258 THE PENOKEB IRON BEARING SERIES. 



ferru'i'inous cherts aud actiiiolite slates are (ifteu identical, the recular 

 laniiiijE of iron ore and silica, the concretions, and the breaking of these 

 concretions and laininte by veinlets, all occurring in both classes of rocks 

 (PI. XXIII, Figs. 3 and 4; PI. xxiv, Figs. 1 and 2). Also the two types of 

 rock are alike, in that they both contain at times residual iron carbonate. 

 It is true that the amount of sidei'ite is not so great in the actinolitic slates 

 as in the ferruginous cherts. A furtlier likeness between the two rocks lies 

 in the occasional pseudomorphous aggregates of actinolite and magnetite 

 after iron carbonate, as exhibited by PI. xxviii, Fig. 1. The only essential 

 difference, then, between the two classes of rocks is the introduction of 

 actinolite; the subordinate differences are the more completely crystalline 

 character of the quartz and the abundance of magnetite. To account for 

 the actinolite is comparatively easy. We have only to suppose that the 

 silica-ljearing waters at the time of the rearrangement and silicification of 

 these rocks united with a portion of the protoxides of calcium, magnesium, 

 and iron, thus ])roducing actinolite. As has been seen by the description, 

 page 210, the order of crystallization of the minerals in these slates is 

 magnetite, actinolite, and quartz. The condition which explains the large 

 amount of magnetite also explains the production of the actinolite. In the 

 ferruginous cherts it has been seen that the oxidation of the carbonate was 

 mainly prior to the rearrangement and introduction of silica; so in the 

 actinolitic slates, the first step in the i)rocess of alteration from the cherty 

 carbonates was the production of magnetite by the decomposition of 

 siderite. That this i-esultant oxide is largely magnetite is proof of an 

 insufficient amount of oxidizing agent. When the rearrangement and sub- 

 sequent introduction of silica occurred a portion of it simply united with 

 the protoxide bases present and thus formed actinolite. An abundance of 

 protoxide of iron was here perhaps as carbonate, and certainly in the mag- 

 netite. In this connection the relations of the actinolite Jind magnetite are 

 of great significance. The actinolite frequently surrounds the magnetite 

 areas, less often penetrates them, and is usually, although not always, 

 associated with them. In the final stage in the process of the foiTnation of 

 this rock the most of the silica separated as (juartz, but apparently actino- 

 lite continued to form to the very last, as it is almost everywhere included 



