THE IKON-BEARING MEMBER. 259 



within the (quartz. To expUiin the differences between these rocks and 

 the ferruginous cherts we need, then, only to suppose one different con- 

 dition — the absence of a sufficient amount of oxygen at the time of their 

 alteration to change all of the iron to the peroxide. As has been sug- 

 gested before, it is possible that those parts of the iron formation in which 

 the actinolitic slates occur were more highly heated than the other parts. 

 The more coarsely crystalline character of the silica so prevalent in tliem. 

 may also be explained by this supposed increase in tem^jerature ; for, as 

 has been shown by laboratory experiments, a temperature considerably 

 above the normal is favorable to the production of coarsely crystalline 

 quartz.' This supposed increase of temperature is rendered plausible by 

 the frequent if not universal association of large quantities of basic erup- 

 tives with the iron formation at the places where the actinolitic riicks occur. 

 Also these cutting or interlaminated eruptives may have excluded to some 

 extent surface water and thus caused the supply of oxygen to be deficient. 



The details b}- which the particular phases of the actinolitic slates 

 have been j)roduced cannot l)e satisfactorily given, for in so few of them 

 is there any remaining iron carbonate. However, taken in connection 

 with their position and intermediate links, there can be little question 

 of their derivation from a cherty iron carbonate. If there is any doubt 

 u])on this point it is set at rest by considering the occurrences in the 

 Animikie series in northeastern Minnesota. That these two iron-bearinsr 

 series are .stratigraphically equivalent can hardly be doubted. It has been 

 seen that in the Animikie series all the transition phases between cherty 

 iron carbonates and the most crystalline actinolitic slates are as perfectly 

 illustrated as are the intermediate phases between the cherty iron carbon- 

 ates and the ferruginous cherts in the Penokee series. 



The origin of these actinolitic slates is analogous to that of the tremo- 

 litic limestones, so M;idely }iresent in the iron-bearing series of the North- 

 west and in later geologic times. Tremoliteis calcium-magnesium-silicate; 

 actinolite is calcium-magnesium-iron-silicate. The tremolitic limestones 

 arc, aside from the tremolite, mostly composed of calcium-magnesium-car- 

 bonate. The original material from which the actinolitic slates were 



' Roscoe and Schorlemmer, Treatise on Chemistry, vol. i, p. 569. 



