THE IRON-BEARING MEMBER. 265 



the Peuokee district is relatively greater abuudauce of magnetite. To 

 describe the ordinary coucretioiiary and brecciated forms would be a repeti- 

 tion of wlint lias already been given as applying to rocks in the Penokee 

 series, so that here all that will be said is in reference to the manner of 

 occurrence of the magnetite. It has been noted that magnetite is occasion- 

 ally pseudomorphous after siderite in the Penokee series. This is often the 

 case in the Animikie series. The formation of magnetite from iron car- 

 bonate, as exhibited in many sections in all its phases, is finely illustrated 

 in PI. XXV, Fig. 2, which shows on one side of the figure oval and rhomljo- 

 hedi'al areas of nearly pure siderite and in the middle of the section similar 

 areas which have completely altered to magnetite. In the intermediate 

 parts of the figure all the transition phases can be made out. In this sec- 

 tion the magnetite has taken its forms from the siderite but it is more often 

 the case that its crystal forms are independent of the original areas of 

 carbonate. In sections in which there is a good deal of both siderite and 

 magnetite, the magnetite is ordinarily found scattered through the section, 

 disconnected with, adjacent to, or included in the exterior parts or even 

 quite to the centers of the siderite areas. The relations are such as to show 

 that the magnetite is in the process of formation from the siderite, but the 

 crystals of the oxide included in it generall}' have their own octahedral form, 

 and their outlines consequently bear no relation to those of the carbonate 

 from which they are derived (PI. xxix. Fig 3). Magnetite and hematite in 

 these sections are most closely associated. Grenerally areas are made up of 

 an intimate mingling of the two. In very thin sections, in transmitted 

 light, the blood red hematite can be seen as spots here and there through the 

 magnetite areas (PI. xxix. Fig. 1) ; or, if the hematite predominates, the 

 reverse is the case. In some cases, however, the relation between these 

 two minerals is somewhat peculiar. The magnetite is found as cores in tha 

 hematite areas. In this case it is either possible that magnetite was origi- 

 nally foi'med from the iron carbonate and that it has subsequentl}- peroxi- 

 dized upon its exterior and thus formed hematite ; or that at first the con- 

 ditions were favorable for the production of magnetite and subse(juently 

 changed so as to produce hematite. 



