272 THE PENOKEE IRON BEARING SERIES. 



and west strike. As used in reference to the dikes, an east and west direc- 

 tion means parallel with the iron formation; a north and south direction, 

 transverse to it. The important thing for the present purpose is not the 

 absolute direction in which the dikes run, but their relations to the contain- 

 ing formation. The dikes vary a good deal in their dip and strike in dif- 

 ferent mines, and the same dike at times in the same mine also varies in dip 

 and strike. However, certain of their elements are quite constant. They 

 always dip to the south, and generally this southern dip or its component 

 transverse to the formation is from 20° to 30°. The northern dip of the 

 iron formation is from 60° to 70°. It follows from this that if the stratified 

 rocks were placed again in a horizontal position the dikes ivoidd he vertical. The 

 true dip of the dikes is, however, usually not exactly transverse to the for- 

 mation, but east of it, so that a component along the dikes, parallel to the 

 strike of the rocks, has usually an eastern pitch. This pitch may be as 

 high as 35°. From this amount it varies to horizontality or even to a 

 western pitch of 10°. The strike of the rocks of the Iron-bearing member 

 in which the ore bodies occur is, throughout most of the distance, north of 

 east. For a short way in the eastern part of T. 47 N., 11. 46 W., Michigan, 

 the strike of the rocks is "nearly east and west; but in passing westward 

 the strike begins to vary to west of south and in the western part of 

 the ore-bearing area is from 25° to 30° west of south (PI. ii). If these rocks 

 were turned back to a horizontal position in a direction at right angles to 

 their strike, the strike of the dikes would be the same as the strike of the 

 rocks of the Iron-bearing member at present, when there is no eastern or 

 western component in the dip of the dikes. When the dip component of 

 the dikes along the formation is east, it must be added to the amount that 

 the iron formation now strikes north of east to })roduce the strike which the 

 dikes would have if the iron rocks were placed in a hoi'izontal position. 

 If, on the contrary, the component along the formation is to the west, 

 it must be subtracted from the amount that the strike of the formation is 

 north of east. The position of these dikes with reference to the foot-wall 

 quartzite will be better imderstood by an examination of Pis. xxx and 

 XXXI. In one mine in which the pitch of the dike is 10° to the west, 

 the Mount Hojie, the strike of the formation is about 10° north of east ; 



