PETEOGRAPHICAL CHARACTERS OF ARCHEAN. 43 



seems to be the result of incipient alteration, since the edges of the flakes 

 are ragged, and in many cases almost the entire biotite of the section is 

 altered to a chlorite, which shows ordinary white to light greenish pleoch- 

 roism, with the simultaneous production of epidote and bundles of needles 

 with high single and double refraction, having yellowish or brownish color. 

 These needles are taken for rutile. The biotite is found usually lying 

 between the feldspar and quartz grains almost as though it had been the last 

 product of crystallization. It has suff'ered crushing with the other minerals. 

 Apatite and zircon were observed in a few crystals. No original iron 

 ore was seen. As intimated above, by the use of the term "epidote-zoisite'' 

 the exact character of this secondary material is not always determinable. 

 In some instances parts of an epidote crystal show the deep blue inter- 

 ference color of zoisite, apparently indicating a mixture of the zoisite and 

 epidote molecules, the latter predominating in the crystals.^ The remaining 

 secondary minerals mentioned as occurring in the granite show their usual 

 characters. 



GNEISSOID BIOTITE-GRANITE, BORDER FACIES OF GRANITE. 



About the central area of biotite-granite just described, and in part 

 forming the border of the Ai-chean area, are rocks having a gneissic 

 structure. With these are associated the biotite-granites. The gneissoid 

 rocks in general are markedly darker in color than the granites, showing 

 normally a rather dark gray. They vary little from one another in texture 

 and are much fmer grained than the granites. The fine-grained condition 

 of these schistose and banded rocks has perhaps a great deal to do with 

 their dark color, though this is primarily owing to the amount of biotite 

 present. 



In some of the specimens the bands can be readily distinguished under 

 the microscope, and are seen to contain a white mica and a nmch smaller 

 amount of biotite. These two minerals are present in fine films between 

 the crushed quartz and feldspar grains, gi^^ng to the rocks a very decided 

 schistose character. These mica folia are much more numerous in certain 

 areas than in others, thus producing a more or less perfect banding. The 

 mica plates are not all regularly parallel, although ordinarily having a 



' On some granites from Britisli Columbia and the adjacent parts of Alaska and the Yukon 

 district, by F. D. Adams : Canadian Record Sci., Sept., 1891, p. 346. 



