The iron ores. Hematite. Pyrite.] 



MINERALOGY AND PETROLOGY. 



965 



FIG. 55. PRIMARY AND SECOND- 

 ARY MAGNETITE IN ROCK NO. 

 1C. 



a. Dense magnetite, originaK ?) 



b. Secondary magnetite, in the lamina- 

 tions of altered pyroxene. 



c. Changed pyroxene (diallage) . 



d. Unchanged pyroxene (diallage). 

 f . Feldspar grains. 



line forms of magnetite (a) are surrounded by feldspar (bytownite) and must have 

 had an early date in the formation of the rock. The secondary magnetite is arranged 



in sheets apparently coincident with the lamellation of 

 the diallage, and penetrated the diallage in proportion as 

 decay of the diallage went on. This alteration probably 

 dated from the cooling period of the rock. It would be 

 interesting to know what relation subsists between the 

 titanium content and the primary or secondary nature 

 of the magnetite, but no investigation has been directed 

 to that point. From the fact that only a trace of titanium 

 is reported by Prof. Dodge* in the analysis of No. 1, where 

 most of the magnetite is of secondary origin, and that 

 Mr. J. H. Kloos reported that he found "no trace of 

 titanium " in the gabbros at Duluth, it appears that tita- 

 nium must be of the magnetite dating from the earliest 

 generation. The general absence of leucoxene in No. 1 

 also accords with this. 

 Magnetite, therefore, so far as it constitutes rock masses in the Archean, or in 

 the gabbro, is the result of secondary causes, and was concentrated probably from 

 other minerals (largely hematite) by some unusual forces acting at epochs of great 

 metamorphism, applied locally to some preexisting rock. Such forces have long ago 

 ceased to operate at those localities, and therefore magnetite is no longer in process 

 of formation, but rather of destruction. 



Hematite. The oldest recognizable oxide of iron, aside from the magnetite (or 

 ilmenite) of the igneous greenstones of the Archean, is hematite. It constitutes 

 masses of great economic value in the Keewatin, as seen at Soudan, Ely and else- 

 where. It is the ore of the jaspilyte, with only microscopic exceptions, where jaspi- 

 lyte becomes so rich in iron as to be called ore. In the Keewatin it has been pre- 

 sumed by some to be secondary after siderite, and this hypothesis has been applied 

 extensively to explain the origin of the hematite ores of the Taconic. Microscopical 

 examination has shown clearly, however, that it existed prior to the associated 

 siderite, earlier than the magnetite and earlier than pyrite, all of which are found 

 to have had a very early origin in those ores. This is discussed in connection with 

 siderite, foregoing, and it is only necessary at this place to call attention to the 

 prior existence of hematite in the jaspilyte of the Keewatin. This is seen in Nos. 

 903 and 907, and better still in Nos. 1565 and 1961. 



Pyrite. The sulphide of iron at no place in Minnesota, so far as known, is so 

 abundant as to constitute rock masses. It is, however, very widely distributed, 



