40 THE CRYSTAL FALLS IRON-BEARING DISTRICT. 



petrographicaIj ciiaractp:rs. 



The rocks of the Archean comprise biotite-granite, gneissoid biotite- 

 g-raiiite, and acid and basic dikes. 



BIOTITE-GRANITE (GRANITITE). 



Tlie rock occupying the main and central part of the Archean area is 

 a biotite-granite. This rock is also found to some extent on the border of 

 the area. The rocks of this kind vary in color from light-gray rocks to 

 those having various tints of red, depending usually upon the degree of 

 alteration. They vary also from medium to coarse grain. Some varieties 

 show a decided porphyritic texture, and in some cases also an approach to 

 a laminated structure. The porphyritic character is due to the presence of 

 large crystals of feldspar, which stand out from the surrounding granitic 

 groundmass, thus producing a typical granite-porphyry. The feldspar phe- 

 nocrysts lie with their longer axes parallel, and thus help to produce an 

 imperfect laminated structure. This parallel structure in the granite- 

 porphyry is apparently analogous to the flow structure of the volcanic 

 rocks, and probably was produced by movements in the magma before it 

 had reached even a viscous state, as we find that the phenocrysts give no 

 evidence of having iindergone excessive mashing or torsion. The different 

 textural varieties grade into one another in such a way as to indicate that 

 they are merely modifications of the same magma. In addition to these 

 textural varieties, which are original, we find in certain places a passage 

 from massive to schistose rocks, in which the schistosity is of dynamic 

 origin, i. e., of secondaiy nature. 



In the thin sections these rocks show the normal granitic texture and 

 the usual mineral constituents which characterize biotite-granites. The 

 chief minerals are orthoclase, microcline, plagioclase, quartz, and biotite. 

 Zircon and apatite are the accessory minerals present, and the secondary 

 minerals include epidote-zoisite, chlorite, muscovite, rutile, and iron pyrites. 



Quartz occurs in grains forming the cement and molding around the 

 other minerals. In one of the granites it has a peculiar saccharoidal char- 

 acter macroscopically, and under the microscope such portions are resolved 

 into very fine aggregates of quartz grains. 



The quartz is also frequently found in round blebs of varying size 

 included in the best crystallized feldspar crystals. Thus the crystallization 



