THE IKON BEARING MEMBER. 267 



iruiv, is cliuniL-teristic. Tlie jictiiiolitL' began to t'oriu at the time of the 

 entrance and rearrangement of tlie silica, the magnetite and siderite fur- 

 nisliing the necessary bases and tlu^ sihca furnishing the acid for its forma- 

 tion. The reactionary nature of this actinoHte is well illustrated by PL 

 XXV, Fig. 4, where the actinolite constitutes an almost solid baud between 

 a cherty nodule composed mostly of silica and the finely laminated slate 

 which largely contains siderite and magnetite. Also actinolite and magnet- 

 ite occur together in well formed concretions. Frequently in the concre- 

 tions a ring of magnetite is inclosed upon both sides by a border of 

 actinolite. Again, in other j^laces, when a magnetite ring is not quite com- 

 2ilete, the actinf)lite border which follows the magnetite throughout the part 

 of the concretion where it is present completes the ring. 



It has been suggested of the actiuolitic slates in the Penokee series 

 that the conditions which explain the presence of the magnetite also explain 

 the appearance of the actinolite. Yet it is not always true that actinolite is 

 accompanied by magnetite ; magnetite may also appear without the ])res- 

 ence of actinolite. In some few of the sections actinolite and a calcium- 

 magnesium-bearing siderite are the only important constituents, silica, 

 however, alwaj's being present ; and the rock then becomes an actinolitic 

 carbonate. It is here to be noted that this rock is an exact analog-ue of the 

 tremolitic limestones ; silica in the case of the limestones has united with 

 the bases present to form tremolite; in a similar wa)' the actinolite has 

 formed in the ferriferous carbonate by the union of silica with the bases. 



General. — It need hardly be said that the rocks of this formation are 

 mainly nonfragmental. It is, however, the case that fragmental material is 

 more widely mingled with the iron formation of the Animikie than of the 

 Penokee series. A good many thin sections contain a considerable amount 

 of fragmental quartz, which instantly exhibits itself in its true character, 

 showing its rounded outlines and frequent enlargements. This occuiTence 

 then fm-nishes another illustration of the ease of separating a fragmental 

 from a nonfragmental quartz when the rock has not been subjected to 

 dynanii(t action and of the fundamental difference of their natures. 



'V\\{' relations ami order of crystallization of the minerals contained in 

 the actinolitic slates and ferruginous cherts are then precisely the same as in 



