686 THE JOURNAL OF GEOLOGY. 



blue quartz, fills cracks in the underlying aspe wedge, often in 

 directions transverse to the banding (Fig. 4). Some of these 

 cracks may be traced for several feet back from the contact. In 

 many places the general course of the contact cuts the banding 

 of the jasper of the wedge at a considerable angle. Altogether 

 the conglomerate is unmistakably basal, and was clearly laid down 

 upon an irregularly eroded surface. 



From this contact for about sixteen feet to the east the 

 jasper wedge comes in. The great mass of this rock cannot be 

 distinguished, either by the eye or under the microscope, from 

 the specular jasper of the Lower Marquette iron-formation. It 

 is very rich in iron ore, the jasper bands often showing the tend- 

 ency to break up into oval-shaped units that indicates an 

 advanced stage towards concentration. Occasionally, however, 

 within the solid body of the jasper, small patches of quartzite 

 may be observed, which occur singly, or several together along 

 the same general line. These patches are not interbanded with 

 the jasper bands, but often cut them off squarely at their mar- 

 gins (Fig. 5). They recall the sand-pockets which occur 

 similarly in the banded ore at the Mountain Iron Mine on the 

 Mesabi range. 1 Here, doubtless, as there, the sand has been 

 introduced from above, at a time subsequent to the development 

 of the banded structure. 



With the exception of these rare quartzite pockets the jasper 

 wedge contains no foreign fragmental material. Through it, 

 however, run several fault lines^ which are indicated by narrow 

 breccias, the materials of which are entirely derived from the 

 jasper itself, and by displacement of the jasper bands at the 

 walls. Some of these breccias run nearly parallel with the 

 banding, while others cut across it at considerable angles. 



The jasper wedge terminates to the east in a very distinct 

 fault breccia some five feet in width, at its widest point north 

 of the Thompson pit. This breccia consists of large angular 

 pieces of jasper (one measured one by three feet), of quartzite, 

 of quartz and of iron ore, and many pebble-like forms of all 



1 Compare Bulletin No. X., Minnesota Geological Survey, p. 205, Fig. 20. 



