72 KEMP'S ORE DEPOSITS. 



Thus the lump magnetite of the Chateaugay mines, in the 

 northern Adirondacks, affords but 50$ iron, yet it is mined and 

 shipped forty miles to Plattsburg, and thence four hundred miles to 

 the furnaces. It has 18.44 SiO 2 and only 0.029 P and 0.052 S. Late 

 operations in the siliceous specular hematites of Pilot Knob, Mo., 

 have utilized low-grade ore rejected in former years. Other in- 

 stances could be cited. On the other hand, a moderate amount of 

 phosphorus is not only no drawback for ordinary foundry irons 

 and such as are to be subjected to tool treatment, but it is a prime 

 necessity. Excessive amounts are desired only for weak but very 

 fluid irons. Considerations like these, which are rather metallur- 

 gical than geological, largely determine the availabilty of a deposit, 

 and to some extent the present locations of the mining districts. 



2.01.03. Iron itself is one of the most abundant and widely dis- 

 seminated elements entering into the composition of the earth. A 

 careful estimate of the probable average composition of the outer 

 crust of the earth afforded Alexander Winch ell (Geological Studies, 

 pp. 19, 20) the results in Column I. Prestwich, the English geologist, 

 gives the figures in Column II. (Geology, Vol. I., p. 10.) 



I. II. 



45 50 



Si 25 Si 25 



Al 10 Al 10 



Fe 8 Ca 4.5- 



Ca 6 Mg , 3.5 



Na 2.5 Na 2 



C, H, S, N, C, Mg, etc.. 1.5 C, Fe, S, Cl 2.4 



K 2 K 1.6 



100. 99.0 



The figures were obtained by taking the general prevalence of 

 the common rocks and estimating on their known compositions. 

 In WinchelPs estimate iron is fourth in abundance, while in Prest- 

 wich's it is ninth. Either case illustrates its great abundance and 

 wide distribution, although the former with much greater em- 

 phasis. 



2.01.04. A general comparison of the tabulated analyses of ig- 

 neous rocks (See Roth's Gesteinsanalysen and Allgemeine Chem. 

 und PhysikaL Geologie) ^hows that the granites contain 0.0-7$ 

 iron oxides, the porphyries 0.0-1 4#, the rhyolites 0.0-8$, the di- 

 orites and diabases 4-16$, the andesites 3-15$, the basalts 12-20$. 



