THE IRON BEARING MEMBER. 271 



cient to say that whether the ore deposits rest upon tlie fraginental quartz- 

 ite or are north of it, they have as their southern boundaries an approxi- 

 mately regular plane, dipping to the north at an average angle of from 60° 

 to 70°. 



Dikes in Iron-bearing member. — The eruptives of the series are treated 

 in another place. They are known to be very numerous in the Iron- 

 bearing member, although their abundance here does not sliow that 

 they are not as plentiful in other members of the series; for mining 

 developments have cut this belt in every direction and thus found the 

 greenstones, the presence of which would not have been suspected from 

 natural exposures. As explained in their description (cha})ter vii), the 

 greenstones of the Iron-bearing member are much altered. Many of 

 them are so decomposed as to be soft friable masses which can he 

 j)icked to pieces with tlie lingers and now contain none of the original 

 minerals that make up ordinary liasic intrusives. They retain, however, 

 distinctly their diabasic structure and can occasionallj^ be traced into com- 

 paratively unaltered phases which are true diorites. These much altered 

 greenstones are known to the miners either as soapstones or as dioi-ite 

 dikes. That they are dikes is manifest from their form as well as by the 

 way in which they cut across the layers of the Iron-bearing member 

 into the foot-wall quartzite. This dike-like character is finely shown by 

 Pis. XXX and xxxi. 



One traveling along the iron range is at once struck liy the con- 

 stancy of the association of the iron ores and these so-called soapstones. 

 At every mine, except those east of Sunday lake, a greater or lesser 

 amount of this soapstone is found among the debris from the mines and 

 oftentimes its quantity is large. This association was found to be so 

 constant that Mr. J. Parke Channing, then inspector of mines for Gogel)ic 

 county, Mich., was employed to work out the relations of the ore bodies 

 and dike rocks. What follows as to the position of the dikes themselves 

 and as to the position of the ore bodies with reference to them is wholly 

 the result of data furnished l)y his investigations. 



The position of the dikes is given with reference to the iron fonnation 

 in which they occm-. Tliis formation has a northern dip and a general east 



