Yol.  49.] 
OF  MUSCOYITE-BIOTITE  GKEISS. 
343 
pleochroic  than,  the  ordinary  form  of  that  mineral.  Most  of  these 
minerals  are  well  known  to  occur  in  cipolino  and  in  varieties  of 
ka  Ik  si  lica  t-h  0  rnfe  Is. 
(10)  Felspar. — Microcline  occurs  in  the  principal  band  of  the 
group  of  calcareous  rocks  mentioned  above.  With  this  exception, 
almost  the  whole  of  the  felspar  developed  in  the  metamorphosed 
rocks  is  plagioclase ;  and,  where  the  identity  of  the  species  can  be 
established,  it  is  mostly  oligoclase.  This  fact  is  of  extreme  import¬ 
ance,  because  the  bulk  of  the  felspar-pebbles  in  the  grits  of  the  less- 
altered  areas  are  also  oligoclase ;  and  these  grits  can  be  clearly  seen 
to  pass  into  gneisses  as  they  approach  the  great  intrusion.  Fresh 
specimens  of  the  oligoclase  are  seen  under  the  microscope  to  be 
water-clear,1  a  feature  which  appears  to  be  characteristic  of  these 
rocks.  The  larger  grains  are  not  only  twinned,  but  the  twinning- 
planes  usually  pass  right  across  the  crystal.  In  the  oligoclase  of  the 
igneous  rocks  the  twinning-planes  frequently  stop  short  of  the  edge 
of  the  crystal,  i.  e.  the  edge  is  more  or  less  completely  untwinned. 
lSTo  trace  of  zonal  banding  has  been  met  with  in  the  felspars  of  the 
metamorphosed  rocks. 
Y.  Hocks  of  the  Metamorphic  Area. 
A  brief  description  may  now  be  given  of  the  principal  types  of 
rock  in  which  the  minerals  above  described  occur.  They  may  be 
divided  into  four  groups  :  firstly,  those  of  the  sillimanite-zone  ; 
secondly,  those  of  the  cyanite-zone  ;  thirdly,  those  of  the  staurolite- 
zone ;  and  lastly,  those  lying  between  the  third  zone  and  the  Great 
Highland  Fault,  as  seen  on  the  banks  of  the  North  Esk. 
(a)  The  Sillimanite-zone. 
Quite  in  the  north-western  corner  of  the  area  here  described  the 
dominant  rock  is  a  coarse  felspathic  gneiss,  of  which  the  felspar 
is  almost  exclusively  oligoclase.  The  felspar  occurs  in  lenticles  of 
variable  size,  more  or  less  associated  with  quartz.  These  lenticles 
are  separated  one  from  another  by  felted  masses  of  mica,  both 
brown  and  white.  As  the  lenticles  get  more  and  more  elongated, 
and  of  greater  thickness  (they  are  at  times  2  feet  thick),  a  rudely 
parallel-banded,  coarse  gneiss  is  produced.  Into  this  the  tongues 
of  igneous  gneiss  are  intruded,  and  so  inextricably  are  the  two 
rocks  interwoven  that  it  frequently  becomes  impossible  to  say  how 
much  of  the  mass  is  igneous  and  how  much  of  metamorphic  origin. 
In  such  rocks  sillimanite  is  rare.  By  an  increase  of  the  silica  or 
alumina,  these  coarse  gneisses  pass  insensibly  into  fine-grained, 
grey,  siliceous  rocks  on  the  one  hand,  or  into  still  gneissose,  though 
less  coarse,  sillimanite-bearing  rocks  on  the  other.  The  former 
siliceous  type  occurs  abundantly  on  the  Driesh,  the  highest  mountain 
1  Messrs.  Harker  and  Marr  have  drawn  attention  to  the  clearness  of  the 
minerals  in  the  metamorphic  rocks  surrounding  the  Shap  Granite,  Quart. 
Journ.  Geol.  Soc.  vol.  xlvii.  (1891)  p.266. 
