226 THE VERMILION IRON-BEARING DISTRICT. 



the intensely fractured rocks of the u-on formation. Through these frac- 

 tiu-es downward-percolating water carried the ore and deposited it upon the 

 impervious bottom of the troughs formed by the intercalated igneous sheets. 

 Hence it comes about that the ore deposits, being derived fi'om only the 

 small quantity of iron formation which occurs between these adjacent 

 sheets, are relatively small, and for the most part disconnected. They 

 occur, however, in a syncline, as does the enormous Chandler-Pioneer 

 deposit of Ely, the essential difFerence being that at Soudan the ore body 

 in the syncline has been separated into a number of irregular bodies, the 

 one above the other, instead of occurring in one continuous ore deposit. 



One might infer from the above statement that these sheets were 

 introduced very regularly into the u'on formation. This would be an incor- 

 rect inference. Anyone familiar with igneous phenomena knows that the 

 dikes and sheets divide more or less frequently at irregular intervals. They 

 have done so in Soudan Hill, so that we may find deposits joining other 

 deposits as a result of the disappearance of the subdividing sheet as we go 

 away from the point where it leaves the main mass, or a large deposit will 

 divide into two or more small ones as a result of the introduction of such an 

 offshoot from a sill. Hence there may be a very remarkable irregularity in 

 the occurrence of the ore deposits formed in such synclinal basins where the 

 impervious bottom is due to the presence of intrusive sills. A further 

 cause of irregularity is the introduction of the more or less vertical dikes 

 which were contemporaneous with the introduction of the sills. These, 

 cutting through both sills and the associated iron formation, have still 

 further tended to subdivide the rocks into masses of varying size. Fur- 

 thermore they, like the iron formation, were much folded, and now occupy 

 various positions and are of greater or less importance in determining the 

 size of the ore bodies. Thus, for example, if a dike should have cut across 

 a sheet at nearly right angles, and in such a way that when the two were 

 folded a pocket with nearly impervious bottom and sides was formed, an 

 ideal condition would have been produced for the deposit of ore, according 

 to methods described by Van Hise in immerous articles to which reference 

 has already been made. The larger the pocket the larger, other things 

 being equal, wotild be the deposit of ore. 



Belonging with the ore deposits occurring within the iron formation 

 are certain small deposits of relatively slight commercial importance, but of 



