THE IRON-BEAEING MEMIIEU. 21 '5 



tlu' ferruginous cherts ut' the hist type, one moi'e tliiiiy to iiuhcate th;i1 ;ill 

 are derived from essentially tlie same original material. The actinohtic 

 ingredient of these rocks presents itself usnalh' in hunches of minute 

 radiating- blades of a jtale greenish tint. Occasionallv tlie in(li\i(hial l)]ades 

 are large enough, as indicated })reviouslv, for tliem to be seen witli the 

 naked eye or with the magnifying glass, l)Ht more usually they are micro- 

 scopic. Kven these radiating bunches of actinolite are aggregated into 

 certain laininie to the apju'oximate exclusion of the other mineral ingredi- 

 ents. They occur also in single l)lades scattered through the thin section, 

 many minute blades being not infretpuMith' inclosed in a single grain of 

 quartz, while other blades or bnnches of blades often traverse a number of 

 quartz individuals. 



One unusual phase of the actinolitic rocks deserves especial description. 

 At Penokee ga}), both on the west an<l on the east side of the fault which 

 there occurs (see PI. xxxvi), the uppermost layers of the Iron-l)earing 

 member are composed of a thin belt of peculiar slate, in which garnet is a 

 ])rominent constituent. This slate is nearly a)|hanitic, of a dark cfilor, 

 banded vaguely with lightei- streaks, and cleaves readil}- parallel to the 

 lamination. Minute individuals of garnet may at times be detected 

 with a strong magnifying glass in the lighter colored streaks, l)ut 

 usually they requii'e the microscope for their detection. Under the 

 microscope this rock is seen to be almost wholly composed of 

 actinolite, magnetite, and garnet. The garnet is in small and M-ry 

 numerous individuals, presenting a pale pinkish tint. In the ordinary 

 light many of these garnets are to be seen provided with crystal out- 

 lines, but in the polarized light this becomes less evident, because many 

 of the garnets are penetrated b\ numerous needles of actinolite, which are 

 frequently very plentiful in the outer portions of the garnet individuals. 

 The actinolite also makes uj) most of the interstitial portions of the thin 

 sections, while the magnetite, in tine particles, at times provided with sharp 

 crystal outlines, is scattered uniformly throughout all portions of the sec- 

 tion, being included within each of the other nunerals. Numerous minute 

 ilakesof biotite occur always in such relation to the actinolite as to suggest 

 their possible derivation from it. ( 'lilorite occurs also in some sections 



