3G8 THE MAKQUETTE lEONB BAKING DISTEICT. 



appears, as in the cases of" the Peiiokee and Animikie series, that when the 

 siderite decoinjoosed there was an abundance of silica present, and conditions 

 not favorable to oxidation, so that the silica united with the iron oxide 

 alone, producing griinerite, or with the iron, calcium, and magnesium 

 oxide, producing a mineral intermediate between griinerite and actinolite. 

 The iron which does not combine with the silica, not being completely 

 oxidized, is in the form of magnetite. This is the first stage of the 

 development of the magnetite-griinerite-schists from the sideritic slates. 



The gruneritic and magnetitic schists (PI. XVIII) may vary from nearly 

 pure griinerite-schists to nearly pure magnetite-schists. However, the 

 more common phase is the griinerite-magnetite-schist. Griinerite, niaguet- 

 ite, and quartz are the three important constituents, but in some areas 

 quartz is in subordinate quantity. The minerals are usually concentrated 

 to some extent into bands, although a layer composed chiefly of any one 

 of the three always includes a greater or less quantity of the other two. 

 In many cases within a felted mass of griinerite or magnetite are found 

 n:iany rhombohedra of siderite, and this siderite has such relations to the 

 griinerite and inagnetite as to suggest that these minerals developed from 

 the siderite. We thus have evidence of the transition of the sideritic slates 

 into a griinerite-magnetite-schist. When the transformation is complete 

 there remains no evidence of the change, as the rock then consists of a 

 completely interlocking crystalline mass of the three minerals, griinerite, 

 magnetite, and quartz. 



Not infrequently with the magnetite is a variable quantity of hema- 

 tite. In some cases this appears to have been an early development, simul- 

 taneous with the magnetite, and in other cases it has resulted from the 

 weathering of the rock, developing either from the magnetite or from the 

 griinerite. Less frequently limonite is found in similar relations. A common 

 hornblende appears in some cases to be separable from the griinerite by a 

 decided pleochroism, and the two often occur in the same section or inter- 

 grown in the same individual. Not infrequently the quartz grains have a 

 peculiar parallel arrangement, with their longer axes in a common direction, 

 and Avith this an undulatory extinction. This is taken as indicating that 

 these rocks have been subjected to stress during or subsequent to the time the 



