336  ME.  G.  BARROW  ON  AN  INTRUSION  [Aug.  1 893, 
to  a  pleochroic  halo.  Apatite  has  not  been  observed,  and  its  absence 
distinguishes  this  brown  mica  from  that,  of  the  newer  (pre-Old  Red 
Sandstone)  granites,  in  which  apatite  occurs  abundantly. 
The  white  mica  contains  8*47  per  cent,  of  potash,  and  only  0*91  per 
cent,  of  soda.  As  it  is  widely  biaxial,  it  may  be  fairly  claimed  as 
typical  muscovite.  The  large  micas  of  the  pegmatites  have  the 
same  composition  as  the  small  individuals  of  the  ‘  permeation-area.’ 
The  plagioclase  has  in  the  majority  of  cases  been  proved  to  be 
oligoclase,  but  it  is  often  impossible  to  determine  accurately  its  true 
nature.  Traces  of  zonal  banding  are  common  in  the  plagioclase  of 
the  larger  masses  of  the  intrusion.  This  plagioclase-banding  is  often 
very  puzzling.  A  part  of  a  crystal  is  well-twinned  oligoclase  ;  the 
rest  may  show  no  twinning  at  all  and  be  water-clear.  The  twinned 
portion  merges  insensibly  into  the  untwinned.  A  common  pheno¬ 
menon  is  a  binary  twin,  with  little  patches  of  plagioclase-banding  in 
one  half  only,  and  from  this  we  pass  to  the  simple  binary  twin, 
identical  as  regards  state  of  decomposition  with  the  undoubted 
oligoclase.  The  passage  strongly  suggests  that  the  binary  twin  is 
not  orrhoclase,  and  for  that  reason  a  note  of  interrogation  is  often 
put  after  4  orthoclase  ’  in  describing  the  specimens. 
The  analyses  of  the  microcline  are  not.  satisfactory.  The  variation 
in  the  proportion  of  potash  to  soda  is  such  as  to  suggest  that  some 
albite  is  invariably  intergrown  with  the  microcline  proper.  The 
individual  crystals  in  the  pegmatite  fringing  the  third  igneous  mass 
are  often  of  enormous  size. 
The  highly  crystalline  condition  of  the  rocks  into  which  the 
above-described  igneous  masses  have  been  intruded  has  been  already 
referred  to.  Their  grain  is  coarser  than  in  most  other  areas,  and 
the  micas  are  especially  large.  The  general  aspect  of  the  north¬ 
western  portion  of  the  district  is  distinctly  gneissose.  Proceeding  in 
a  south-easterly  direction,  this  gneissose  character  is  gradually  lost, 
the  rocks  become  finer  in  grain  and  more  like  the  ordinary  schists 
of  the  Southern  Highlands.  Eventually  they  assume  the  phase  of 
phyllites  in  the  case  of  the  originally  fine-grained  sediments,  and  of 
more  or  less  crystalline  arkose  grits  in  the  case  of  the  coarser  deposits. 
This  decrease  in  crystalline  aspect  is  most  rapid  when  measured 
in  a  south-easterly  direction  from  the  pegmatites,  which  have  been 
proved  to  form  the  outer  edge  of  the  intrusion.  Eurther,  the 
variation  in  crystalline  aspect  has  been  found  to  be  accompanied 
over  a  large  area  by  a  change  in  the  minerals  of  which  the  rocks 
are  composed.  This  is  especially  well-marked  in  the  case  of  the 
aluminous  silicates  (sillimanite,  c}Tanite,  and  staurolite),  minerals 
which  have  been  clearly  recognized  as  the  products  of  thermo- 
metamorphism  (so-called  contact-metamorphism).  In  the  area  in 
question  the  three  minerals  mentioned  above  characterize  three 
more  or  less  distinct  zones,  the  boundaries  of  which  are  sensibly 
parallel  with  a  line  joining  the  most  southerly  or  south-easterly 
exposures  of  the  igneous  intrusion  at  the  surface.  Thus  the  distri¬ 
bution  of  the  minerals,  as  well  as  the  crystalline  phase  of  the  rocks, 
