Yol.  49.]  CRYSTALLINE  SCHISTS  OF  THE  MALVERN  HILLS.  403 
across  the  microscopic  section.  And  (2)  that  we  can  observe  under  a 
high  power  that  the  thinnest  visible  cracks  are  occupied  by  brown 
matter  (iron  oxide  ?)  in  minute  particles.  This  film  has  apparently 
been  sufficient  to  prevent  the  coalescence  of  the  sides  of  the  crack 
when  plasticity  supervened. 
If  the  biotite  be  regarded  as  an  original  constituent  of  the  rock, 
the  planes  of  discontinuity  must  have  been  formed  after  it :  and,  as 
the  cracks  and  flakes  often  coincide,  the  mica  must  frequently  have 
been  torn  and  distorted.  But  such  is  not  the  case.  While  the 
hornblende  is  more  or  less  decomposed  and  deformed,  the  biotite  in 
the  cracks  is  idionrorphic  and  clear,  frequently  showing  in  section 
lath-shaped  forms,  rectangular  or  oblique,  with  the  ordinary 
parallel  cleavage. 
The  difference  between  this  schist  and  the  hornblende-gneiss 
gives  us  a  hint  of  the  important  part  in  metamorphism  played  by 
mineral  solutions  infiltrated  along  cleavage-planes  and  cracks.  The 
absence  of  cracks  in  the  hornblende-gneiss  probably  accounts  for 
the  slight  degree  of  mineral  change.  Water  was  of  course  present, 
for  chlorite  has  been  produced.  But  the  rock  which  was  converted 
into  biotite-gneiss,  traversed  as  it  was  with  countless  cleavage- 
planes,  must  have  been  saturated  like  a  sponge.  This  principle  is 
of  wide  application. 
Muscovite-chlorite  Gneiss  (from  Diorite). — This  schist  is  not  of 
wide  distribution  at  Malvern.  I  showed  in  Ho.  II.  (p.  480)  that 
in  or  near  a  granite-diorite  complex,  muscovite  was  sometimes 
formed  from  the  felspar  of  the  diorite,  and  not  biotite  from  the 
chlorite. 
Sericite-gneiss  and  Muscovite-gneiss  (from  Diorite). — The  transi¬ 
tion  which  will  now  be  described  is  one  of  the  most  interesting 
that  I  have  studied.  At  one  end  of  the  series  we  have  a  granitoid 
diorite;  at  the  other,  a  fine-grained  schist  of  acidic  composition. 
The  typical  sections  have  been  repeatedly  visited  during  three 
years,  and  39  slices  of  the  rock  have  been  examined. 
The  schist  occupies  the  western  side  of  the  long  spur  running 
from  the  summit  of  Ragged  Stone  Hill  in  a  southerly  direction 
to  the  village  of  White-leaved  Oak.  The  band  is  several  yards 
broad,  but  possibly  extends  farther  westward  under  the  Hollybush 
Sandstone.  Towards  the  eastern  slope  of  the  spur,  the  schistose 
structure  becomes  fainter,  and  the  rock  resembles  a  roughly  lami¬ 
nated  grit.  -In  the  quarry  at  the  village,  this  gritty  material 
graduates  into  the  diorite.  The  complete  transformation  is  seen  in 
section  in  this  quarry.  The  description  will  be  taken  from  east  to 
west,  that  is,  from  the  diorite  through  the  grit  into  the  schist,  a 
distance  of  about  50  yards.  The  structural  planes,  whether  in 
schist  or  grit,  dip  at  a  high  angle  to  the  north  of  east. 
We  work  along  the  northern  side  of  the  quarry,  commencing 
where  an  irregular  buttress  is  thrust  forward,  the  rock  to  the  east 
being  decomposed  and  of  no  interest  in  the  present  investigation. 
The  projecting  mass  is  about  two  yards  wide  ;  in  the  middle  of  it  is 
the  diorite,  a  vein  of  granite  runs  up  the  eastern  side,,  and  several 
