity to resist 

 mechanical wear and chemical solution they are segregated in alluvial 

 deposits, in the sands and gravels of streams, and on beaches of lakes 

 and oceans, because the other constituents are transported, worn, or dis- 

 solved more readily than is the resistant valuable product. In the pro- 

 duction of residual deposits the capacity of the valuable material to resist 

 solution is of dominating importance, and in the production of beach 

 and stream deposits the valuable material must have power to resist both 

 solution and wear. 



The ores produced by mechanical concentration comprise both metals 

 and oxides. The important metals belonging' to this group are gold, plati- 

 num, iridium, and osmium. Practically all of the last three metals which 

 have been exploited have been derived from placers. But of the metals 

 produced by processes of sedimentation gold is the one of dominating 

 importance. To the end of the nineteenth century a very large fraction 

 of all the gold extracted from the earth by man was derived from placers. 



The oxides which occur as mechanical concentrates are those of tin 

 (cassiterite) and iron (magnetite). For the first of these compounds the 

 placer deposits are of great importance, but for the iron oxides are relatively 

 unimportant. Although much cassiterite has been taken from veins, for 

 many years the chief supplies of tin ore have come from the East Indies — 

 the so-called Straits tin — and all except a relatively insignificant amount 

 of this cassiterite has been derived from mechanical concentrates. 6 The 

 Malay Peninsula now produces almost 50 per cent of the tin of the world; 



• «Schrader, F. C, and Brooks, A. H., Some notes on the Nome gold region of Alaska: Trans. Am. 

 Inst. Min. Eng., vol. 30, 1901, pp. 236-247. 



kRolker, C. M., The production of tin in various parts of the world: Sixteenth Ann. Kept. U. S. 

 Geol. Survey, pt. 3, 1S95, pp. 458-492. 



