OE  MUSCOYITE-EIOTITE  GNEISS. 
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Vol.  49.] 
would  fall  out  and  leave  hollows.  Moreover,  in  many  cases  the 
pseudomorphs  after  staurolite  are  often  harder  than  white  mica.  In 
staurolite  and  cyanite  the  replacement  starts  from  the  outside,  and 
then  frequently  proceeds  into  the  interior  along  cracks  so  as  to  leave 
cores  of  the  unaltered  mineral,  thus  reminding  one  of  the  well- 
known  mode  of  alteration  of  olivine  into  serpentine.  In  the  case 
of  sillimanite  in  quartz  the  reverse  happens.  The  sillimanite,  where 
massed,  practically  reaches  the  outside  of  the  quartz-lenticle  and 
decomposes  ;  but  the  projecting  needles  round  the  edges  of  a  felted 
mass  are  buried  in  quartz,  and  so  protected.  In  consequence,  a 
1  shimmer  ’-aggregate  may  be  proved  to  be  sillimanite  by  carefully 
examining  its  edges  for  the  ends  of  the  needles  under  a  high  power. 
The  fine  yfos^r-aspect  of  the  sillimanite  may  often  be  recognized 
under  crossed  nicols,  even  when  complete  alteration  has  taken  place. 
(5)  Micas. — Brown  and  white  micas  occur  more  or  less  abund¬ 
antly  in  nearly  every  rock  in  the  district.  In  the  most  altered  areas 
the  crystals  are  usually  large,  and  arranged  parallel  to  the  foliation- 
planes.  They  are  rarely,  if  ever,  idiomorphic.  The  brown  mica 
has  been  isolated  and  analyzed.  It  is  rich  in  ferrous  oxide,  and  low 
in  ferric  oxide  and  magnesia.  As  it  is  also  approximately  uniaxial, 
it  may  be  classed  as  haughtonite.  Seen  in  sections  at  right  angles 
to  the  basal  plane,  it  is  always  strongly  pieochroic.  There  are  two 
well-marked  tints  observed  in  such  crystals  :  the  first  is  pale  brown, 
changing  to  blackish  brown  on  rotating  the  nicol  90° ;  and  the  second 
is  very  pale  red-brown,  changing  to  a  deep  red- brown.  This  mineral 
has  a  curious  mode  of  occurrence  in  the  less  highly-altered  schists. 
Oval  grains  of  brown  mica  are  seen  in  the  angles  of  the  puckers  of  fine 
schists,  so  that  the  basal  cleavage  of  the  mineral  is  at  right  angles  to 
the  foliation-planes.  The  puckering  may  be  frequently  traced  through 
the  grains  by  means  of  minute  inclusions  of  iron  ores.  A  special  point 
of  importance  is  the  occurrence  in  many  cases  of  numerous  pieochroic 
spots  in  the  red-tinted  micas  (rare  in  the  brown).  In  sections 
parallel  to  the  vertical  axis  these  spots  are  almost  invisible  when  the 
short  axis  of  the  polarizer  lies  at  right  angles  to  the  trace  of  the 
cleavage,  and  the  crystal  is  very  pale  in  colour.  When  the  short 
axis  is  parallel  to  the  cleavage,  the  spots  are  deep  black  and  the  mica 
is  deep  red-brown.  These  spots  are  seen  to  have  tiny  grains  in 
their  centre :  in  some  cases  these  are  iron  ores  ;  in  others  they  have 
been  extracted  and  proved  to  be  epidote.  I11  a  thin  section  the 
epidote-grains  can  be  distinguished  from  grains  of  zircon  by  the  fact 
that  the  latter  (in  igneous  rock  at  least)  always  have  a  black  ‘  skin’ 
round  them,  independently  of  the  pieochroic  halo  ;  but  this  black 
‘  skin  ’  does  not  appear  to  ever  surround  the  inclusions  of  epidote. 
This  type  of  mica,  with  its  pieochroic  spots,  has  been  so  often 
recorded  in  contact-rocks,  such  as  those  of  Shap  and  New  Galloway 
in  this  country,  and  those  of  the  Britanny  granites  in  France, 
that  it  may  be  fairly  claimed  as  a  ‘  typical  mineral  ’  produced  by 
thermometamorphism.  In  the  most  highly-altered  rocks  white  mica 
often  occurs  in  considerably  larger  flakes  than  brown  ;  but  in  the 
finer  sericite- schists  it  is  always  the  smaller  of  the  two  minerals. 
