284 THE VERMILION IRON-BEARING DISTRICT. 



process of folding was inaugurated between Archean and Lower Huronian 

 time; but since the present attitude of the troughs in which the main ore 

 deposits are located was mainly produced by the folding of the Lower 

 Huronian, the replacement certainly occui'red, for the most part, after 

 Lower Huronian time. Since there are no pre-Cambrian deposits later 

 than the Lower Huronian in this part of the district, the determination of 

 the time of the replacement process can not be more accurately made. The 

 process begun shortly after Lower Hiironian time doubtless has continued, 

 perhaps with interruptions, to the present time. 



METHODS OF MINING IN THE VERMILION DISTRICT. 



All of the ores of the Vermilion district are at present obtained by 

 means of underground workings. The underground work follows one of 

 two systems — either that known as the "overhand stoping" system, or 

 that known as the "caving" system. Both systems have been modified in 

 certain particulai-s, according to the peculiarities of the deposit or in 

 agreement with the ideas of the management as to the most economical 

 methods of exploitation. It would therefore be impossible to give in this 

 place detailed descriptions of all the methods in use, as this would iuA^olve 

 practically a description of the system followed in every mine in the 

 district; but a brief space will be devoted to a description of the methods 

 used at two of the typical deposits — the mines at Soudan, which use a 

 system of overhand stoping, and the Chandler mine, of Ely, which is 

 exploited by means of the caving system. 



Li the mines at Soudan the system of overhand stoping is best 

 developed, and therefore these mines, and especially the workings of No. 

 8 shaft, may be regarded as a type of this system. The description which 

 follows is in part an abstract of papers by Bacon" and by Denton'' and of 

 the statements of Mr. F. Ahbe, sometime milling engineer in charge of the 

 mine, and in part is the result of personal observation by the author. 



The ore at Soudan, which is a hard, blue hematite, occurs in great 

 irregular bodies of more or less lenticular shape, which dip steeply to the 



"Development of Lake Superior iron ores, by D. H. Bacon: Trans. Am. Inst. Min. Eng., Vol. 

 XXI, 1892, pp. 299-304. 



b Elements of methods of metal mining based upon Lake Superior practice, by F. W. Denton: 

 Engineers' Year Book, University of Minnesota, Vol. IV, 1896, pp. 49-67. 



