134 THE CRYSTAL FALLS IRON-BEARING DISTRICT. 



ellipsoid from arounil which it was broken. This is essentially an exceed- 

 ingly fine-grained quartz rock, with chlorite flakes and black ferruginous 

 specks scattered through it, and here and there an irregular oval siderite 

 grain remaining, ^^er3'^ few and unimportant grains of epidote were also 

 noticed. This rock represents nearly the last stage in the process of silici- 

 fication bv which the siderite has been replaced, and a part, probably the 

 greater part, of its iron content oxidized. Some chlorite and epidote has 

 been produced, clearly from the lime and magnesian impurities in the 

 siderite. Essentially the same process of silicification has been described 

 bv Van Hise in his various articles on the Penokee-Gogebic and Marquette 

 iron ranges, to which references have been so frequently made. I have 

 desired especially to call attention to it here, however, on account of the 

 fact that it shows the possibility of the production of an iron ore from an 

 original eraptive rock by the combined processes of carbonation and silici- 

 fication. It is true that the end product in the case described does not 

 contain enough iron to be an ore deposit, but that is a mere detail. May 

 not this serve also to some extent to explain the numerous clearly marked 

 belts of magnetic attraction which occur throughout this area of altered 

 basalts, in which little of the original magnetite remains unaltered to exert 

 an influence upon the magnetic needle"? To explain this we must suppose 

 the influence to be exerted by secondary magnetite accumulated along cer- 

 tain lines. The magnetic lines traced out agree in a very marked way 

 with what has been determined to be the trend of the lava flows and tuff 

 beds. The condition which would determine the presence of such a line 

 of carbonation, if we may so put it, may be the presence of a scoriaceous 

 lava flow or a bed of tuff, which offers exceptional facilities for the passa'ge 

 of carbonate-bearing waters. It is thus intimated that there is possibility 

 of finding purely local ore bodies of small size even in the midst of this 

 volcanic area. 



The process of silicification is generally considered as a deep-seated 

 one, occurring far below the outer weathering zone. 



When the rocks exhibiting these various phases of silicification are 

 exposed in the zone of weathering, certain interesting results are obtained 

 which are worth noticing. Rocks are produced from these which upon the 

 surface strongly resemble amygdaloids, but in which the pseudo-amygda- 

 loidal cavities are of purelj'' secondary origin. For instance, when the 

 siderite mass has become only partially replaced by silica, weathering 



