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DR.  C.  CALLAWAY  ON  THE  ORIGIN  OF  THE 
[Aug.  1893, 
(A)  Simple  Schists. 
The  following  are  the  most  important  varieties  : — 
Hornblende-gneiss  (from  Diorite).  This  schist  involves  the  mini¬ 
mum  of  metamorphism.  The  constituents  have  been  re-arranged, 
and  quartz  has  been  generated.  The  change  is  too  well-known 
to  need  description. 
Biotite-liornblende  Gneiss  (from  Diorite ). — Between  bands  of  the 
gneiss  just  named  and  the  biotite-gneiss  which  often  forms  the 
core  of  a  veined  plexus,  the  rock  becomes  progressively  compressed, 
and  biotite  begins  to  appear.  A  slide  (391) 1  offers  the  following 
characters  : — 
The  hornblende  has  lost  its  clearness  and  cleavage,  is  irregularly 
stained  with  iron  oxide,  and  is  drawn  out  in  the  direction  of 
schistosity  into  irregular  ribbons  or  even  strings.  Sometimes, 
however,  it  retains  perforations  and  traces  of  sinuate  contours. 
The  felspar  is  not  so  clear,  but  is  here  and  there  clouded  by 
partial  decomposition.  Quartz  invades  it  in  aggregates  with 
mosaic  polarization.  Quartz  also,  or  a  water-clear  mineral  like 
quartz,  usually  occupies  the  perforations  in  the  hornblende,  whether 
within  the  crystal  or  at  the  margin. 
The  biotite  is  well  characterized.  It  is  strongly  pleochroic, 
changing  from  light-brown  to  nearly  black.  The  laminae  lie 
parallel  to  the  general  schistosity  of  the  rock.  A  little  chlorite  is 
associated  with  this  mica.  The  felspar  and  quartz  are  perfectly 
moulded  upon  both  biotite  and  chlorite,  as  well  as  upon  the 
distorted  hornblende:  a  flake  of  mica,  or  chlorite  banded  with 
mica,  having  sometimes  one  extremity  in  the  middle  of  a  felspar- 
crystal,  and  the  other  end  in  the  middle  of  another  crystal. 
The  slide  is  traversed  by  a  large  number  (from  12  to  20)  of 
cracks  (shear-planes  ?)  running  roughly  with  the  foliation.  They 
cut  through  crystals  of  felspar  and  granules  of  quartz,  and  are 
clearly  connected  with  the  distortion  of  the  hornblende,  for  the 
latter  suffers  unusual  elongation  where  they  pass  through  it,  and  it 
is  often  drawn  out  at  the  end  into  a  long  tail  occupying  a  crack. 
The  formation  of  the  biotite  is  also  dependent  upon  the  cracks.  A 
large  proportion  of  it  is  seen  to  lie  either  in  a  crack,  or  in  the  line 
which  a  crack  would  take  if  produced.  Or  a  mica-flake  tails  out  in 
an  attenuated  film  of  brown  matter,  which  is  continued  along  a 
crack.  I  cannot  resist  the  conclusion  that  this  mica  has  been 
formed  by  the  action  upon  the  decomposed  hornblende  of  mineral 
solutions  infiltrated  into  the  planes  of  discontinuity. 
But  we  are  met  with  an  apparent  difficulty.  The  felspar  and 
quartz  must  have  been  in  a  plastic  state  subsequent  to  the  distor¬ 
tion  of  the  hornblende  and  the  genesis  of  the  mica,  for  the  former 
two  are  moulded  upon  the  latter  two  minerals.  How  is  it  then  that 
the  cracks  still  remain?  The  answer  is  (1)  that  to  a  large  extent 
they  have  been  obliterated :  none  of  them  can  be  traced  all  the  way 
1  These  numbers  indicate  throughout  the  numbers  of  the  slides  in  my  own 
collection. 
