1 882. IT Trans. N. V. Ac. Set. 



since they may all have a shallow water origin, deposited by marine 

 currents differing in force and direction at different times. The absence 

 of such mechanical deposits from the sedimentary formations is not 

 established, since even some of the hematites and bog ores may re- 

 present the results of oxidation and hydration of original magnetite 

 sands ; while, on the other hand, the huge beds of menaccanite found in 

 Canada and elsewhere, which have not yet found their parallel among 

 the sedimentary strata, may indeed show a certain difference of conditions 

 during the deposit of the ancient crystalline rocks. The comparative 

 regularity of the thin alternating laminae, which make up the ferruginous 

 jasper schists of Huronian terranes, appears analogous to that every- 

 where prevailing in a cross section of beach sands along our oceanic 

 border ; while the mountain masses of this rock, with laminee of magne- 

 tite, or of octahedra of martite after magnetite, or of hematite in triangu- 

 lar scales after martite — all these ores being remarkably free from cal- 

 cium phosphate — are most simply explained by the accumulations upon 

 a sea bottom, strewn alternately with fine silicious silt, and with octahe- 

 dra of magnetite. The presence of manganese oxide in iron ores may 

 often be due, as at the Buckhorn mine m North CaroHna,* merely to the 

 decomposition of a manganese garnet originally concentrated as a me- 

 chanical sediment, while the concentration of titanic acid and of alu- 

 mina, in many iron ores, is perfectly explained by the accumulation of 

 heavy sands of menaccanite, corundum, etc. In a magnetic iron ore of 

 S. G. 4.5, occurring in serpentine at South Ham, Quebec, Canada, the 

 percentage constitution reported t(Fe 44.69, Cr 'O^ 8. 31, Ti O'' 21.64^ 

 is equivalent to a mixture of three minerals known to occur in iron 

 sands: magnetite, 50, chromite, 13, and menaccanite, 37 per cent. 



The consideration of these facts would lead to the conclusion that 

 the mode of genesis of a bed of magnetic iron ore may be often deter- 

 mined by the following diagnosis : 



When the ore presents structural characteristics allied to those of 

 limonite, or when hematite occurs in included masses, or when the ore 

 contains a notable amount of phosphorus, a chemical origin is prob- 

 ably indicated. 



When the ore is almost free from phosphorus or is rich in titanic or 

 chromic acid or alumina, or closely associated or mixed with granular 

 garnet or olivine, a mechanical origin may be inferred. 



Mr. N. F. Darton stated that he had found over two per cent, of 

 chromic acid in the limonite ores of Staten Island, and Mr. N. L. BrIT- 

 TON, from 1.6 to 3 per cent, in the ores of the same region. 



* W. C. Kerr, Geol. of N. C, I. 222. 



t B. J. Harrington, Can. Nat., 1881, IX, 309. 



