36 Van Hise — Iron Ores of the Penokee- Gogebic Series. 



may be as high as 35°. From this amount it varies to hori- 

 zontally, or even to a western pitch of 10°. The position of 

 these dykes with reference to the foot-wall quartzite will be 

 better understood by figs. 2 to 8. 



The thickness of the dykes varies greatly, running from 

 those of but a few inches in thickness, to those nearly 90 feet 

 thick, fig. 4. In most of the mines in which the ore-deposits 

 £re of any magnitude the dykes are six feet or more in 

 thickness ; while it is noticeable that the three largest 

 mines have dykes of considerable thickness. At least one 

 dyke has been found in connection with every deposit 

 west of Sunday Lake, with the single exception of the 

 Ironton-Puritan ore-body, and it is possible that when the 

 workings of these mines penetrate deeper they will come in 

 contact with a dyke — one is known to come to the surface 

 three or four hundred feet west. As to the three mines east 

 of Sunday Lake, it has already been noted that their character 

 is quite exceptional. From the eastern pitch of the dykes, it 

 is evident that they must, if they continue in their observed 

 directions, reach the surface to the west of the present work- 

 ings of each of the mines. As these workings are but a few 

 hundred feet deep, it follows that when these dips are high the 

 dykes would reach the surface but a short distance from the 

 ore- deposits. It thus becomes probable that there are as many- 

 dykes in the lower horizon of the iron-bearing member as are 

 seen in all of the different mines, while doubtless there are 

 many more. In some mines there are as many as three or four 

 parallel dykes. In those cases in which there are several dykes 

 in a single mine, one is generally known as the main dyke. 

 The smaller ones are in some cases clearly offshoots from the 

 larger, the actual connections between them being traced. 



Position of the ore in reference to the dykes. — The ore has 

 been spoken of as resting upon the fragmental quartzite as a 

 foot-wall, and in exceptional cases as resting upon a non-f rag- 

 mental quartz-rock, which belongs in the iron-beariug member, 

 but which nevertheless forms a foot-wall for the ore deposits, 

 dipping north with the formation. From the description of 

 the position of the dykes and the quartzites, it is evident that 

 the two rocks form Y-shaped troughs, which have at the apices 

 right angles, and the south arms of which are nearer vertical 

 than the north arms ; the first being upon an average 20° to 30° 

 from a vertical, while the second is from 20° to 30° from a hori- 

 zontal position. The relation is that of a right angled trough 

 tilted toward the north until it lacks 20° to 30° from having 

 its arms in horizontal and vertical positions. In one or two 

 mines, for a short distance, these troughs do not incline either 

 east or west, but at most of them, from what has gone before, 



