SOUDAN FORMATION. 227 



considerable scientific interest. These deposits are those which have the 

 fii'st mode of occurrence mentioned above. They are surrounded on all 

 sides by jasper. Hence then- occurrence does not depend on the formation 

 of a synclinal trough with impervious bottom, as in the previously con- 

 sidered cases. These deposits, as a rule, form so-called chhnneys of 

 ore, having in general, as the name indicates, a rectangular outline, and 

 their origin is evidently due to the presence of fracture planes or zones 

 in the jasper. Along these zones water has percolated and has produced 

 the ore bodies from the iron formation by the well-known process of 

 replacement, the ore diminishing in richness as the distance from these 

 fractures increases. 



ORIGIN OF THE ORE DEPOSITS. 



Having now described the manner of occurrence of the ore deposits 

 and shown their relation to the geologic structure, we are prepared in the 

 light of the facts given to consider their origin. The origin of ore bodies 

 completely surrounded by jasper obviously depends on the occurrence of 

 fractures, for the ore is confined to the vicinity of the fractures and 

 diminishes in richness as the distance therefrom increases. The importance 

 of these fractures as channels for downward-descending waters is also 

 obvious. Hence the connection between the occurrence of the deposits and 

 the action of percolating water is shown. This interdependence is further 

 impressed upon one when a study is made of the cross section of the narrow 

 ore deposits of Soudan, and also of the cross section of the Ely trough. It 

 will be readily recognized that the plane of contact between the two forma- 

 tions will, as a rule, perform the same function as any large, continuous 

 fracture in the formation itself, in that the plane of contact will permit more 

 readily the passage of water, since it has a continuous line of weakness, 

 than will the small, discontinuous fractures which may exist in the forma- 

 tion. Hence the ore deposits in this case will be confined more or less 

 closely in their occurrence to this plane of contact. Where the converging 

 streams of descending waters meet, at the bottom of the trough, the action 

 has been most intense, and consequently the largest bodies have been 

 accumulated there. Theoretical considerations show that if such a trough or 

 other favorable place is to contain a large body of high-grade ore it should 



