THE IKON BBAKING MEMBER. 293 



feet the duta at present are too scant to indicate. I am inclined to 

 believe, liowever, that tliey niaj' be depended npon to continue for a 

 C(.>nsiderable depth. While they may extend in imiinpaired richness and 

 magnitude to a depth as great as can be penetrated ])}■ workings, it is cer- 

 tain that they do not continue to an indefinite distance. There is also a 

 possibility that tlie deposits ma}' become somewhat poorer comparatively 

 near the surface; for it may be that percolating waters, since the termina- 

 tion of the glacial epoch, have been able to remove from the upper parts of 

 the deposits a small percentage of silica. Siich a removal, even to the 

 extent of 5 per cent or less, woiild have an important influence upon the 

 value of the deposits.' 



Emmons on ore deposits. — In thi^ connection it is of interest to com- 

 pare the conclusions reached with those of Emmons^ as to the origin of 

 the silver-lead deposits of Leadville, Colorado. He finds that the ore 

 deposits there did not form in preexisting cavities, but by a gradual 

 replacement of the rock materials by substances brought in solutions, and 

 also that tliese solutions did not come up from l^elow, l)ut liave reached 

 their immediate locus by passing downward through the rocks above. In 

 his discussion upon oi-e deposits in general he maintains that a like origin 

 is much more common than has been believed. It will be seen that our 

 own conclusions as to the origin of the iron-ore deposits of the Penokee- 

 Gogebic series, arrived at independently of the publication of Mr. 

 Emmons's monograph, are in exact harmony with his general conclusions. 



Iron ores in other imrts of lake Superior eonntry. — Before closing this 

 chapter some allusion must be made to llie nature and origin of tlie iron 

 ores wliicli are found in other districts of the lake Superior countr}'. Large 

 deposits of ore arc found in rocks remarkably like those of the Penokee- 

 Gogebic series in the Vermilion lake, Marquette, and Menominee disti-icts. 

 Recent investigations of these districts have shown that the pitching ore 



' Since the above was written development lias extended in the larger mines to a depth of sev- 

 eral linndred feet, and .as yet there is no apprcoiahle diminution in tli(^ size or richness of tlie ore 

 deposits. 



''Ennnons, Samncl Fr.anklin: Monograph IT. S. Gcol. Survey, 188t), pp. 375-379, vol. .\ii, Geol- 

 ogy and Mining Industry of Leadville. Also Structural Relations of Ore Deposits; Trans. Am. Inst. 

 Min. Eng., vol. xvi, 1S88, pji. 801-839. 



