Yol.  49.] 
OF  MTJSCOYITE-BIOTITE  GFTEISS. 
333 
(4557)  from  a  specimen  taken  near  the  western  side  of  the  mass 
shows  the  following  characters : — The  rock  is  a  fine-grained,  grani¬ 
toid  gneiss,  composed  of  quartz,  plagioclase,  a  little  orthoclase  (?), 
and  two  micas.  There  is  no  microcline.  Its  structure  is  more  like 
that  of  a  normal  granite  than  the  last.  The  plagioclase  does  not 
show  the  lenticular  form,  but  is  rather  rounded  in  outline.  The 
micas  are  in  distinctly  thicker  flakes,  especially  the  muscovite,  which 
shows  a  tendency  to  idiomorphism.  The  dark-bordered  inclusions 
in  the  biotite  are  hardly  so  numerous  and  are  extremely  small, 
still  some  can  be  clearly  made  out  to  he  zircon.  Irregular  grains  of 
garnet  are  also  met  with.  There  are  no  apatite-inclusions.  The 
quartz  fits  into  the  rest  of  the  rock  as  in  a  granite.  A  point  of 
special  importance  is  the  occurrence  of  micropegmatite.  The  rock 
is  essentially  a  foliated  granite,  and  the  foliation  is  even  less  marked 
under  the  microscope  than  in  the  hand-specimen.  Several  slides 
have  been  cut  from  this  mass  and  they  are  mostly  similar  to  the 
above.  On  approaching  the  eastern  edge  the  rock  has  rather  a  pink 
tinge,  and  after  one  has  crashed  up  a  number  of  fragments  it 
becomes  clear  that  this  part  of  the  foliated  granite  contains  a 
considerable  amount  of  microcline,  a  mineral  hitherto  absent.  It 
has  already  been  stated  that  pegmatite,  in  which  the  hulk  of  the 
felspar  is  microcline,  is  often  largely  developed  here. 
The  points  in  which  this  area  differs  from  the  former  may 
therefore  be  summarized  as  follows : — White  mica  is  more  abundant 
than  brown;  micropegmatite  occurs  throughout;  the  structure  is 
more  granitic,  and  microcline  begins  to  appear  near  the  edges  of  the 
mass. 
The  third  mass  varies  considerably  in  character.  Small  portions 
of  the  central  and  west-central  parts,  where  the  outcrop  is  broad, 
closely  resemble  the  area  last  described ;  but  the  main  mass  has 
a  different  character,  well  represented  by  a  specimen  (4234) 
from  the  eastern  side  of  Ben  Tirran.  It  is  composed  of  quartz, 
oligoclase,  orthoclase  (?),  microcline,  muscovite,  and  biotite.  The 
quartz  contains  4  hairs,’  and  inclusions  with  bubbles.  The  4  hairs  ’ 
may  occasionally  be  seen  crossing  the  junction  of  two  quartz-grains 
without  interruption  or  distortion.  Zircon  occurs  as  an  accessory. 
Some  of  the  oligoclase  shows  fairly  well-marked  zonal  banding. 
This  felspar  and  microcline  are  roughly  equal  in  amount.  The 
white  micas  .are  usually  much  larger  than  the  brown.  Inclusions 
of  zircon  and  garnet  occur  only  in  a  few  crystals  of  biotite  ;  the 
smaller  individuals,  which  much  predominate,  usually  contain  none. 
Towards  the  edge  of  the  intrusion  a  further  increase  of  potash- 
felspar  takes  place.  Thus  a  specimen  from  a  point  about  a  mile 
south-east  of  Loch  Brandy  (4232)  is  composed  of  microcline,  oligo¬ 
clase,  quartz,  muscovite,  and  biotite.  This  is  distinctly  less  foliated 
than  the  two  preceding,  and  the  microcline  exceeds  the  oligoclase 
in  amount.  Still  closer  to  the  edge  a  specimen  (4236)  taken  from 
Bough  Craig,  about  a  mile  east  of  Clova  Hotel,  shows  under  the 
microscope  orthoclase,  microcline,  oligoclase  (rare),  -  quartz,  and 
muscovite.  The  quartz  contains  4  hairs  ’  and  inclusions  with 
