122 C. R. Van Ilise — Iron Ores of Michigan. 



recomposed rocks. It is reasonable to suppose that a thin 

 stratum of the conglomerate overlying the richer part of the 

 iron-bearing formation may have been nearly, if not quite as 

 heavy in its iron content as the formation from which it was 

 derived. In these cases a part of the iron of the ore-deposit 

 at least is a direct mechanical detritus and differs therefore in 

 its genesis from the remainder of the ores ; but all have alike 

 been affected by a secondary concentration-, for the process of 

 replacement has affected the siliceous detritus in the same way 

 that it has the laminated jasper in the original formation. 

 When the ore-bodies partly occupy the place of the quartzite- 

 conglomerate and partly that of the ore-formation proper, as a 

 consequence of the secondary concentration, these two forma- 

 tions, although of widely different geological age, have been 

 welded together. This is well shown at the Kloman mine. 



(£.) Deposits resting upon soap-rock which grades into 

 massive diorite. — Here are to be placed many of the deposits 

 of soft ore and some of the hard ores. The masses of soap- 

 rock may follow somewhat closely the lamination of the ore- 

 formation or they may cut across it. In either case the deposit 

 follows along the contact plane, the impervious soap rock 

 always being below the iron ore and above it the fractured and 

 porous jasper or chert (see fig. 1, p. 123). Not infrequently a 

 mass of soap-rock or diorite forms a synclinal trough in which 

 the ore body rests, when the maximum thickness of the ore is 

 likely to be at the lower part of the synclinal (fig 5). Sometimes 

 a small dike of soap-rock or paint rock shoots off from the large 

 body and cuts into the ore (fig. 6). At other times the soap- 

 rock bulges into the ore as though it had been bent into sharp 

 corrugations. That the soapstone upon which the ores rest, 

 actually grades into diorite is shown by many drill holes at the 

 Lake Superior mine and very clearly at the Salisbury mine, as 

 well as at other localities. 



In some cases the first and second classes of deposits ap- 

 proach close to each other and might be mistaken for one con- 

 tinuous deposit of the same character, the two occupying 

 opposite sides of a synclinal. This is shown by fig. 7, which 

 shows upon one side of the synclinal the following section : 

 diorite grading upward into soapstone, ore, jasper, quartzite- 

 conglomerate. Upon the other side of the synclinal the sec- 

 tion is : jasper, ore and quartzite-conglomerate. 



(3.) Deposits resting upon dikes of soap rock which follow 

 along or cut across the ore-hearing formation. — The ores here 

 belonging, like those of the second class, are usually soft. 

 They may occur upon one side only of a dike rock, or, when 

 it is vertical or nearly so, upon both sides. When two dikes 

 are not far distant from each other the whole or part of the 



