SEGREGATION OF CHEMICAL PRECIPITATES. 1037 



DIVISION A. ORES PRODUCED BY PROCESSES OF SEDIMENTATION. 



Since ores produced by processes of sedimentation do not properly 

 fall within the scope of a treatise on metamorphism, they will be considered 

 only to the extent necessary to show their relation to the division of ores 

 somewhat fully discussed — that produced by processes of metamorphism. 



The ores produced by processes of sedimentation may be divided into 

 two groups — those formed by chemical precipitation, and those formed by 

 mechanical concentration. 



ORES FORMED BY CHEMICAL PRECIPITATION. 



Ores produced by chemical precipitation comprise some of the iron, 

 manganese, and aluminum deposits. 



Bog deposits of iron ore, of recent origin and doubtless in some cases 

 within older rocks, are the direct results of chemical precipitation. The 

 processes by which these develop — the leaching of the metal from the land 

 areas', and its transportation and precipitation as limonite in shallow bodies 

 of water — have all been fully treated in connection with the development 

 of iron carbonate and limonite (see pp. 824-829, 842-845), therefore the 

 process will not be here again described. Some manganese deposits are, 

 in part at least, the direct result of chemical precipitation, the lines of 

 development being the same as those of limonite. 



Hayes holds that certain bauxite deposits also are the result of chem- 

 ical precipitation. According to his views the aluminum for some of the 

 bauxite deposits of Arkansas was taken into solution by underground water, 

 was brought to the surface by springs which issued into a shallow sea, 

 where the aluminum and iron oxides were chemically precipitated. (See 

 Chapter XI, p. 985.) Since these deposits are considered in the chapter 

 on the redistribution of the elements nothing further will be said here in 

 reference to them. 



Aluminum and iron are the most abundant of the metals, while man- 

 ganese is rather plentiful. Again the law of mass action applies. The 

 abundant metals are those which are likely to be thrown down as chemical 

 .sediments in sufficient amount to constitute ore deposits. 



