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PROF.  T.  G.  BONNET  ON  BIOTITE  AND 
[Feb.  1893, 
macroscopically,  resembles  generally  that  of  the  garnet-bearing 
schist;  it  has  a  similar  sheen,  but  contains,  instead  of  garnets, 
numerous  dark  actinolites,  rather  roughly  formed,  which  are 
commonly  from  |  inch  to  |  inch  long  and  about  yL  inch  thick. 
These  lie  mostly  in  the  general  direction  of  the  cleavage-foliation,  but 
some  make  high  angles  with  it.  They  present  a  certain  resemblance 
to  the  still  larger  green  actinolites  in  the  garnet-bearing  schist,  so 
abundant  on  the  southern  slopes  of  the  St.  Gothard  massif ,  but  in 
the  present  case  both  the  mineral  and  the  rock  have  a  4  dirtier  ? 
aspect.  The  matrix  under  the  microscope  (allowing  for  a  clear 
streak  mostly  occupied  by  quartz,  biotite,  and  paragonite  (?),  no 
doubt  a  rolled-out  vein)  does  not  differ  much  from  that  of  the 
ordinary  black-garnet  schist,  except  that  the  quantity  of  opacite  is 
less,  and  of  quartz  more,  in  proportion  to  the  white  mica.  But  the 
Fig.  2. — Hornblende-crystal  ( ivith  some  biotite )  including  curved 
stratulce  of  opacite.  x  30. 
streaks  of  opacite  often  exhibit  sharp  flexures.  Films  of  biotite 
occur  here  and  there  in  parts  of  the  slide,  grains  and  plates  of  iron 
oxide,  and  small  narrow  prisms.  These  last  have  an  oblique 
extinction,  give  fairly  bright  colours  with  polarized  light,  and  are 
probably  kyanite ;  a  few  small  grains  of  a  plagioclase  felspar  can 
also  be  recognized.  On  closer  study  we  perceive  that  in  parts  of 
the  slide  where  the  opacite  and  mica  are  less  abundant,  the  inter¬ 
stitial  material,  on  applying  crossed  nicols,  exhibits — over  spots 
of  some  size,  perhaps  y1-  inch  wide — a  uniformity  of  tint  and  of 
extinction,  as  though  the  opacite  and  mica,  with  some  granular  quartz, 
