106 
PEOF.  T.  G.  BONNET  ON  BIOTITE  AND 
[Feb.  1893, 
film  of  a  very  minute  white  mica,  forming  a  kind  of  ‘  varnish/ 
The  garnets,  about  025  inch  in  diameter,  have  been  affected  by 
pressure — but  still  retain  a  fairly  regular  dodecahedral  form.  One 
difference  only  attracts  the  eye,  most  conspicuous  on  the  clean-cut 
surface,  that  here  and  there  flakes  of  biotite  (sometimes  nearly  the 
above  length)  occur,  often  parallel  with,  but  occasionally  athwart 
the  direction  of  cleavage. 
The  matrix  of  the  rock,  when  a  thin  section  is  examined  under 
the  microscope,  indicates  the  effects  of  severe  pressure,  and  resembles 
my  specimen  from  the  Val  Canaria  more  closely  than  those  from 
Tal  Piora.  It  is  like  a  mass  of  matted  fibres,  which  lie  roughly 
in  one  direction.  These  are  fiakelets  of  a  colourless  mica,  among 
which  is  scattered  much  opacite  in  small  rods  or  plates,  down  to 
the  most  minute  dust ;  the  latter  has  a  slightly  streaky  arrange¬ 
ment.1  Part  of  one  garnet  is  on  the  slide  :  its  margin  is  rather 
irregular  :  the  ‘  sides  '  of  the  grain  are  bounded  by  a  narrow  zone  of 
cleaner  mica,  brown  and  white.  Streaky  opacite  is  continued  through 
the  grain,  bending  slightly  out  of  the  usual  direction,  but  is  almost 
absent  for  a  space  of  nearly  *01  inch  from  the  margin.  In  the 
garnets  from  the  other  localities  lines  of  opacite  can  also  be  seen. 
Films  of  biotite  also  may  be  noted  in  the  groundmass  of  these 
rocks,  suggestive  of  a  crystallization  of  later  date  than  the  pressure. 
In  this  slide  the  matter  appears  to  be  beyond  doubt.  Here  the 
biotite  occurs  in  crystalline  grains,  rarely  in  scattered  films.  The 
basal  planes  of  the  mineral  sometimes  almost  lie  in  the  plane  of 
cleavage-foliation,  but  are  often  athwart  it,  and  once  at  right  angles 
to  it.  The  grain  is  elongated,  in  the  former  case,  in  the  direction 
of  the  basal  plane,  when  it  is  nearly  free  from  enclosures ;  in  the 
latter  case  the  elongation  is  perpendicular  to  the  basal  plane,  and 
the  streaks  of  opacite  pass  through  the  mineral,  though  the  quantity 
seems  diminished.  The  last-named  grain,  a  portion  of  which  is 
figured  (see  fig.  1,  p.  107),  is  as  nearly  as  possible  ’175  inch  long,  at 
right  angles  to  the  basal  plane.  For  slightly  more  than  one  third  of 
its  length,  it  is  about  *03  inch  broad.  It  is  then  invaded  by  a  streak 
of  matrix,  which  enters  through  a  gap  in  one  side  nearly  *02  inch 
wide,  but  part  of  the  crystal  forms  a  continuous  strip,  though  in 
it  the  mica  for  a  space  almost  disappears,  as  if  there  had  been  an 
insufficient  supply  of  the  requisite  constituents  (the  mineral  being 
well  developed  on  either  side  of  the  gap).  The  streaks  of  opacite 
pass  through  the  biotite,  though  in  the  clearer  portions  they  are 
diminished  in  quantity,  the  finer  lines  disappearing,  but  twice  or 
thrice  a  thin  strip  of  biotite,  bounded  by  two  cleavage-planes, 
interrupts  them  more  or  less  completelv.  Generallv,  however,  these 
planes  can  be  seen  to  cut  right  through  the  1  dustv  ’  streams  without 
producing  any  effects. 
The  relations  of  these  biotite-crystals  to  the  matrix  seem  to 
place  it  beyond  doubt  that  they  have  been  completelv  reconstituted, 
if  not  wholly  formed,  since  the  epoch  when  the  rock  was  modified 
1  Other  silicates  and  probably  quartz  are  also  present,  but  as  these  are 
unimportant  for  my  purpose  I  have  passed  them  over. 
