254 THE PHYSICAL MANIFESTATIONS OF WEATHEKINa 



stream has left behind, and which afterwards, when the water 

 has changed its course, has been again covered by new layers of 

 mud, clay, and sand. It is this gravel bed which the natives 

 call nellan, and from which they chiefly get their treasures of 

 precious stones.^ The same process in states bordering along 

 the Appalachian Mountain system in North America has given 

 rise to auriferous sands, as well as to sands bearing monazite, 

 zircons, and other valuable minerals, which become segregated 

 merely through their greater density and power to resist decom- 

 position. The stream tin ores of the Malayan Peninsula, the 

 diamond-bearing gravels of Brazil, and indeed placer deposits in 

 general are illustrative of this same principle. The very soil 

 itself, although so indispensable to human existence, is but an 

 incidental and transitory phase of rock-weathering, as has been 

 made sufficiently apparent in previous pages. The deposits of 

 kaolin in western Pennsylvania and nothern Delaware, as else- 

 where noted, are but decomposed highly feldspathic gneisses 

 and conglomerates, while the phosphate deposits of middle Ten- 

 nessee are insoluble residue left by the leaching out of the cal- 

 cium carbonate from phosphatic limestones.^ 



The Clinton iron ores of Alabama are, according to I. C. 

 Russell,^ insoluble residues left by the leaching out of the lime 

 from a ferriferous limestone. The same agencies that were 

 instrumental in bringing about the corrosion of the Shenandoah 

 limestones of Virginia transformed the disseminated sulphides 

 of zinc into carbonate and silicates and left them to accumulate 

 with the clay residue in the irregular pits and cavities with which 

 the surface abounds. Here again weathering has acted as a proc- 

 ess of concentration and rendered available ores originally too 

 widely disseminated to be of value. A perhaps still more im- 

 portant illustration is offered in the secondary enrichment of 

 ore bodies, and particularly those of copper, through a down- 

 ward leaching, by meteoric waters, and a redeposition of the 

 dissolved material at the permanent water level. It is by this 

 same leaching action on aluminous limestones that is formed the 

 so-called ''rottenstone'' so commonly used in polishing brasses 

 and other metals. 



^ Nordenskiold, Yoyage of tlie Yega. See also Judd, On the Eubies of 

 Burma, etc., Pliilos. Trans. Boyal Soe. of London, Yol. OLXXXYII, 1896, 

 p. 151. 



2 J. M. Safford, American Geologist, October, 1896, p. 261. 



3 Bull. 52, U. S. Geol. Survey. 



