Trans. N. V. Ac. Set. 16 Oct. 23, 



DISCUSSION. 



Mr. A. A. JULIEN pointed out that the theory thus presented by Dr. 

 Newberry was a new one, and might be designated as the " Lake ore 

 theory," in contradistinction from the marsh or bog ore theory now 

 commonly accepted. The difference is sharply defined in the following 

 quotations from the Manual of Geology by Prof. J. D. Dana. In his 

 discussion of the Upper Silurian deposits, that author states (p. 231) : 

 " the beds of argillaceous iron ore . . . could not have been formed 

 in an open sea, for clayey iron deposits do not accumulate under such 

 circumstances. They are proof of extensive marshes, and, therefore, 

 of land near the sea level. The fragments of crinoids and shells found 

 in these beds are evidence that they were, in part at least, salt water 

 marshes, and that the tides sometimes reached them." Again he re- 

 marks (p. 357), concerning the American continent during the Carbon- 

 iferous age : " It may have been long a region of barren marshes, and 

 in this condition it might have received its iron ore deposits, as now 

 marshes become occupied by bog ores." 



In reference to the Laurentian deposits (p. 155), he states : " Lime- 

 stone strata occurred among the alternations, and argillaceous iron ores, 

 though vastly more extensive." And again, " the argillaceous iron ore 

 has become the bright hematite or magnetite, and it is banded by or 

 alternates with schist and quartz, etc., which were once accompanying 

 clay and sand-layers." 



However, it is hard to conceive the growth of crinoids and shells in 

 salt water marshes, even if reached by the tides, in the free abundance 

 indicated by the crowded organic lorms in the Clinton ore beds, and of 

 these the lake ore theory presents a more satisfactory explanation. It 

 also well accounts for the origin of the enormous ore deposits of hema- 

 tite, of magnetite still retaining a pseudomorphous structure after 

 limonite, and of all iron ores rich in calcium phosphate. This theory 

 is not necessary to account for the presence of pyrite, since the presence 

 of any vegetable matter, e.g., algae growing and decaying in sands or 

 mud, is always likely to produce the de-oxidation of sulphates and iron 

 oxide ; nor does it account at all for the abundance of titanic acid and 

 alumina in many iron ores. 



On the other hand, the mechanical accumulation of iron sands, by agi- 

 tation of the waves and currents, would appear to account more simply 

 for the smaller but more numerous deposits thickly dispersed through- 

 out the crystalline rocks, varying in size from scattered grains to little 

 flakes and lenses a few centimeters in length, and even thick ore beds 

 many meters in thickness and diameter. The close association, often 

 observed, of materials of widely different specific gravity, e. g., shale 

 and limestone with the magnetic ores, need not appear anomalous. 



