194 THE PENOKEE lEON-BEAKING SERIES. 



of liaving been disrupted and the broken fragments receniented. Both the 

 regular and irregularly laminated phases are found in large areas. 



The uniformly laminated phase to which the name ferrwjinous slate 

 is given has a uniform texture and even lamination, and varies in color 

 from yellow to deep red, the change in color being due to different 

 oxides of iron. This phase might be taken for a slaty iron ore, but its 

 low specific gravity and its large content of silica, as shown by analysis, 

 exclude it from the iron ores. The silica and iron oxide are so uniformly 

 mingled and the silica is in such small particles that it is not macroscop- 

 ically visible. 



The second phase, the ferruginous cherts, is distinguished from the first 

 phase by the greater concentration of the iron oxides. They occur in 

 irregular bands and areas, interlaminated with chert layers of greater or 

 less purity. There are all gradations in exposure, from the regularly 

 laminated slates of the first phase through the ferruginous cherts to areas 

 in which the iron oxide is concentrated in such large masses as to be 

 workaljle deposits of iron ore. In general the texture of the chert is 

 aphanitic; occasionally it presents a chalcedony-like appearance. In many 

 instances there is present a more or less porous texture, minute cavities 

 occurring throughout the specimens. Also a minute brecciatiou of the 

 rock is not unfrequently perceptible to the naked eye, though this peculiar 

 characteristic comes out more prominently in the thin section, as subse- 

 quently explained. When perceptible to the naked eye, it shows itself as 

 an irregular mottling of dark and light gray, or of black and gi'ay; the 

 a})pearance being that of more or less angular fragments iml)edded in a 

 lighter colored matrix. The following are the results of specific gravity 

 determinations of the ferruginous cherts: 2-65, 2-67, 2-G9, 2-7fi, 2-90, 2-92, 

 2"95, 3"21, 325, 3"26, 339. Of these amounts those above three represent 

 tlie more ferruginous kinds. 



The rocks belonging to the third ty})e, the actinolitic and magnetitic 

 slates, are in the main very dark colored, being often black. Occasion- 

 ally, however, in Ixmds the color is lighter, in which case the light and dark 

 colored bands present a striking striped appearance in natural exposure. 

 The grain is usually a very fine one, being at times quite aphanitic. Occa- 

 sionally each of the main individual constituents mav be detected with the' 



