THE IRON BEARING MEMBER. 257 



completed l)etV>re the latter ended. The an-angement of the iron oxide and 

 quartz in rough layers in the rock and in concentric belts in the cavities 

 go to show this. The cavities generally have as an exterior casing an iron 

 oxide. This was formed at the stage in which the iron carbonate was 

 partly decomposed and partly removed. Later, the solutions })assing 

 through the rock.s ])egan to carry silica, and finally the silicitication 

 was tlie cliiet" i-caction; as a result of which, the layers of quartz crys- 

 tids formed within tlie cavities and in veins in the cracks, oftentimes 

 entirely filling them. In jjroducing the many peculiar phases and mixtures 

 of chert and iron oxides the conditions must have varied in different places. 

 With the stages of growth of one particular form, the concretions, we are 

 familiar. In them the series of operation seems to have been exactly as 

 described above. 



The actinoUtic slates. — The field relations of the actinolitic "slates are 

 such as to show that they, like the feiTUginous cherts, are almost certainly 

 derived from iron carbonates. There is not here the intimate association 

 Itetween tliese two types of rock that there is between the cherty carbonates 

 and the ferruginous cherts. The latter are for many miles directly overlain 

 by the carlionates. Within their mass they contain large exposures of this 

 rock, so that between the two, both in exposure and in thin section, we 

 have ti'ansitions showing all the intermediate phases. The ferruginous 

 cherts and iron carbonates which occupy the central portion of the iron 

 formation grade into the actinolitic slates to the east and west. This 

 transition is slow, occupying in each case several miles of distance ; but 

 above the actinolitic slates no carbonate is found; also within the ))elt 

 itself there are no bodies of this material; so that the only proof of the 

 derivation of tliis rock from the cherts and carbonates lies in the facts that 

 the tw() classes of rock occupy the same horizon, and in the passage from 

 one to the other there are transition phases. 



The actinolitic slates are not so difi'erent from the ferruginous cherts in 

 their essential characteristics as might at first be thought. In mineral con- 

 stitution the two types of rock are the same, except that in the actinolitic 

 slates the additional mineral actinolite is present and magnetite more 

 plentiful. In the peculiar arrangement of the iron oxide and silica the 



MON XIX 17 



