METAMORPHISM OF SEDIMENTARY ORES. 1043 



process which is concerned in their segregation. Almost any one of these 

 deposits illustrates the idea. For instance, a sedimentary deposit of bog 

 iron or manganese is finally concentrated by precipitation of these materials 

 in a marsh or lagoon. But the earlier stages of the work of segregation are 

 by underground water, and if this part of the work were considered the ores 

 would be classified as metamorphic. Again, the mechanical concentrates 

 are not mainly segregated from the sparsely dispersed metal in the original 

 rocks, but are segregated from previous segregations, which may be the 

 result of igneous, sedimentary, or metamorphic processes, or some combina- 

 tion of the two or three. To illustrate, the gold placers of the Sierra 

 Nevada chiefly derived their gold from the vein deposits of the same 

 mountain system. The segregation of the gold within these veins is 

 the most important part of the process and it will be considered under the 

 work of underground water, but the last process — mechanical concentra- 

 tion — produced the placers and it must be taken as the basis of classi- 

 fication. Similar statements could be made with reference to the residual 

 cassiterite of the Malay Peninsula or the residual iron ore of Iron Mountain, 

 Missouri. 



DIVISION B. ORBS PRODUCED BY IGNEOUS PROCESSES. 



Ores produced by igneous processes, like those produced by processes 

 of sedimentation, do not properly come within the scope of this treatise 

 upon metamorphism. They will, therefore, be considered only so far as 

 necessary to show their relation to the ores produced by processes of meta- 

 morphism. 



Ores directly produced by igneous processes are regarded as a small 

 division by those who have most closely studied them. The recent advo- 

 cates for a direct igneous origin for certain ore deposits include Vogt, Beck, 

 Kemp, Spurr, and others. Emmons" has also favored the idea of at least a 

 first concentration of the metallic contents of ores by processes of differen- 

 tiation of igneous rocks, more particularly the basic rocks. Spurr has 

 recently well summarized the principles which result in the segregation of 

 ores by magmatic processes. 6 



"Emmons, S. F., Geological distribution of the useful metals in the United States: Trans. Am. 

 Inst. Min. Eng., vol. 22, 1894, pp. 53-95. The mines of Custer County, Colo. : Seventeenth Ann. Rent. 

 U. S. Geol. Survey, pt. 2, 1896, pp. 470-472. 



*Spurr, J. E., A consideration of igneous rocks and their segregation and differentiation as related 

 to the occurrences of ores: Trans. Am. Inst. ilin. Eng., vol. 33, 1903, pp. 2SS-341. 



