266 THE PENOKEE IRON BEARING SERIES. 



The aetinolitic slates. — (PI. xxv. Figs. 3 and 4; PI. xxix, .Figs. 2 and 3.) — 

 The most important addition to our knowledge of the origin of iron-forma- ' 

 tion rocks given by the Animikie series is from the actinoHtic slates. It has 

 been seen that in the Penokee series the transition phases were somewhat 

 sparingly present. This is not the case in the Animikie. The man}" phases 

 of unaltered carbonate, ferruginous slate, and ferruginous chert contain actin- 

 olite and magnetite. Different sections exhibit these two minerals in qiiautity 

 from minor to chief constituents. There are kinds which are mainly com- 

 posed of actinolite and siderite, others which are mainly composed of actin- 

 olite and iron oxide including magnetite, ami still others which are com- 

 posed of actinolite, iron oxides, and chert ; the latter including the most con- 

 cretionary and brecciated forms. 



The evidence that the actinolite slates have been formed from cherty 

 iron carbonates is like that which proves the ferruginous cherts and the fer- 

 ruo^inoiis slates to be of this orio^in. 



It has already been shown that the magnetite is largely a secondary 

 product of siderite, being very frequently pseudomorphous after it. Also, 

 when not thus pseudomorphous, its relations are often such to the siderite 

 as to show that it is secondary to it. The relations of magnetite and the 

 other oxides of iron in the concretions have already been given. The 

 growth of these concretions has been traced out step liy step precisely as 

 in the Penokee series ; so that there is no doubt that the magnetite in them 

 is secondary to the siderite. The proportion of this mineral derived from 

 siderite is so great that it is exceedingly probable that it is of secondary 

 origin even in those cases in which this can not be demonstrated. 



The secondary character of the actinolite can not alwa}'s be clearly 

 shown ; but the cases in which its origin can be made out are so numerous, 

 that the conclusion is reached that all of it is of the same nature. That the 

 silica in the actinolite slates has been extensively rearranged and more 

 coarsely crystallized, with the probable addition of silica to that which the 

 original rock contained, can not be doubted. The way in which the silica 

 cuts the other minerals is nicely illustrated by PI. xxix, Figs. 1 and 2. The 

 actinolite constantly cuts the quartz. The association of the actinolite with 

 the magnetite and siderite, that is, with minerals which contain j)rotoxide of 



