552 THE MAEQUETTE IRON-BEARING DISTRICT. 



the quartz of the original grains is almost entirely gone. It is an inter- 

 esting inquiry, upon which, however, little direct evidence can be brought 

 to bear, as to how far the concentration in the conglomerates has depended 

 upon the new growth of iron ore about rolled nuclei of iron sand. Bearing 

 possibly upon the question is the fact that the crystalline plates of specular 

 hematite are frequently and perhaps always coarser in the slate ores that 

 occur in the contact zone than in those belonging lower down within the 

 iron-bearing member. 



The process of concentration in the Republic area has not proceeded 

 indiscriminately throughout the iron-bearing member. The distribution 

 of the important ore deposits shows that it has been localized in accord- 

 ance with certain physical conditions. The main facts of distribution are 

 (1) that the ore bodies occur within or not far below the contact between 

 the upper quartzite and the iron-bearing member; (2) that they occur in 

 pitching synclines in the vicinity of the greater orogenic disturbances; 

 (3) the larger bodies usually have a basement of soaprock. These relations 

 of distribution are so constant that they must be regarded as necessary 

 conditions. It is evident that the first two conditions were such as to pro- 

 mote comparatively free circulation. In the contact zone the loose textvire 

 of the conglomerates afforded connecting open spaces through which waters 

 could readily pass It is equally evident that the general breaking up 

 attending sharp folding in the undei'lying iron formation w^ould not only 

 open channels for percolating waters but would also rediice the siliceous 

 bands to a condition in which they could be readily attacked. The third 

 condition was favorable to the concentration of the iron-bearing percolating 

 waters. 



From the relations which the ore deposits bear to the structure produced 

 at the time of the later folding, it clearly appears that much of the concen- 

 tration has been effected since Upper Marquette time; but it does not follow 

 that some of the iron oxide of the deposits was not already in existence at 

 the time of the Upper Marquette transgression. If, as there is strong reason 

 for believing, the specular character of the hematite of the jaspers and of 

 the rich ore deposits of the Republic area is connected with differential 

 movements of bed on bed, produced at the time of the latest folding, it is 



