348 
ME.  G.  BAREOW  ON  AN  INTRUSION" 
[Aug.  1893, 
microscopic  section  (5105)  was  made  from  the  edge  of  one  of  these 
aggregates.  The  cyanite  is  clear  and  glassy,  and  only  one  little 
spot  shows  the  blue  colour.1  It  is  set  in  a  matrix  of  chlorite  (after 
biotite),  white  mica  and  iron  ores,  associated  with  a  curious  dusty 
material,  probably  quartz  full  of  minute  inclusions.  Some  of  these 
are  large  enough  to  be  made  out  as  garnets,  but  most  of  the  dust  is 
black  and  probably  consists  of  iron  ores. 
Now,  comparing  the  rocks  of  this  neighbourhood  with  those  seen 
about  Loch  Lee  and  the  watershed  of  Glen  Clova,  there  is  a  very 
great  difference  between  the  matrix  in  which  the  cyanite  is  set  in 
the  southern  and  northern  areas.  In  the  latter  the  micas  are  fre¬ 
quently  large,  and  each  individual  may  be  easily  seen  by  the  unaided 
eye ;  so  that  it  is  not  correct  to  say  that  any  eyanite-bearing  rock 
is  less  crystalline  than  any  sillimanite-bearing  rock.  But  it  is 
absolutely  true  that  a  specimen  of  the  former  is  invariably  less 
crystalline  than  a  specimen  of  the  latter  lying  between  that  cyanite- 
rock  and  the  nearest  outcrop  of  igneous  gneiss. 
In  addition  to  the  eyanite-bearing  rocks  described  above  as 
occurring  in  this  zone,  there  are  some  gneisses,  quartzites,  and  grits. 
The  former  are  felspathic  in  the  northern  area  ;  but  though  lenticles 
of  felspar,  or  quartz  and  felspar,  are  often  met  with,  the  individual 
grains  are  much  smaller  than  those  composing  similar  lenticles  in 
the  sillimanite-zone  close  by.  The  quartzites  are  thin  bands  occur¬ 
ring  near  the  Clova  Hotel  and  in  Glen  Effock.  They  have  a  very 
bedded  look,  due  to  the  presence  of  thin  layers  of  white  and  brown 
mica.  But  these  layers  have  nothing  to  do  with  original  bedding, 
for  they  are  entirely  parallel  to  the  planes  of  strain-cleavage.  The 
grits  are  especially  interesting.  On  the  eastern  side  of  Loch 
Brandy,  as  well  as  in  Ben  Beid  on  the  west,  there  are  some  rather 
gneissose  felspathic  rocks,  which  from  their  weathering  appear  to 
be  intensely  altered  felspathic  grits,  traces  of  the  incompletely- 
absorbed  felspar-pebbles  being  still  visible.  Moreover,  at  the  former 
locality  these  pass  gradually  into  more  siliceous  rocks,  in  which  much- 
deformed  quartz-pebbles  can  be  recognized  in  a  highly  siliceous 
matrix  of  quartz-schist.  The  high  percentage  of  silica  was  not 
favourable  to  the  development  of  new  minerals,  and  the  original 
clastic  character  of  the  rock  can  still  be  made  out.  Again,  starting 
from  the  southern  side  of  Glen  Effock  and  proceeding  along  the  low 
ridge  between  the  head  of  Keeny  Burn  and  the  main  stream  of  the 
North  Esk,  this  phenomenon  is  repeated  far  more  slowly.  The  rocks 
gradually  become  less  gneissose,  and  traces  of  pebbles  are  seen  here 
and  there,  every  little  infolded  patch  with  an  originally  siliceous 
matrix  being  an  unquestionable  pebbly  grit.  Gradually  even  the 
felspathic  grits  show  clearly  their  original  clastic  character,  till,  as 
we  approach  Bulg,  the  whole  mass  of  the  group  of  pebbly  grits  is 
clearly  recognizable. 
On  Cairn  Caidloch,  a  mountain  due  south-east  of  Loch  Lee,  is  a 
mass  of  pure  white  quartzite.  On  breaking  the  loose  blocks,  they 
1  The  blue  colour  is  hardly  ever  seen  in  microscopic  slides ;  when  it  is,  the 
colour  occurs  only  in  isolated  spots,  like  the  yellow  specks  in  cordierite,  etc. 
