352 
ME.  €r.  EAREOW  ON  AD?  INTRUSION 
[Aug.  1893, 
Moreover,  it  appears  that  the  entire  area  is  formed  of  some  seven, 
or  at  most  eight  bands  of  rock,  not  one  of  which  need  originally 
have  been  of  any  great  thickness.  The  detailed  evidence  for 
these  statements  is  too  long  and  complicated  to  be  given  here,  and 
must  be  reserved  for  the  official  Survey  memoir  on  the  district. 
VII.  Evidence  oe  Progressive  Metamorphism. 
The  green-mica  quartzite,  which  has  been  so  often  referred  to,  is 
seen  on  the  eastern  flank  of  Bulg.  Ascending  this  hill  and  keeping 
along  the  ridge  overlooking  Keeny  Burn,  one  meets  with  the 
quartzite  at  intervals  infolded  in  staurolite-schist.  On  the  same 
ridge  farther  north  it  can  be  followed  continuously  for  some  miles, 
while  the  metamorphism  in  the  surrounding  rocks  is  seen  to  be 
steadily  increasing.  Quartzite  traverses  the  zone  of  cyanite-schists, 
and  finally  at  East  Craig,  near  the  head  of  Inchgrundle  Burn,  it 
enters  the  sillimanite-zone,  where  it  is  penetrated  by  the  third  mass 
of  the  great  igneous  intrusion.  Thus  this  quartzite,  which  is  easily 
recognized  and  is  not  liable  to  undergo  important  alteration  in  con¬ 
sequence  of  its  composition,  may  be  followed  through  the  various 
metam orphic  zones.  The  band  of  rock  adjacent  to  the  quartzite,  at 
the  point  farthest  from  the  igneous  mass,  is  a  staurolite-schist.  As 
we  approach  the  intrusion,  it  becomes  first  of  all  a  cyanite-gneiss, 
and  finally,  close  to  the  contact,  a  coarse-grained  sillimanite-gneiss. 
A  more  conclusive  proof  of  progressive  metamorphism  in  one  and 
the  same  bed  is  not  easily  conceivable. 
YIII.  General  Conclusions,  and  Summary  op  Results. 
In  concluding  this  account  of  the  thermometamorphism  or  the 
South-eastern  Highlands,  there  are  two  features  that  call  for  special 
explanation.  These  are,  firstly,  the  great  extent  of  the  area  affected, 
and  secondly,  the  intensity  of  the  alterations  produced. 
Much  light  is  thrown  on  the  first  of  these  by  the  wide  distribution 
of  the  pegmatites.  The  connexion  between  these  pegmatites  and 
the  muscovite-biotite  gneiss  has  been  established  in  the  first  part 
of  this  paper.  They  have  been  shown  to  represent  that  part  of  the 
magma  which  consolidated  last,  and  penetrated  farthest  into  the 
overlying  rocks.  Their  occurrence  in  any  area  may  therefore  be 
reasonably  supposed  to  indicate  the  presence  of  an  underlying  mass 
of  muscovite-biotite  gneiss ;  as  they  increase  or  decrease  in  quan¬ 
tity,  it  becomes  obvious  that  this  gneiss  is  nearer  to  or  deeper  below 
the  surface.  But  the  area  over  which  they  occur  is  so  great  that, 
viewed  on  a  large  scale,  the  upper  surface  of  this  igneous  mass 
must  be  roughly  horizontal.  And  so  we  are  led  to  the  conclusion 
that  these  gneisses  occur  in  huge  sills  or  laccolites  having  approxi¬ 
mately  horizontal  upper  surfaces.  That  this  is  the  true  explanation 
of  their  mode  of  occurrence  is  rendered  almost  certain  by  the  fact 
that  we  have  already  mapped  a  sill  of  muscovite-biotite  gneiss, 
closely  resembling  the  gneiss  of  the  permeation-area  in  its  mode  of 
intrusion,  in  which  both  the  top  and  the  base  keep  at  a  fairly 
