344 
ME.  G.  B ARROW  ON  AN  INTRUSION 
[Aug.  1893, 
in  the  district.  A  microscopic  section  (4881)  shows  this  rock  to  he 
composed  of  quartz,  felspar,  biotite,  and  muscovite.  .  Structural!}7  it 
is  a  fine  gneiss  or  schist ;  but  the  amount  of  free  silica  in  the  domi¬ 
nant  rock  of  this  mountain  places  the  sedimentary  origin  of  the 
6chist  beyond  reasonable  doubt. 
The  most  gneissose  phase  of  the  sillimanite-bearing  rocks  is  seen 
close  to  the  outcrop  of  the  limestone,  in  the  crags^south  of  the 
stables  on  the  Lee  Water.  A  microscopic  section  (4531)  shows  it 
to  be  composed  of  plagioclase  (mostly  eligoclase),  quartz,  and  ‘white 
and  brown  mica.  The  micas  occur  in  films,  and  aie  associated  v  ith 
a  considerable  amount  of  sillimanite,  which  interlocks  with  the 
frayed  edge  of  the  brown  mica.  A  few  garnets  are  also  present. 
These  three  types,  the  coarse  felspathic,  the  siliceous,  and  the 
coarse  sillimanite-bearing  gneiss,  are  very  abundant  in  the  western 
part  of  the  sillimanite-zone.  A  less  coarsely  gneissose  phase  occurs 
in  the  great  pass  of  the  Lnic-h,  above  Loch  Lee,  and  extends 
almost  down  to  the  Loch.  A  specimen  (4544)  taken  from  Craig 
Maskeldie  is  a  puckered,  grey,  gneissose  rock  in  which  segregation 
of  felspar-len tides  is  still  fairly  well-marked,  and  the  surface  is 
coated  with  quartz  sillimanitise.  It  is  composed  of  quartz,  felspar, 
sillimanite,  muscovite,  biotite,  garnets,  and  iron  ores  (sillimanite- 
gneiss).  Farther  down  the  glen,  and  farther  from  an  outcrop  of 
the  igneous  gneiss,  the  same  band  shows  still  less  segiegation  of 
the  felspars,  and  a  smaller  quantity  of  sillimanite  ;  indeed,  Mr.  Teall 
describes  one  specimen  (4541)  as  a  schist  containing  sillimanite 
rather  than  a  gneiss.  ...  . 
Some  garnetiferous  varieties  of  the  sillimanite-bearing  rocks  are 
seen  on  the  top  of  the  crags  above  Loch  Brandy,  and  stretching  thence 
down  a  narrow  ridge  to  the  Unich.  The  first  of  these  is  a  very  coarse 
carnet-schist  (?  gneiss)  with  minute  patches  of  quartz  sillimanitise. 
Proceeding  towards  the  igneous  gneiss  which  crops  out  close  to  the 
Unich  a  segregation  of  the  components  takes  place.  Siliceous 
patches  with  a  silky  lustre  are  interlaminated  with  more  garneti¬ 
ferous  portions  of  the  rock.  A  microscopic  section  of  the  foimer 
(4540)  shows  sillimanite-needles  embedded  in  quartz. 
is  a  perfectly  typical  specimen  of  faserJciesel  or  quartz  silli¬ 
manitise.  Close  to  the  Unich,  where  it  is  joined  by  the  Longsbank 
Burn,  is  a  considerable  mass  of  extremely  garnetiferous  coarse¬ 
grained  rock,  somewhat  gneissose  in  aspect,,  and  associated  with 
much  sillimanite.  Seen  in  section  (4o45)  it  is  composed  of  quartz 
with  a  little  felspar,  white  and  brown  mica,  and  large,  irregularly 
shaped  wine-clear  garnets.  Iron  ores  and  sillimanite  also  occur. 
Some  of  the  sillimanite  is  enclosed  in  garnet,  but  most  of  it  is  of  the 
filamentous  type,  fraying  out  on  the  edges  and  interlocking  with 
brown  mica.  "  The  garnets  in  this  case  form  nearly  one  halt  of  the 
rock,  which  is  obviously  so  rich  in  silicate  of  alumina  and  iron  as  10 
place  its  sedimentary  origin  beyond  dispute. 
Some  sillimanite-bearing  rocks,  with  a  peculiar  mode  of  weather¬ 
ing,  occur  on  the  hill  about  £  mile  E.N.E.  of  the  church  of  Loch 
Lee.  In  these  the  quartz  sillimanitise  projects  from  the  weathered 
