548 THE MARQUETTE IRON-BEARING DISTRICT. 



the lower series; others again are parth' in both. In form the bodies 

 are exceedingly iiTegular. but they may be described in general terms as 

 pod-like in shape, the two horizontal dimensions being usually very much 

 smaller than the third, which follows down roughly parallel to the dip plane 

 of the quartzite, often pitching to one side of the vertical plane normal to the 

 dip plane. Of the two horizontal dimensions, the longer is usually parallel 

 with the stratification, and the shorter normal to it. 



Where the ore deposits are wholly contained within the lower series, 

 the contacts between them and the rock of the inclosing iron formation are 

 usually as follows: The siliceous bands in the ferruginous rock become 

 separated into lenses by the encroachment of the adjoining iron bands at 

 frequent intervals along their length, and grow narrower. The siliceous 

 material of which they are composed becomes mixed with a larger propor- 

 tion of the iron oxides, and as the boundaries of the rich ore are ajjproached 

 the bands separate into oval-shaped units. These finally disappear partly 

 or entirely, and the adjacent iron oxides fill the whole volume of the banded 

 rock. Such passages from the banded rock of the iron formation to rich 

 ore take place both along and across the strike. Sometimes the passage is 

 very gradual, lea\'ing a large zone of more or less lean ore between the 

 rich ore and the rock; but often it is very sudden, and the line of demar- 

 cation is sharp. Such sudden passages have been observed at the line of 

 cross joints, along which minute faul tings have taken place. It is unusual 

 to find any ore deposit, however, that is directly surrounded hj the iron- 

 formation rock on all sides. Generally on either the hanging or foot wall 

 soaprock intei'veues between, somewhere along the surface of contact; and 

 in these cases the iron formation is usually present on one side, Avhile the 

 rich ore comes up to the soaprock on the other. These bodies of soaprock 

 have already been refeiTed to as old dikes and intrusive sheets of igneous 

 material. 



Of the deposits entirely within the upper series two classes may be 

 distinguished. The first are those deposits which lie at the base of the 

 upper series, and reall}' represent an enriched, very ferruginous phase of 

 the basal conglomerate. At many localities the Upper Marquette conglom- 

 erate is made up of siliceous pebbles embedded in a cement of iron ore, part 



