Yol.  49.]  CRYSTALLINE  SCHISTS  OF  THE  MALVERN  HILLS.  411 
biotite  pass  into  flakes  of  white  mica,  whose  cleavage-lines  are 
often  indicated  by  brown  iron  oxide.  The  gradation  between  grit 
and  schist  seen  in  this  slide  is  unmistakable. 
The  explanation  that  I  have  given  of  the  genesis  of  this  schist 
may  throw  light  upon  certain  cases  in  which  authors  have  asserted 
an  epiclastic  structure  in  schists.  It  is  obviously  needful  in  such 
cases  either  to  trace  the  origin  of  the  fragments,  or  to  establish 
reliable  tests  for  distinguishing  between  epiclastic  and  cataclastic1 
structures. 
Fig.  4  (Slide  347). 
The  sericite-gneiss.  Year  the  centre  of  the  figure  is  a  little  of  the 
fragmental  ‘quartz-felspar5  still  remaining. 
In  summarizing  the  series  of  changes  by  which  diorite  is  con¬ 
verted  into  sericite-gneiss,  we  observe  that  hornblende  passes  into 
chlorite,  epidote,  calcite,  and  iron  oxide,  where  the  crushing  is  only 
moderate.  In  the  more  highly-sheared  rock,  epidote  is  scarce  or 
wantiug.  The  most  important  part  in  the  metamorphosis  is  taken 
by  the  chlorite :  by  a  process  of  reconstruction  it  becomes  either 
biotite  or  white  mica,  and  the  biotite  also  changes  to  white  mica  in 
the  ultimate  stage.  The  white  mica  is  either  in  twinned  crystals 
(muscovite)  or  in  undulating  bundles  (sericite  ?).  The  felspar  is 
first  crushed  into  fragments,  which  are  broken  up  smaller  and 
smaller.  Some  of  it  passes  into  quartz  :  or  a  mosaic  structure,  in 
which  it  is  often  impossible  to  distinguish  between  quartz  and  fel¬ 
spar,  is  gradually  produced.  The  fragments  are  then  softened,  and 
1  Terms  proposed  by  ZSIr.  J.  -J.  H.  Teall,  F.E.S.  (Geol.  Mag.  for  1887, 
p.  493).  1  Epiclastic,’  formed  upon  the  earth's  surface  ;  ‘  cataclastic,’  produced 
during  deformation. 
2  e  2 
