Yol.  49.] 
OF  irrSCOTITE-BIOTHE  GXEISS. 
339 
inch,  long  ;  cut  off  two  slips  along  the  length  of  the  pencil,  so  that 
the  flat  faces  are  parallel  and  the  distance  between  them  is  roughly 
half  that  of  the  diameter  of  the  pencil ;  then  round  the  ends  a  little, 
and  a  fair  model  is  produced  of  all  the  isolated  crystals  of  cyanite 
in  the  district.  This  rounding  of  their  angles  and  ends  is  a  never- 
failing  character.  As  the  crystals  become  smaller,  the  tendency 
to  roundness  in  outline  becomes  more  marked ;  but  while  the 
brachypinacoid  (0 1 0)  is  often  obliterated  in  this  manner,  the 
macropinacoid  (1  0  0)  never  is  quite  lost.  All  crystals  of  cyanite 
cleave  with  extreme  facility  parallel  to  the  latter  face,  and  fairly 
well  parallel  to  the  former.  They  break  easily,  but  at  irregular 
intervals,  nearly  at  right  angles  to  the  macropinacoid,  across  the 
length  of  the  prism.  These  planes  of  fracture  are  the  well-known 
4  gliding-planes  ’  parallel  to  the  basal  pinacoid.  Further,  these 
evenly-distributed  crystals  are  never  twinned,  so  far  as  has  been 
observed  in  this  district.  An  interesting  feature  in  the  larger 
isolated  crystals  is  that,  instead  of  showing  the  typical  blue  colour 
to  which  the  mineral  owes  its  name,  they  seem  to  be  coated  with  a 
black  dust,  having  the  appearance  of  graphite  or  4  black  lead.’  As 
such  crystals  are  extremely  abundant  over  large  areas,  they  may  be 
conveniently  designated  ‘  black-leaded,’  to  distinguish  them  from 
the  normally  blue-coloured  specimens.  The  graphitic  aspect  of  the 
black  dust  is,  however,  deceptive  ;  for,  on  examining  a  cleavage-flake 
under  the  microscope,  it  is  seen  that  these  crystals  have  a  vast 
number  of  grains  (sometimes  crystals  ?)  of  iron  ore  scattered  through 
them  in  a  fairly  even  manner.  It  is  impossible  not  to  feel  that  the 
structure  thus  produced  is  of  the  same  character  as  those  described 
by  Dr.  Salomon,  as  occurring  in  the  inclusions  in  the  tonalite  of 
Adamello  ;  and  these  4  black-leaded  ’  cyanite-crystals  exhibit 
4  typical  contact-structure.’  Sections  of  this  mineral  under  the 
microscope  are  easily  recognized.  In  the  first  place,  they  have  a 
clear  glassy  appearance,  very  rarely  showing  the  blue  colour  of  the 
large  crystals.  Then  their  high  refractive  index,  which  makes  their 
borders  and  characteristic  cross-jointing  or  cleavages  stand  out  in 
bold  relief,  assists  largely  in  the  determination.  A  section  parallel 
to  the  length  of  the  crystals  extinguishes  almost  straight,  if  it  be 
also  parallel  to  the  face  010,  and  at  about  30°  if  parallel  to  the 
face  of  easiest  cleavage  (10  0).  Under  crossed  nicols  they  give 
moderately  high  polarization-tints.  It  may  be  here  added  that 
the  isolated  crystals  tend  to  lie  with  their  broadest  face  (1  0  0)  pa¬ 
rallel  to  the  dominant  foliation  of  the  rock ;  but  it  is  doubtful  how 
far  this  holds  good  in  the  case  of  very  small  crystals.  Cyanite 
also  occurs  as  segregations  or  aggregations.  In  both  cases  the  blue 
colour  is  well  shown ;  but  by  far  the  most  beautiful  crystals  occur 
in  quartz-segregations,  from  which,  unfortunately,  it  is  impossible 
to  extract  them,  as  they  break  so  much  more  easily  than  quartz. 
My  colleague,  Mr.  J.  J.  H.  Teall,  F.E.S.,  in  addition  to  kindly 
helping  me  with  the  petrological  descriptions  of  the  slides,  has  further 
drawn  my  attention  to  some  very  remarkable  literature  bearing 
on  the  points  here  discussed,  and  has  shown  me  the  following 
