﻿Iron Ores of the Lake Superior region. 271 



be recleposited in various places as it became farther oxidized, 

 thus making ore bodies and various impregnations, and again 

 forming the coloring matter of the jasper, the finely interlami- 

 nated hematite of which is taken to be mainly the result of a 

 secondary infiltration following the banding of the original 

 rock, though we may imagine the hematite bands to have 

 formed at times by direct oxidation from iron carbonate seams. 



(4.) In other places, however, instead of leaching it out more 

 or less completely, the silicifying waters seem to have decom- 

 posed the iron carbonate in place, producing a magnesia-silicate, 

 or a magnesia-iron-silicate, the excess of iron oxidizing imper- 

 fectly and separating out as magnetite, and the excess of silica 

 crystallizing finally as a minutely interlocked quartz ground 

 mass. Thus have arisen most of the actinolitic magnetite- 

 schists. The various intermediate forms met with between the 

 magnetitic and jaspery or cherty hematitic schists may be ex- 

 aed by the carrying out simultaneously of the processes of 

 removal of the carbonate and of its decomposition in place. 



(5.) The bodies of rich ore have probably had different 

 origins in different cases. Some of the red hematites seem to 

 have resulted from a direct oxidation in place of the original 

 carbonate. This is demonstrable in the cases of some of the 

 thinner seams of hematite by the retention of the crystalline 

 form of the carbonate, but most of the red ores and specular 

 ores, between which there are many gradation varieties, seem 

 rather to be the result of the concentration of the iron by 

 leaching. It is quite noteworthy that, except in the case of 

 certain magnetite mines, all the ore bodies are associated with 

 great developments of jaspery or cherty silica, or both: i. e., 

 thev occur where the silicirication has been most thorough, and 

 where there has been the greatest amount of leaching of the 

 iron carbonate. Some of the magnetite mines appear to be 

 working on what are merely richer parts than usual of the 

 magnetitic schists : but there are other cases of magnetite de- 

 posits, as also of deposits of specular ore. for which I do not 

 now see any satisfactory explanation, though the difficulty 

 comes chiefiy, I suspect, from lack of sufficiently close acquain- 

 tance with them. 



(6.) Some of the silicifying process went on before the fold- 

 ing of the formations : but some also afterwards, and to the 

 latter period appear to me to belong the larger bodies of crys- 

 talline ore. the crumpling and shattering of the layers afford- 

 ing the best conditions for the action of the silicifying waters. 



It is not supposed that the theory thus advanced will not 

 require modification in the future. Such a necessity is to 

 be expected, and modifications, even to complete abandon- 



