THE NATURAL HISTORY SURVEY. 47 



and is unattacked by acids. Muscovite is the most common of 

 the micas, since it is the most stable. Other forms, like biotite, 

 decompose readily, and pass into muscovite. It is an essential 

 constituent of mica schist, and a prominent constituent of 

 gneisses. 



Biotite. (Al, Fe) 2 (Mg, Fe) 2 (K, H) 2 (Si O 4 ) 3 Monoclinic. 



Crystal habit similar to that of muscovite. Often in scales 

 disseminated through the containing rock, sometimes in mas- 

 sive aggregations. Cleavage plane is parallel to the base, and 

 as in muscovite highly perfect. The planes of separation 

 shown by percussion and gliding planes shown by pressure are 

 similar to those of muscovite, but differently oriented in regard 

 to the optic axes. The lustre is pearly to submetallic when 

 black. The lateral surfaces, when smooth, are vitreous, the 

 colors are always dark from green to black, deep black in thick 

 crystals and even with thin laminae are brown with transmitted 

 light. Transparent to opaque. Pleochroism strong on section 

 at right angle to the base. Sections such as are common in 

 petrographical slides parallel to the c axis show a greater 

 change of color than does hornblend being pale yellow to red 

 for vibrations parallel to c and brown for vibrations at right 

 angles to this direction. Microscopic inclusions are often sur- 

 rounded by pleochroitic halos. The axial plane parallel to the 

 chief percussion line helps distinguish light-colored biotite 

 from the darker shades of muscovite. Further, the axial angle 

 is so small that this species often appears uniaxial. Before the 

 blowpipe in a closed tube water is given off and on charcoal 

 there is a strong reaction for iron. It is completely decom- 

 posed by sulphuric acid. Biotite is common in the granites 

 and in the syenites and diorites of the region. 



Phlogopite. Composition identical with biotite, Monoclinic. 



save that the amount of Fe is always small. 



The crystal habit of this mineral resembles that of biotite, 

 and the same may be said of its cleavage, tenacity, hardness 

 and lustre. It is somewhat lighter in weight, and much lighter 

 in color, than biotite. It often has copper-like reflections from 

 the cleavage surfaces, and exhibits a lustre varying from pearly 

 to submetallic. Quite commonly it encloses regularly arranged 

 microlites of epidote, augite, rutile needles, which form an 

 angle of 60 with each other, and cause star-like rays to appear 

 when a section is viewed through transmitted light (asterism.) 



