340 
ME.  G.  BAEEOW  ON  AN  INTETJSION 
[Aug.  1893, 
passage  in  Dana’s  4  System  of  Mineralogy,’  6th  ed.  1892,  p.499  : — 
44  Vernadsky  shows  that  cyanite  is  transformed  at  1320°-1380° 
into  sillimanite  ”  (Bull.  Soc.  Franc.  Mineral,  vol.  xii.  p.  447, 
1889,  and  vol.  xiii.  p.  256,  1890).  Thus  the  experience  of  the 
laboratory  furnishes  an  explanation  of  the  fact  that  sillimanite 
characterizes  the  inner,  and  cyanite  the  outer,  zone  in  an  area  of 
thermometamorphism.  Further,  we  obtain  a  general  idea  of  the 
high  temperature  to  which  great  masses  of  the  Central  Highland 
rocks  have  been  raised.  This  temperature  was  probably  greater 
than  that  suggested  by  Vernadsky’s  experiments  ;  for  as  the  specific 
gravity  of  sillimanite  (3*23)  is  less  than  that  of  cyanite  (3-56-3-60), 
the  temperature  required  to  effect  the  change  would  almost  certainly 
be  increased  by  pressure. 
(3)  Staurolite. — In  addition  to  the  zones  already  described,  there 
is  a  third  still  farther  removed  from  the  igneous  rocks,  characterized 
by  the  presence  of  staurolite.  This  is  the  best  marked  of  all ;  for, 
with  the  exception  of  an  interval  of  a  few  hundred  yards,  the  zone 
is  nowhere  interrupted  by  siliceous  rocks  throughput  a  distance  of 
nearly  10  miles.  This  is  probably  due  to  the  fact  that  the  zone 
very  nearly  corresponds  with  the  actual  outcrop  of  a  particular  bed. 
Moreover,  if  a  line  be  drawn  joining  up  all  the  most  southerly  out¬ 
crops  of  the  intrusion,  then  this  zone  is  roughly  parallel  with  that 
line.  In  hand-specimens  staurolite  usually  weathers  out  as  yellow- 
tinged  crystals.  In  the  more  crystalline  areas  these  are  of  large 
size,  and  twinning  is  the  rule,  the  exception  being  to  find  untwinned 
crystals,  in  which  respect  staurolite  is  the  reverse  of  cyanite.  For 
some  undiscovered  reason,  twinning  takes  place  so  that  the  crystals 
cut  one  another  at  an  angle  of  60°.  In  other  districts  in  the  Central 
Highlands  the  twins  frequently  assume  the  form  (roughly)  of  a 
Maltese  cross  ;  but  here  they  rarely  do  so.  The  crystals  may  be 
seen,  from  the  part  projecting  from  the  rock,  to  be  mostly  eight- 
sided  or  six-sided.  Under  the  microscope  they  vary  in  colour  from 
pale  yellowish-brown  to  red,  and  are  distinctly  pleochroic.  Their 
refractive  index  is  a  little  higher  than  that  of  quartz,  and  they  are 
much  cracked,  the  cracks  standing  out  in  strong  relief.  The  mineral 
is  composed  of  silica,  alumina,  and  iron,  thus  differing  from  silli¬ 
manite  and  cyanite  in  composition,  and  approaching  the  garnets. 
It  is  essentially  an  unstable  mineral,  in  marked  contrast  with  its 
ally,  the  garnet,  which  is  unusually  stable.  It  is  rare  to  find  a 
crystal  of  staurolite  which  has  not  decomposed  more  or  less  to  a 
4  shimmer ’-aggregate ;  and  if  a  weathered-out  crystal  be  pounded 
up,  scarcely  a  single  grain  will  be  found  to  possess  the  optical  pro¬ 
perties  of  the  original  mineral. 
(4)  4 Shimmer’ -aggregates. — Sillimanite,  cyanite,  and  staurolite  all 
tend  to  pass  over  to  a  fine-grained  material  giving  a  4  shimmer  ’  of 
high  polarization-tints  under  cross  nicols.  Dr.  Barrois,  op.  jam  cit ., 
says  they  have  been  altered  ( epigenise )  into  white  mica.  But  though 
the  shimmer  of  colour  may  be  due  to  very  minute  scales  of  this 
mineral,  they  must  be  cemented  by  some  hard  substance,  possibly 
quartz;  otherwise  the  replacing  material  would  not  project,  but 
