TUE BIOTITEGKANITES. 173 



replacing- the material of the latter. A large, cloudy ortluiclase may in 

 many cases be found com[)letely saturated with clear, colorless microcline 

 substance. There is no sharp line of contact between the two feldsjiars, 

 but tliev seem to g-rade into each other. As the microcline replaces the 

 orthoclase it absorbs the alteration products of this mineral, the resulting 

 new felds})ar thus being- free from inclusions, while the original feldsi)ar is 

 full of them. 



Dr. Williams, in his report on the Marquette greenstones, refen-ed to 

 the microcline in the granites as more probably the effect of pressure twin- 

 ning in orthoclase than the product of chemical alteration. To the writer it 

 appears more probable that the microcline is all, or nearly all, new material, 

 produced bv chemical agencies. In e^^dence of this view, and against that 

 Avhicli regards the mineral simply as a pressure-twinned orthoclase, we would 

 cite the freshness of the latter mineral as compared with the orthoclase, its 

 freedom from inclusions, its irregular occurrence within the orthoclase 

 grains, and its existence in large quantity in veins and as the cement of the 

 crushed mosaic. 



Fresh plagioclase is also a common new product in some sections. It 

 occurs as grains among the crushed materials, and sometimes it surrounds 

 cloudy feldspar as a clear, colorless zone. Its twinning bars are commonly 

 much bent, and nearly always they present a few or more of the usual 

 features due to movement. 



The epidote grains in the mosaic need no description. They are very 

 light in color, and therefore show no pleocliroism. The muscovite that is 

 in some cases associated with the biotite or chlorite is found with these 

 minerals only where they are in the mosaic aggregate, and then only 

 wdiere in contact with orthoclase, a large mass of chlorite in some cases 

 being separated from the orthoclase by a rim of muscovite. This mineral 

 is also present in laminar aggregates of flakes, which in some slides })ene- 

 trate the mosaic, but which in most slides separate it from the unfractured 

 original granitic components. 



The mosaic of fractured minerals and new products is always more or 

 less schistose. This structure is ])roduced by the lengthening of the frag- 

 ments in a common direction, and by the development of the chlorite and 



