410  X>E.  C.  CALLAWAY  ON  THE  OEIGIN  OF  THE  [Allg.  I  893, 
apparently  belongs  to  the  sericite  family.  The  quartz-felspar  con¬ 
tains  few,  if  any,  cavities.  It  is  largely  penetrated  by  microliths  o 
the  mica,  which  often  forms  an  interrupted  sheath  to  individual 
granules,  representing,  it  may  be,  the  original  fragments.  ihe 
process  of  reconstruction  is  most  marked  in  the  ferruginous  matter. 
Besides  opaque  granules,  there  are  several  linear  aggregations  o 
iron  oxide  in  regular  forms,  many  of  which  are  seen  by  their 
cubic  crystallization  to  be  magnetite. 
Some  confirmatory  proof  of  the  secondary  origin  of  the  white 
mica  will  here  be  added.  The  calcite  liberated  during  the  decompo¬ 
sition  of  the  diorite  often  appears  in  considerable  quantities  m  the 
schist.  It  is  in  single  granules,  definite  forms,  or  venules.  When 
the  venules  cross  the  foliation,  they  are  often  shifted  in  the  direction 
of  foliation  at  the  junction  of  the  quartzose  folia  with  the  mica-tolia. 
Here  and  there  the  continuity  of  a  vein  is  broken  by  a  flake  or 
bundle  of  white  mica,  but  is  renewed  along  the  same  line  at  the 
other  side  of  the  flake.  It  would  therefore  seem  that  some  part  of 
the  schist-making  process,  and  the  production  of  some  at  least  of 
the  mica,  took  place  after  the  liberation  of  the  calcite.  In  slide  4zS 
the  evidence  of  the  secondary  origin  of  the  mica  is  of  a  different 
kind.  A  venule  of  calcite  runs  with  the  foliation,  at  first  tapering 
almost  to  a  line,  then  swelling  out  into  a  thick  lenticle.  On  each 
side,  the  venule  is  sheathed  in  bundles  of  white  mica,  whose  kminee 
follow  precisely  the  contours  of  the  enclosed  calcite,  bending  inwards 
at  the  narrow  neck,  and  curving  outwards  to  enclose  the  lenticular 
head.  The  curvature  of  the  mica  is  rendered  more  distinct  by  a 
number  of  enclosed  scales  of  iron  oxide  (seen  in  section),  which 
lie  in  the  general  direction  of  the  cleavage,  and  thus  appear  to  flow 
round  the  contours  of  the  calcite.  In  the  same  slide  is  seen  another 
lenticle  of  calcite  wrapped  round  by  a  sheath  of  mica.  The  mica, 
being  thus  moulded  to  the  calcite,  must  be  of  contemporaneous  or 
posterior  origin.  . 
Another  series  of  slides,  taken  from  the  summit  of  the  hill,  points 
to  a  somewhat  different  process  of  schist-making  from  that  just  de¬ 
scribed.  The  chlorite  is  first  converted  into  biotite,  and  this  passes 
into  a  fibrous  or  scaly  white  mica.  These  changes  are  described  in 
No.  II.  (p.  488).  The  felspar  is  sometimes  replaced  by  a  highly- 
refracting  mineral  with  low  interference-colours,  and  occasionally  a 
columnar  structure  :  it  may  be  zoisite.  This  mineral  does  not, 
however,  appear  in  the  completed  schist,  in  which  the  felspar  is 
represented  by  the  usual  mosaic. 
A  slide  (803)  from  this  locality  shows  a  very  interesting  passage 
between  the  grit  and  the  schist.  The  field  is  mostly  occupied  by 
fragments  of  felspar,  the  highly-refracting  mineral,  and  water- 
clear  mosaic,  immersed  in  a  groundmass  of  chlorite,  passing  into 
biotite  with  a  distinctly  lamellar  structure.  The  whole  has  a 
schistose  appearance,  which  is  accentuated  by  numerous  subparallel 
cracks,  more  or  less  marked  by  the  presence  of  biotite  and  iron 
oxide.  Towards  one  margin  the  schistosity  becomes  more  pro¬ 
nounced.  Spindle-shaped  mosaics  set  in ;  and  the  chlorite  and 
