Van Rise — Iron Ores of the Penokee-Gogebic Series. 35 



which dip with the formation and are known as foot-walls. 

 Whether the ore-deposits rest upon the f ragmen tal quartzite or 

 are north of it, they have then as their southern boundaries a 

 plane dipping to the north at an angle of 60° to 70°. 



DyTi.es in the iron-bearing member. — Mining developments 

 have shown that the iron-bearing member is cut by numerous 

 greenstones, the presence of which would not have been sus- 

 pected from natural exposures. These greenstones are much 

 altered ; many of them are so decomposed as to be soft friable 

 matter which can be picked to pieces with the fingers, and 

 which now contain none of the original minerals which com- 

 pose ordinary basic eruptives. They retain, however, dis- 

 tinctly their diabasic structure, and occasionally can be traced 

 into comparatively unaltered phases which are true diorites. 

 These altered greenstones are known to the miners, either as 

 soapstones, or as diorite dykes. That they are dykes is mani- 

 fest from their shape, and the way in which they cut across 

 the layers of the iron-bearing member being traced at times 

 into the foot-wall quartzite. This dyke-like character is well 

 shown by figs. 2 to 7. The association of the ores and these 

 soapstones was found to be so constant, that Mr. J. Parke 

 Channing, Inspector of Mines for Gogebic County, Mich., 

 was secured to work out the relation of the ore-bodies and 

 dyke-rocks. What follows as to the position of the dykes 

 themselves, and as to the position of the ore-bodies with refer- 

 ence to them, is wholly the result of data furnished by his in- 

 vestigations. 



The position of the dykes is given with reference to the iron 

 formation in which they occur. This formation has a northern 

 dip and a general east and west strike. As used in reference 

 to the dykes, an east and west direction means parallel with 

 the iron formation, a north and south direction transverse to it. 

 The important thing for the present purpose, is not the abso- 

 lute direction in which the dykes run, but their relations to the 

 containing formations. The dykes vary a good deal in their 

 dip and strike in different mines, and the same dyke at times 

 in the same mine also varies in dip and strike. However, 

 certain of their elements are quite constant. The dykes 

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

 component transverse to the formation, is from 20° to 30°. 

 The northern dip of the iron formation has been said to be 

 from 60° to 70°. It follows from this, that if the stratified 

 rocks were placed again in a horizontal position, the dykes 

 would be vertical. The true dip of the dykes is usually, how- 

 ever, not exactly transverse to the formation, but east of it ; so 

 that a component along the dykes, parallel to the strike of the 

 rocks, has usually an eastern pitch (figs. 2 and 5). This pitch 



