SEGREGATION OF MECHANICAL CONCENTRATES. 1039 



the adjacent islands, Banca and Billiton, produce about 25 per cent. 

 Doubtless this tin is of the same origin as that of the Malay Peninsula. 

 Thus it appears probable that substantially 75 per cent of the tin of the 

 world is derived from mechanical concentrates." 



In a small way magnetite beach sands have been utilized. Those of 

 New Zealand are best known. 6 



An excellent illustration of an iron-ore deposit produced by mechanical 

 concentration was that of Iron Mountain, Missouri, where considerable 

 masses consisting of well-rounded bowlders of hematite" were found scat- 

 tered over the mountain as a residual deposit and buried in pre-Cambrian 

 ravines under Cambi'ian and Silurian deposits. 



METAMORPHIC ALTERATIONS OF SEDIMENTARY ORES. 



No sooner are ore deposits produced by processes of sedimentation 

 than they are subject to the metamorphic processes, and thus are modified 

 to a variable extent. Sedimentary deposits, whether originally formed in 

 the belt of weathering or in the belt of cementation, later may be in the 

 belt of weathering, in the belt of cementation, in the zone of anamorphism, 

 or partly in one and partly in another. In each case the deposit is 

 subject to the metamorphic processes of the horizon in Avhich it may be. 

 Where a deposit is in the belt of weathering, the material may be dis- 

 solved to some extent and the metal carried downward into the belt of 

 cementation. If a deposit is in the belt of cementation valuable minerals 

 may be added to or taken away from it as a consequence of the work of 

 underground water. If the material is partly in the belt of weathering and 

 partly in the belt of cementation a portion of the material dissolved in the 

 belt of weathering may be carried downward and deposited in the belt of 

 cementation, and thus the lower part of the deposit enriched at the expense 

 of the upper part. If the material reaches the zone of anamorphism it may 

 there undergo the alterations of that zone. After having been subject to 

 the metamorphic effect of a certain belt or zone a deposit may be trans- 

 ferred to another belt or zone and thus the alterations of this zone be 



"Penrose, R. A. F., jr., The tin deposits of the Malay Peninsula, with special reference to the 

 Kinta district: Jour. Geol., vol. 11, 1903, pp. 135-154. 



' J Birkinbine, John, The production of iron ores in various parts of the world: Sixteenth Ann. 

 Rept. U. S. Geol. Survey, pt. 3, pp. 29-186, 186. 



"Nason, F. L., Iron ores of Missouri: Geol. Survey Missouri, vol. 11, 1S92, pp. 27-3 



