1030 A TREATISE ON METAMOKPH1SM. 



SOURCE OF THE METALS. 



The nature of the rocks which contribute the metallic salts has been 

 much discussed. With Sandberger," I have little doubt that the metallic con- 

 stituents of ores are in large part derived from the igneous rocks which have 

 been intruded into or extruded upon the lithosphere. In my paper on 

 "Some principles controlling the deposition of ores," as originally published, 6 

 I strongly advocated the idea that igneous rocks are the direct source of 

 some ores, that they are the ultimate source of all ores, and that the heat of 

 the igneous rocks is of fundamental importance in the segregation of the 

 ores. The igneous rocks as a source of metallic ores are especially impor- 

 tant at periods of exceptional volcanism. At such times there rise from 

 the lower parts of the lithosphere, and possibly to some extent from the 

 centrosphere, enormous masses of igneous rocks which are injected into 

 the zone of fracture or brought to the surface. While, daring the middle and 

 later portions of geological time the magmas which came into the zone of 

 fracture or were spread over the surface of the earth were derived from 

 unknown depth, it is entirely possible that, during the early stages of the 

 earth's history, magma very generally existed near or at the surface, and 

 that such magma may have furnished a portion of the metals which now 

 occur in ore deposits, although in such cases doubtless the metals have 

 been distributed and redistributed again and again. From the earliest 

 geological time the igneous rocks have been the original source of the 

 sedimentary rocks, although their immediate source may have been other 

 sedimentary rocks. From the sedimentary and igneous rocks metamorphic 

 rocks are produced. Therefore the original source of all ore deposits is 

 believed to be magma. 



I have further held that there is every gradation between igneous action 

 and aqueous action, or, as I have stated it "under proper conditions water 

 and liquid rock are miscible in all proportions." c I believe that there are 

 complete gradations between purely igneous pegmatitic dikes and purely 

 aqueous vein material. (See pp. 720-728.) Moreover, in previous papers'* 



« Sandberger, F., Untersuchungen iiber Erzgiinge, Wiesbaden, 1885, 2d Heft', pp. 159-431. 



(-Trans. Am. Inst. Min. Eng., vol. 30, 1901, pp. 27-177. 



''Van Hise, C. E., Principles of North American pre-Cambrian geology: Sixteenth Ann. Rept. U. S. 

 Geol. Survey, pt. 1, 1896, p. 687. 



''Van Hise, C. E., Metamorphism of rocks and rock flowage: Bull. Geol. Soc. Am., vol. 9, 1898, 

 ;>. 299. Also this monograph, Chapter VII, pp. 566-569; Chapter VIII, pp. 659-660. 



