342 
ME.  G.  BARROW  ON  AM  INTRUSION 
[Aug.  1893, 
Oval  grains,  similar  to  those  of  brown  mica,  are  rare  •  but  when 
they  do  occur  the  rocks  containing  them  are  always  intensely 
crystalline.  The  only  inclusions  of  importance  in  this  mineral  are 
minute  flecks  of  brown  mica  lying  parallel  to  the  basal  cleavage  of 
the  enclosing  mica.  Chemical  analysis  shows  that  potash  and  soda 
occur  in  the  proportion  of  7*56 :  3*2  per  cent.,  so  that  the  soda  is 
considerably  higher  than  in  the  muscovite  of  the  igneous  rocks.  The 
hite  mica  of  these  highly-altered  rocks  is  frequently  more  or  less 
embedded  in  the  brown,  in  such  a  way  that  the  basal  plane  of  the 
latter  makes  an  angle  of  about  60°  with  the  former.  When  the 
muscovite  appears  in  section  as  a  number  of  tiny  laths,  these  laths 
bear  the  same  relation  to  the  brown  mica  as  the  felspars  do  to  the 
augite  in  an  ophitic  dolerite. 
(6)  Garnets. — The  garnets  in  this  area  do  not  differ  from  those  so 
abundant  all  over  the  Highlands,  until  we  approach  one  of  the 
igneous  masses,  or  are  well  within  the  area  of  intense  thermometa¬ 
morphism.  In  this  case  they  are  of  a  weak  port-wine  colour,  and 
remarkable  for  their  transparency  ;  such  garnets  do  not  habitually 
show  the  usual  crystalline  form  (dodecahedron)  when  large,  but  are 
often  like  drops  of  transparent  red  gum.  Both  under  the  micro¬ 
scope  and  in  the  hand-specimen,  they  exactly  resemble  the  garnets 
which  Miss  Gardiner  has  shown  to  be  produced  by  the  intrusion  of 
the  Galloway  granite  in  the  surrounding  rocks. 
(7)  Tourmaline  or  Schorl. — This  mineral  has  not  been  largely 
developed ;  but  there  seems  good  ground  for  stating  that  when 
it  occurs  in  the  pegmatites  it  is  also  developed  in  the  adjacent 
schists.  The  number  of  instances  is  too  small  to  enable  one  to  speak 
confidently.  The  schorl  of  the  igneous  rock  is  brown  in  thin  sections ; 
it  is  dark  olive-green  in  the  schists  (slide  4865).  The  blue  schorl 
or  the  limestone  is  not  specially  connected  with  the  pegmatites. 
(8)  Quartz. — Over  an  area  of  at  least  60  square  miles  the  larger 
grains  of  quartz  possess  a  curious  character.  Under  crossed  nicols 
they  do  not  extinguish  as  a  whole ;  but  neither  can  the  extinction 
be  fairly  called  undulose,  There  is  a  difference  of  several  degrees 
in  the  extinction-positions  of  different  parts  of  a  large  grain.  Thus 
a  grain  will  break  up  sometimes  into  five  or  six  parts  ;  while  half 
are  dark,  the  remainder  are  very  nearly  so  ;  but  there  is  no  definite 
boundary  to  the  patches,  which  fade  into  one  another.  This  phe¬ 
nomenon  is  so  constant  in  the  area  surrounding  the  intrusions,  and 
so  uncommon  elsewhere,  that  it  seems  unreasonable  to  doubt  that 
this  curious  optical  property  has  been  produced  by  intense  heating. 
As  is  well  known,  the  optical  properties  of  many  minerals  are  liable 
to  be  entirely  changed  by  being  kept  at  a  high  temperature  for  a 
considerable  time. 
(9)  Lime-silicates. — A  distinct  group  of  minerals  has  been  deve¬ 
loped  in  the  calcareous  rocks  of  this  district.  The  chief  members 
of  this  group  are  malacolite,  a  green  pyroxene,  zoisite,  epidote, 
idocrase,  and  sphene.  A  variety  of  pale-brown  mica  also  occurs, 
but  not  in  sufficient  quantity  to  enable  us  to  determine  its  compo¬ 
sition;  it  has  a  somewhat  different  appearance  from,  and  is  less 
