Mica-Slates of the Penokee- Gogebic Iron Series. 455 



It is important here to recall the chemical changes which 

 occur in the alteration of orthoclase and oligoclase to chlor- 

 ite, mur,covite and biotite. The average content of silica of 

 the following minerals is taken from Dana's System of Min- 

 eralogy : orthoclase, 65 per cent ; oligoclase, 62 per cent ; 

 muscovite, 45 per cent; biotite, 40 per cent; chlorite, 25 

 to 30 per cent. Evidently where the alterations of orthoclase 

 and oligoclase to muscovite, biotite, and chlorite have taken 

 place so extensively as in the rocks under discussion, it is not 

 difficult to explain the presence of the silica which has enlarged 

 the fragments of quartz, replaced those of feldspar, and sepa- 

 rated as independent interstitial quartz. One of these altera- 

 tions is stated by Tschermak * to occur as follows: " Wenn 

 man die dreifache Formel des Feldspathes 3(K 2 . A1 2 3 . 6Si0 3 ) 

 mit jener des daraus entstandenen Glimmers K 2 0. Al 2 3 . 2Si0 2 

 + 2(H 2 0. A1 2 3 . 2Si0 2 ) vergleicht, so eigibt sich, dass von der 

 urspriinglichen Menge 6Si0 2 nur 2Si0 2 in die neue Verbindung 

 iibergehen und 4Si0 2 tibrig bleiben." In farther speaking of 

 the alteration of orthoclase to muscovite, Tschermak also ob- 

 serves : ''Der neu enstandene Muscovit ist dfters auch von 

 Biotit (Magnesiaglimmer) begleitet." 



For the iron of the biotite in the rocks under consideration, 

 it is not difficult to account. Pyrite, marcasite and ferrite 

 are quite widely present in these rocks as accessory constituents. 

 Often the relations of the pyrite or marcasite and biotite (folia 

 of the latter surrounding particles of the former) is such as to 

 lead to the supposition that the former minerals have furnished 

 the iron necessary for the transformation from feldspar to bio- 

 tite. At all events they indicate a quite sufficient supply of 

 iron. 



For a part at least of the magnesium of the biotite, it seems 

 that we must look to some source extraneous to the feldspar 

 fragments ; i. e., we must regard it as having been supplied by 

 percolating waters. That calcium-bearing and magnesium- 

 bearing waters have been present in these rocks is evident from 

 the occasional presence of secondary calcite and dolomite. The 

 case is the same as that presented by the replacement of feldspar 

 by chlorite, which has commonly taken place in the greywackes 

 of this and other regions in the Lake Superior country. 

 Analyses of three of the biotite-schists gave an average content 

 of MgO of 2*22 per cent, which if entirely contained in the 

 biotite would correspond to a probable proportion of that 

 mineral of from 10 to 20 per cent. 



The change from fragmental quartz-feldspar rocks to quartz- 

 ites and greywackes in this belt having been produced by 

 metasomatic changes, as indicated above, there is a strong pre- 

 sumption that the mica-schists and slates are of the same origin • 

 * Lehrtmch der Mineralogie, Zweite Auflage, page 462. 



