MUSEUM OF COMrARATIVE ZOuLOGY. 31 



Intended in his statement, as these dctrital rocks apparently form but 

 a small portion of his " quartzites." These of course mark old beaches 

 water-worn after the jasper and ore were in situ, in nearly their present 

 condition, and, if the logic of the geologists of the Michigan and Wiscon- 

 sin surveys were carried out, these unconformable detrital formations 

 would mark a new geological age. 



One difficulty feund in mining the iron ore has arisen from the schist 

 being found in large masses, broad at the upper part of the mine, but 

 tapering out to thin wedge-shaped masses below, which are left without 

 support when the ore is removed. This renders one wall, and some- 

 times both, unsafe, no one foreseeing when the support to the treacher- 

 ous schist will be removed. This structure evidently is consonant with 

 the theory of the eruptive origin of the jasper and ore. They break 

 obliquely up through the schist, and send off branches, which, pursu- 

 ing the same general course, leave wedge-shaped masses between them 

 and the trunk. The ore when removed allows that whicli has been 

 supported by it to fall. This very cutting across the lamination, how- 

 ever slightly, would tend to let all severed masses slide out, even if 

 they were cut on one side only. Figure 4, taken from an actual 

 section in an old working at the southeastern end of the Cleveland 

 mine, Ishpeming, shows the phenomenon very well, and its cause. The 

 branches of oi-e are about two to three inches wide here, the main body 

 being about twelve inches in thickness, and the relations can be well 

 studied. In several places near this point the irregular wavy line of 

 contact between the schist and ore can be seen; and all bodies of this 

 or the jasper were found on close examination not to coincide with the 

 lamination, however much they may appear to do so. 



At the upper portion of the Jackson mine, Negaunee, the jasper and 

 hematite were seen to cut across and obliquely up through the schists. A 

 vertical section as shown by the former mining done at this point is given 

 in Figure 5. The jasper also curves in a similar manner at right angles to 

 this nearly east and west section. While this (the figure, not the actual 

 occurrence) could be explained easily by sedimentation, it is fatal to the 

 view of conformable deposition. In pit Xo. 3 of this mine (Jackson) 

 the ore breaks irregularly through the schist, forming a brecciated-look- 

 ing mass, while in other cases it runs up into the schist ending in irreg- 

 ular knobs. Figures 6 and 7 show some of the occun-ences observed 

 and figured from the pit walls. Figure 8 represents a section about 

 forty feet in height at the west end of Xo. 7 pit in the same mine. The 

 schist shows bending and dislocation as represented in the curve b, c, 



