202 THE PENOKEE IKON-BEARING SERIES. 



ferruirindus slates which are one of the characteristic i-ocks of tlie second 

 type beh^w described. Accompanying this oxidation of tlie siderite is 

 generally a rearrangement, and apparently often an introdnction of silica 

 from extraneous sources. The silica in these altered rocks, instead of being 

 the spotty chert, characteristic of the unaltered rock, is frequently more 

 coarsely crystalline, and often in combination with the iron oxides has a 

 concretionary and brecciated appearance. When this rearrangement and 

 introduction of silica is carried to the extreme, with accompanying changes 

 in tlie iron oxide, the cherty iron carbonate passes into the second phase of 

 the sect)nd type of rocks — the feiTuginous cherts. 



Tlie numerous veins and fissures which cut through the rock form a 

 cliaracteristic feature of the iron carbonates. These veins are of greatly 

 varying widths, one of them frequently breaking into several smaller veins, 

 and in some cases the sections are cut by a system of ramifying veins. 

 They are generally composed of the same minerals which make n\) the 

 section in which they are contained; that is, chert, siderite, and some- 

 times chlorite. Upon the whole, silica is nnu'li the more abundant filling. 

 This silica is frequently easily separated from the remainder of the silica 

 in the sections by being coarsely crystallized, or by having the radial 

 fibrous arrangement of chalcedony, and in including little iron oxide. Not 

 unfrequently, however, the veins contain a large amount of siderite. This 

 siderite, like the silica, is usually purer and more coarsely crystallized than 

 the original siderite. 



Microscopical character of the ferruginous slates and ferruffinous cherts 

 (PI. xxii). — The general macroscopic characters of this type of rock have 

 been indicated above. It has also been said alread)' that they are in the 

 main composed of iron oxides and silica of a nonfragmental nature; that 

 the rocks of this type present a gradual transition into those of the third 

 type by an increase in the ainou'iit of actinolite and magnetite, and that 

 similarly they present transitions into the carbonates of the first type and 

 into the hematites of the iron-ore bodies themselves. In addition to the 

 predominating siliceous groundmass, there is always present more or less 

 iron oxide, which may be magnetite, hematite, or tlie liydrated oxide of 

 iron, or two or all tlirc*^ of these together. Accessory to these two prime 



