33G THE PENOKEE IRON P.EAEING SERIES. 



erals have furnished the iron necessary for the tnnistoi-mation trom feldspar 

 to biotite. At all events, they indicate a sufficient supply of iron. 



For a part at least of the niagiiesiuni of the biotite and chlonte, it 

 seems that we must look to some source extraneous to the feldspar frag- 

 ments; i. e., we must regard it as having been supplied by some other 

 mineral or by percolating- waters. That calcium-bearing and magnesium- 

 bearing waters have been present in these rocks is evident from the occa- 

 sional presence of secondary calcite and dolomite. Partial 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.^ 



(2) Muscovitic and biotitic grayivacke (PI. xxxiii. Fig. 1). — Macroscop- 

 ically, this rock is gray, medium grained, and massive. It breaks with a 

 conchoidal fracture. Under the microscope large fragments of feldspar, 

 with the alteration and replacement pi-oducts of the latter, compose the rock. 

 Most of the feldspar is orthoclase, although both microcline and jilagioclase 

 are present. Mucli of it is quite fresh, many individuals showing no altera- 

 tion further than a slight kaolinization. " Other feldspar fragments, however, 

 include in each many grains of quartz, or a single large reticulating quartz 

 individual and numerous flakes of muscovite and biotite. Here the quartz, 

 muscovite and biotite are jilainly replacements and alteration products of 

 the feldspar. In rare cases this alteration has proceeded so far as to leave 

 but irregular, partly replaced and altered cores of feldspar which are entirely 

 surrounded with the secondary quartz, muscovite, and biotite. 



The finer grained parts of the rock are composed of finely crystalline 

 quartz, a portion of which may be clastic; of feldspar, the jM-oportion 

 being smaller than in the coarser parts, proljably on account of the more 

 extended alterations in the small particles; and of biotite and muscovite. 

 The mica is here clearly also to a very large extent secondary to feldspar, 

 while there is little doubt that the small remaining fraction of the mica is of 

 the same origin. Scattered through the finer portions of the section are 



■Lehmann, inhis work ou tho "Eutstchung der altkrystallinischen Schiefergesteine," dcmon- 

 Btrates the lormation of abundaut sooondary biotite and other minerals as accompanying metamor- 

 phoses by folding. His work does not state, however, from what the biotite developed. 



