Vol.  49.] 
OF  MUSCOVITE-BIOTITE  GNEISS. 
335 
of  the  finer  material  consists  of  potash-felspar  and  quartz,  the 
minerals  of  later  consolidation.1  By  applying  these  facts  to  the 
distribution  of  the  minerals  in  the  area  under  consideration,  we  see. 
that  those  which  are  most  abundant  in  the  north-west  belong  to  the 
first  phase  of  consolidation,  while  those  which  are  most  abundant  in 
the  south-east  belong  to  the  second  phase.  Thus  the  earlier-formed 
minerals  appear  to  have  been  retained  or  strained  off,  while  the  still 
liquid  potash-bearing  material  travelled  on  in  a  south-easterly 
direction.  Hence  it  may  be  inferred  that  the  source  of  origin  lies 
to  the  north-west,  and  that  the  intrusion  ends  with  the  formation 
of  the  masses  of  pegmatite,  which  are  the  acid  or  potash-bearing 
residuum.  Further,  the  fact  that  in  one  area  the  intrusion  is  minutely 
subdivided  while  in  the  south-east  it  hangs  together,  finally  consoli¬ 
dating  as  aplite,  suggests  that  the  rocks  to  the  north-west  must  have 
had  a  higher  initial  temperature  than  those  farther  south-east.  It 
will  be  found  that  this  view  is  strongly  corroborated  by  the  evi¬ 
dence  adduced  later  on.  The  high  temperature  was  probably  due 
to  the  intrusion  of  slightly  older  gneisses,  of  the  same  type  as  the 
muscovite-biotite  gneiss,  in  which  white  mica  is  rare  or  absent.  It 
was  intended  to  give  a  brief  account  of  the  intrusion,  but  the  idea  has 
been  abandoned  on  account  of  the  writer’s  reluctance  to  lengthen 
what  is  already  a  long  paper. 
The  igneous  origin  of  these  pegmatites  having  been  placed  beyond 
reasonable  doubt,  a  few  additional  details  may  be  given  here  as  to 
their  chief  petrograplrical  characters.  They  are  most  commonly 
composed  of  quartz,  microcline,  and  muscovite,  and  the  smaller 
veins  are  nearly  all  of  this  type.  An  important  feature  is  that 
there  are  always  at  least  two  faces  of  the  prism  developed  in  the 
last-named  mineral,  often  more ;  but  we  know  of  no  case  in  which 
all  six  sides  are  shown  in  a  large  crystal.  Moreover,  the  white 
mica  is  not  often  in  immediate  contact  with  the  microcline.  It  is 
usually  separated  from  the  latter  by  a  very  variable  amount  of 
quartz.  The  other  types  of  pegmatite  are  a  rude  kind  of  graphic 
granite,  quartz-microcline  rock  common  in  the  larger  veins  and 
masses,  and  a  quartz-muscovite  rock  which  occurs  only  in  small 
patches.  In  addition  to  these  normal  constituents,  garnet  and 
schorl  are  associated  with  the  pegmatites.  The  former  is  locally 
abundant,  though  absent  over  considerable  areas  ;  the  latter  also  is 
abundant  locally,  and  is  the  black  variety,  brown  in  thin  section. 
We  may  conclude  the  petrographical  description  of  this  intrusion 
with  a  short  account  of  some  of  the  special  features  of  the  minerals 
which  enter  into  the  composition  of  the  rocks.  The  brown  mica 
has  been  isolated  and  analysed.  The  iron  occurs  as  FeO  =  20*87 
and  Feo03  =  2*56  per  cent.  The  magnesia  averages  only  4*32  per 
cent.  As  the  mineral  is  approximately  uniaxial,  the  mica  may  be 
fairly  classed  as  haughtonite.  The  inclusions  in  it  are  grains  of 
garnet  and  zircon,  and  the  latter  has  always  a  black  ‘  skin  ’  in  addition 
1  See  a  paper  by  the  present  writer  on  the  ‘  Origin  of  certain  Gneisses,’  Geol. 
Mag.  for  1892,  pp.  64-65. 
