338 
ME.  G.  BARROW  OX  AX  IXTEUSIOX 
[Aug.  1893, 
metamorpliisin  dne  to  the  Galloway  Granite.1  Her  investigations 
have  an  important  bearing  on  the  origin  of  the  crystallization  of 
the  Highland  schists.  Sillimanite  is  shown  to  occur  close  to  the 
granite  both  in  altered  grits  and  shales,  and  is  associated  with  quartz 
and  two  micas.2 * * 
In  the  area  under  consideration  sillimanite  occurs  in  two  phases. 
First  and  by  far  the  most  abundant  mode  of  occurrence  is  that 
known  as  quartz  sillimanitise  or  faserkiesel.  This  substance  con¬ 
sists  of  minute  glassy  needles  of  sillimanite  embedded  in  quartz. 
The  centre  of  a  patch  is  seen,  under  the  microscope,  to  be  made  up 
of  a  great  number  of  massed  needles.  AY  here  the  needles  are  fresh, 
they  produce  the  effect  of  ice-needles  in  snow ;  that  is,  the  centre 
of  a  patch  is  opaque  snow-white.  Away  from  the  centre  the  needles 
decrease  in  number,  and  at  the  edges  isolated  needles  are  seen.  Under 
crossed  nicols  the  mineral  gives  high  polarization-tints,  and  the  usual 
linear  arrangement  is  well  brought  out  in  the  centre  of  the  mass, 
giving  the  whole  a  more  intense  ^user-appearance.  The  patches 
of  faserkiesel  weather  out  on  the  surface  of  the  rocks.  They  appear 
as  quartz  with  a  silky,  pearly,  or  nacreous  lustre,  the  tint  varying 
from  pure  white  to  yellowish  or  greenish.  The  aspect  of  a  fresh 
hand-specimen  is  perfectly  characteristic,  and  quite  unmistakable 
after  it  has  once  been  clearly  recognized.  The  second  mode  of- 
occurrence  of  the  mineral  is  that  in  which  it  is  more  or  less  inde¬ 
pendent  of  quartz.  The  needles  in  this  case  are  arranged  in  wavy 
threads  and  films,  and  somewhat  resemble  spun  silk.  They  are 
massed  in  the  film  except  at  the  ends,  where  the  needles  diverge, 
and  are  fewer  in  number,  thus  producing  the  impression  of  a  frayed 
edge.  This  frayed  edge  interlocks  with  a  similar  edge  of  some  other 
mineral,  usually  brown  mica.  Sillimanite  in  this  form  occurs  chiefly 
in  small  patches,  and  is  not  often  recognizable  on  a  hand-specimen. 
(2)  Cyanite  or  Disthene. — Like  the  last-described  mineral,  this  is 
also  a  silicate  of  alumina,  and  is  met  with  abundantly  on  weathered 
faces  of  the  rocks  in  which  it  occurs.  The  detailed  mapping  has  shown 
that  a  more  or  less  interrupted  belt  of  cyanite-bearing  rocks  follows 
the  south-eastern  edge  of  the  ‘  sillimanite-zone  ’  in  such  a  way  as  to 
form  an  outer  zone  of  metamorphism.  The  interruptions  of  the  belt 
are  due  to  its  being  crossed  by  rocks  that  are  not  likely  to  contain 
free  silicate  of  alumina  in  any  great  quantity,  such  as  siliceous  rocks ; 
for  these  zones  are  not  rigidly  coincident  with  the  strike  of  the  rocks 
of  the  district.  The  general  trend  of  this  zone  is  shown  in  the 
Map  (PI.  XV.),  and  specially  good  places  for  collecting  are  indicated 
by  the  letter  c.  Cyanite  occurs  macroscopically  in  two  forms  :  first, 
as  single  crystals  scattered  fairly  evenly  through  the  mass  of  the  rock, 
and  secondly,  as  more  or  less  segregated  or  aggregated  crystals.  The 
single  crystals  show  ver}7  little  variation  in  form.  Thej-  may  be  briefly 
described  as  follows  : — Take  a  piece  of  common  lead-pencil  half  an 
1  Quart.  Journ.  Geol.  Soc.  vol.  xlvi.  p.  569. 
2  Since  I  wrote  the  above,  Mr.  W.  M.  Hutchings  has  kindly  lent  me  some 
slides  of  highly-altered  ash,  taken  a  few  feet  from  the  edge  of  the  Shap  Granite. 
In  these  sillimanite-needles  are  seen,  embedded  in  quartz. 
