(J. JR. Van Tlise — Iron Ores of Michigan. 119 



is almost encircled by a series of mines, which lie contiguous 

 to the outer border or in the valleys between outlying bluffs. 

 In many cases the greenstone in nearing the ore formation 

 grades into a laminated rock, which has undergone profound 

 alteration as a result of leaching and shearing. Thus changed, 

 it is known as soap-rock, or when stained with iron oxide, as 

 paint-rock. Numerous sections and diamond drill holes have 

 so frequently shown all gradations between the massive diorite 

 and the schistose soapstone and paint rocks as to leave no 

 doubt whatever of their actual continuity. Beside the large 

 masses of greenstone within or associated with the iron-bearing 

 formation are numerous dikes of the same material. These 

 sometimes run nearly parallel to the lamination of the ore- 

 formation, but more frequently cut across it at a greater or 

 lesser angle. These dikes are usually altered throughout, so 

 that in general they have not been recognized as intrusive 

 rocks. Like the altered parts of the greenstone bosses, they 

 are called soap-rock or paint-rock. That they are really igneous 

 is shown by their structural relations, and by the fact that 

 occasionally they are traced to and found to be offshoots from 

 the larger masses of greenstone. These soapstones, as indi- 

 cated by their name, have a greasy feel, are very soft, have 

 frequently a mottled gray and white color, although often they 

 are deeply stained with red oxide of iron. In chemical and 

 mineral composition these soapstones differ greatly from the 

 original diabases. They have lost nearly all of their alkalies 

 and very often are now largely a hydrated silicate of mag- 

 nesium and aluminium. 



The formation overlying the ore-deposits belongs to the 

 Upper Marquette series and is, as has been said, a quartzite- 

 conglomerate, the material of which is very largely derived 

 from the immediately subjacent formation. The heavily con- 

 glomeratic part in some cases is but a few feet in thickness, in 

 others is hundreds of feet thick. The detritus of the quartzite- 

 conglomerate is nearly like the material of the underlying 

 formation, except that within the former no large fragments of 

 pure ore have been observed. This conglomerate suggests 

 that the ore-formation was in approximately its present condi- 

 tion before the deposition of the Upper Marquette series. 



General structure of district. — Before dealing with the ore- 

 deposits it is necessary to speak of the general structure of the 

 Marquette district. This can best be appreciated by examin- 

 ing Brooks' geological map.* The Republic tongue is a sim- 

 ple synclinal, the sides of which are nearly vertical, showing 

 that the formation has been sharply folded upon itself. The 



* Atlas accompanying the Geology of Michigan, PI. 3. 



