1034 A TREATISE ON METAMORPHISM. 



zinc of the Mississippi Valley, from which 90 per cent of the zinc and more 

 than 20 per cent of the lead of the United States are mined, have certainly 

 not derived their metals directly from igneous rocks. It is rather probable 

 that the metals for these deposits, originally in rocks of unknown origin, 

 have been taken into solution in the belt of weathering, have been trans- 

 ported to the sea, and there precipitated in minute quantities at the time the 

 magnesian limestone formations of the Mississippi Valley were built up. 

 The immediate source of the metals for the present deposits is agreed by 

 all who have closely studied them to be the Cambro-Silurian limestones. 



The conclusion is reached that these rocks are really the source of the 

 lead and zinc, because almost everywhere through the great Cambro-Silurian 

 limestones small amounts of these metals are found. In cracks and crevices 

 at innumerable localities there are very minor segregations of galena and 

 blende, but not in sufficient amount to constitute ore deposits. Further- 

 more, Robertson has made analyses of the fresh solid Cambro-Silurian 

 limestone where it showed no galena or blende to the eye, and very small 

 amounts of lead and zinc were found." 



Thus the reasons leading to the conclusion that the Cambro-Silurian 

 limestone is the source of the lead and zinc ores of the Mississippi Valley 

 are precisely the same as those leading to the conclusion that the Keweena- 

 wan lava is the main source of the copper deposits of the Lake Superior 

 region. 



If these ore deposits in limestones are derived from the sediments, 

 why should it be assumed, in cases of ore deposits in sedimentary rocks, 

 especially in limestones cut by igneous rocks — as in the copper districts 

 of Arizona and New Mexico, in the Leadville, Tenmile, and Aspen 

 districts of Colorado, in the Mercur district of Utah, in very many other 

 places in the United States, and at a greater number in other parts of 

 the world — that the associated igneous rocks, frequently subordinate in 

 volume, constitute the sole source^ of the metals lead, copper, gold, zinc, 

 and iron! 



The foregoing facts show that the source of the metal for an individual 

 district is not to be ascribed a priori to igneous, sedimentary, or metamor- 

 phic rocks, but can be determined only after an inductive investigation of 

 the facts. The metal of a district may be derived from the late igneous 



"Winslow, Arthur, Lead and zinc of Missouri: Missouri Geol. Survey, vol. 7, 1894, p. 480. 



