406  DR.  C.  CALLAWAY  ON  THE  OBIGIN  OE  THE  [Atlg.  1 893, 
much  obscured  by  opaque  matter  in  the  form  of  disseminated  dust, 
which  must  be  iron  oxide  liberated  from  the  hornblende.  The 
chlorite,  which  is  in  irregular  strings  and  patches,  is  in  part  similar 
to  the  last ;  but  some  of  it  is  more  strongly  pleochroic  (sap-green 
to  pale  yellow-green),  and  shows  higher  interference-colours*  Here 
and  there  the  green  shades  into  brown,  and  the  interference-colours 
rise  still  higher,  so  that  there  is  apparently  a  passage  into  black 
mica.  Under  crossed  nicols,  most  of  the  chlorite  presents  a  con¬ 
fused  appearance  of  aggregate  polarization  ;  but  this  is  seen  to  be 
due  to  the  presence  of  calcite  scattered  through  the  chlorite  in 
numerous  minute  forms.  The  epidote  requires  no  special  notice. 
Much  of  the  felspar  is  in  large  crystalline  forms,  which  are  more 
or  less  cracked  and  broken,  the  fragments  being  sometimes  drawn 
away  from  each  other.  The  cracks  are  filled  in  with  chlorite, 
calcite,  and  the  opaque  dust.  This  felspar  is  clearer  than  the  last. 
Only  a  small  proportion  of  it  shows  twinning  of  any  kind.  The 
bulk  of  the  felspar  polarizes  in  a  rather  irregular  manner,  some 
of  the  fragments  showing  several  patches  of  different  shades  of 
colour  with  vaguely  sinuate  margins,  suggesting  an  approach  towards 
a  mosaic  structure. 
This  rock  is  traversed  by  cracks,  roughly  parallel  to  the  schist- 
osity,  and  more  or  less  occupied  with  the  products  of  decomposition. 
There  has  been  considerable  strain  in  the  mass  subsequent  to  the 
secondary  consolidation,  both  felspar  and  calcite  displaying  undu¬ 
lating  extinction  during  rotation  of  the  microscope-stage. 
No.  470.  Three  or  four  yards  farther  up  the  slope. — The 
crushing  is  greater,  and  the  parallelism  of  the  constituents  more 
marked.  The  chlorite  passes  more  frequently  into  biotite.  The 
slide  is  traversed  by  several  cracks  filled  with  opaque  brownish 
matter  (limonite  ?),  and  where  these  cracks  pass  through  chlorite, 
they  are  margined  by  a  narrow  band  of  the  mica,  as  if  the  mica  had 
been  formed  out  of  the  chlorite  by  the  addition  of  constituents 
supplied  in  the  cracks.  The  felspar  is  not  so  clear  as  in  the  last 
slide.  In  many  of  the  fragments  it  passes  into  a  water-clear 
mineral,  with  the  double  refraction  and  interference- figure  of 
quartz. 
No.  473  (fig.  2).  About  3  yards  farther  west. — The  ground- 
mass  of  the  rock  is  chiefly  opaque  matter  and  the  brownish  mineral 
which  replaces  the  chlorite.  I  have  spoken  of  this  mineral  (under 
No.  466)  as  suggesting  biotite.  It  has  the  precise  habit  of  the 
chlorite,  occurring  in  patches  and  strings ;  but,  here  and  there, 
where  the  mineral  is  brownest,  an  approach  to  a  fibrous  structure 
is  seen,  and  the  absorption  in  these  fibres  is  greatest  when  they  are 
parallel  to  the  principal  section  of  the  polarizer.  Under  crossed 
nicols,  the  extinction  is  parallel,  and  the  interference-colours  are 
high.  5  In  hand-specimens,  the  glimmer  of  mica  cannot  be  detected  ; 
but  as  the  mineral  has  not  developed  distinct  lamellae,  with  cleavage, 
and  barely  even  a  fibrous  structure,  we  should  hardly  expect  to 
find  reflecting-surfaces.  I  therefore  provision  ally .  describe  this 
mineral  as  biotite.  Some  chlorite  is  associated  with  it. 
