334 
MR.  G.  BARROW  ON  AN  INTRUSION 
[Aug.  1893, 
bubbles.  Magnetite  occurs  as  an  accessor}’.  Thus  the  brown  mica 
is  almost  absent,  and  oligoclase  is  quite  subordinate  in  amount, 
white  mica  and  potash-felspar  being  dominant.  Finally  the 
borders  of  the  mass  are  fringed  with  pegmatite,  of  which  there  is 
a  vast  quantity  on  the  south-eastern  side,  and  of  this  rock  micro- 
cline  is  the  chief  component.  The  pegmatite  here  often  assumes 
the  ‘  graphic  ’  structure,  and  where  this  is  the  case  the  amount  of 
muscovite  is  always  small.  A  section  (4233)  shows  microc-line 
and  albite  in  microperthitic  intergrowths,  with  some  quartz.  The 
4  graphic’  structure  is  seen  in  portions  only  of  the  slide. 
The  special  points  to  be  noticed  in  this  area  are  as  follows  • — The 
centres  of  the  broader  portions  contain  oligoclase  to  the  exclusion  of 
microcline,  and  are  more  foliated  than  the  rest  of  the  mass  (which 
is  essentially  a  slightly  foliated  granite,  and  contains  the  two  felspars 
in  nearly  equal  parts)  ;  white  mica  is  here  much  in  excess  ;  while 
close  to  the  edges  microcline  is  the  dominant  felspar,  and  muscovite 
the  dominant  mica. 
A  typical  specimen  (4241)  of  the  fourth  mass,  taken  from  the 
head  of  Kennel  Burn,  may  be  described  as  a  fairly  coarse  granitic 
rock,  composed  of  quartz,  microcline,  plagioclase,  and  two  micas. 
Microcline  is  the  dominant  felspar.  A  tendency  on  the  part  of  the 
micas  to  flow  round  the  plagioclase  is  well  marked  in  a  few  places, 
otherwise  the  structure  approximates  very  closely  to  that  of  a 
normal  granite.  On  the  south-east  of  the  mass  all  trace  of  foliation 
disappears,  and  the  rock  is  an  aplite.  This  last,  from  its  composition, 
probably  represents  the  pegmatite,  which  now  forms  a  fringe  on  the 
northern  and  western  edges.  The  feature,  then,  of  this  fourth  mass 
is  that  microcline  much  exceeds  plagioclase,  and  brown  mica  occurs 
only  in  very  small  quantity. 
The  facts  enumerated  above  conclusively  prove  that  this  great  in¬ 
trusion  becomes  steadily  more  and  more  acid  as  we  follow  it  from 
the  north-west  to  the  south-east ;  and  further,  that  the  acid  cha¬ 
racter  of  each  outcrop  is  more  pronounced  on  its  southern  and 
eastern  edges  than  in  the  rest  of  the  mass ;  except  perhaps  in  the 
case  of  the  fourth  mass,  which  is  very  acid  throughout.  This 
conclusion  may  be  tabulated  as  follows  : — - 
Korth- western  Area 
South-eastern  Area 
J  1 .  Maximum  plagioclase,  minimum  microcline. 
\  2.  Maximum  biotite,  smallest  white  micas. 
J  1.  Minimum  plagioclase,  maximum  microcline. 
\  2.  Minimum  biotite,  largest  white  micas. 
The  meaning  of  this  distribution  of  the  component  minerals  is  made 
intelligible  by  examining  one  of  the  larger  apophyses  of  any  of  the 
post-metamorphic  Highland  granites  shown  in  Sir  Archibald  Geikie’s 
Geological  Map  of  Scotland  (2nd  ed.  1892).  At  the  junction  with 
the  granite  the  dyke  is  very  coarse  in  texture,  but  farther  from  the 
parent  mass  it  gradually  becomes  less  and  less  coarse,  till  eventually  it 
consists  of  porphyritic  crystals  set  in  a  fine-grained  matrix.  These 
crystals,  which  belong  to  the  earlier  phase  of  consolidation,  consist 
of  bipyramidal  quartz,  oligoclase,  and  brown  mica,  while  the  bulk 
