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MR.  G.  BARROW  OX  AX  INTRUSION 
[Aug.  1893, 
pegmatite,  which  occurs  in  large  masses.  In  the  neighbourhood  of 
Millden  the  intrusion  is  represented  solely  by  large  masses  of 
pegmatite. 
Thus  it  appears  that  the  intrusive  rock  is  most  gneissose  in 
the  north-western  portion  of  the  area,  and  that,  as  we  proceed 
towards  the  south-east,  the  gneissose  character  is  gradually  lost, 
while  the  amount  of  pegmatite  is  seen  to  increase  until  at  last  it 
occurs  as  isolated  masses,  and  thus  becomes  the  sole  representative 
of  the  intrusion  in  that  area.  It  must  not  be  supposed  that  these 
isolated  pegmatites  are  restricted  to  this  district.  Starting  from 
the  South  Esk,  they  may  be  met  with  at  intervals  at  least  as  far 
west  as  Pitlochry ;  and  they  are  well  known  also  on  the  east  coast, 
a  few  miles  north  of  Stonehaven. 
III.  Petrological  Characters  oe  the  Igxeotts  Pioces. 
The  gneiss  of  the  permeation-area,  when  unweathered,  has  a 
grey  colour.  On  a  surface  cut  at  right  angles  to  the  foliation,  the 
felspars  are  seen  to  be  somewhat  rounded  or  pointed-elliptical  in 
shape.  A  section  made  from  a  specimen  taken  from  the  head  of 
the  Lee  Water  shows  the  following  features  (4557  A)1  : — It  is 
composed  of  quartz,  felspar,  muscovite,  and  biotite.  The  lenticular 
crystals  of  felspar  are  mostly  plagioclase  (usually  oligoclase)  ;  there 
is  no  microcline,  and  it  is  doubtful  whether  there  is  any  orthoclase. 
A  feature  in  the  plagioclase  is  the  very  irregular  way  in  which  the 
polysynthetic  twinning  is  rendered  visible  under  crossed  nicols. 
Part  of  a  lenticle  may  show  none,  while  in  another  portion  of  the 
same  individual  it  is  well  marked  ;  moreover,  part  of  a  crystal  may 
be  water-clear,  while  the  rest  is  more  or  less  clouded.  The  micas 
do  not  show  any  definite  form,  and  the  brown  mica  is  not  much 
less  in  amount  than  the  white.  The  former  contains  numerous 
inclusions  of  small  zircons,  with  marked  pleochroic  halos,  and 
irregular  patches  of  garnet,  with  a  very  fissured  appearance. 
Apatite-inclusions  appear  to  be  totally  absent.  The  areas  of  quartz, 
of  which  there  is  a  considerable  amount,  break  up  under  crossed 
nicols  into  small,  irregularly-shaped  patches  with  a  tendency  to 
undulose  extinction.  Small  allotriomorphic  garnets  are  scattered 
throughout  the  slide. 
The  aspect  of  the  different  minerals  of  the  gneiss  under  the 
microscope  suggests  that  the  crystals  of  earlier  consolidation 
underwent  an  appreciable  amount  of  comminution ;  for,  on  the 
whole,  they  distinctly  differ  in  size  and  shape  from  those  seen  where 
the  intrusions  are  more  or  less  massive.  But  there  is  no  evidence  of 
crushing  after  consolidation. 
Thus  it  appears  that  in  this  area  plagioclase  is  the  dominant 
felspar ;  microcline  is  absent ;  and  the  white  mica  is  about  equal 
in  quantity  to  the  brown. 
In  the  second  area,  where  the  intrusions  are  thicker,  a  slide 
1  The  numbers  of  slides,  referred  to  throughout  this  paper,  are  those  of  the 
slides  preserved  in  the  Museum  of  Practical  Geology,  Jermyn  Street. 
