Vol.  49.]  CRYSTALLINE  SCHISTS  OF  THE  MALVERN  HILLS.  413 
paper,  partly  because  the  minerals  composing  a  stratified  rock  are 
usually  more  stable  than  the  constituents  of  diorite. 
The  evidence  for  the  exotic  origin  of  the  granite  is  given  in  No.  IT. 
(pp.  494-496).  Prof.  John  Phillips,  writing  of  these  Malvern  rocks, 
also  regarded1  the  “  posteriority  of  felspathic  and  granitic  veins  to 
the  masses  they  traverse”  as  “  quite  certain.” 
(«)  Schists  of  Primary  Injection. 
Duplex  Dio  rite-gneiss. — This  gneiss  is  of  no  great  importance. 
It  is  (or  was)  well  seen  in  the  large  quarries  at  North  Malvern.  The 
medium  black  diorite  is  interlaced  by  the  coarse  grey  variety,  and 
a  moderate  amount  of  compression  has  produced  a  rough  parallelism 
of  the  veins. 
Granite-diorite  Gneiss  (No.  I.,  p.  533). — It  is  necessary  for  the 
production  of  this  gneiss  that  the  granite-veins  should  be  compara¬ 
tively  few,  and  that  there  should  be  moderate  pressure  with  little 
shearing.  Countless  gradations  may  be  traced  between  a  rock  in 
which  there  is  merely  a  rough  parallelism  of  the  veins  and  a  typical 
gneiss  with  one  or  two  micas,  so  that  it  is  impossible  to  draw  a 
sharp  line  between  a  granite-diorite  gneiss  and  a  true  mica-gneiss. 
Banded  Biotite-gneiss. — This  is  a  rock  in  which  bands  of  quartz- 
felspar  (the  injected  granite-veins)  alternate  with  seams  of  mixed 
biotite,  felspar,  and  quartz  (the  altered  diorite).  To  produce  a 
biotite-gneiss  out  of  a  decomposed  diorite,  the  chlorite  must  be 
mainly  converted  into  biotite,  the  disseminated  iron  oxide  must  be 
either  crystallized  or  absorbed,  and  quartz  must  be  liberated.  All 
these  changes  are  known  to  frequently  occur  at  Malvern.  The  con¬ 
version  of  chlorite  into  biotite  and  the  liberation  of  quartz  from 
felspar  are  extremely  common.  The  recrystallization  of  iron  oxide  is 
seen  in  the  section  at  Eagged  Stone  Hill,  already  described,  and  has 
been  observed  by  other  workers.  The  re-absorption  is  frequently 
illustrated  in  the  present  series  of  papers. 
The  evidence  for  the  conversion  of  diorite  interlaced  with 
granite-veins  into  a  banded  biotite-gneiss  is  of  the  most  positive 
and  direct  kind.  One  of  the  best  examples  is  seen  in  the  quarry  at 
the  southern  end  of  Swinyard’s  Hill.  The  section  is  given  in 
No.  II.  (p.  482),  and  the  chief  mineral  changes  are  there  described. 
I  cannot  too  emphatically  insist  upon  the  completeness  of  the 
gradation  between  diorite  and  gneiss.  As  we  approach  the  plexus 
of  granite-veins,  the  hornblende  changes  to  chlorite  and  the  chlorite 
to  biotite.  The  felspar  becomes  invaded  by  quartz,  and  gradually 
assumes  the  normal  corroded  appearance.  The  parallel  structure 
grows  more  and  more  distinct,  and  near  the  veins  aggregation  sets 
in.  Even  before  we  reach  the  plexus  we  obtain  at  contact  with 
granite-veins  a  partially  gneissic  structure,  a  sort  of  foliated  kersan- 
tite,  seams  of  which  cannot  be  distinguished  from  some  of  the  bands 
in  the  very  core  of  the  plexus. 
It  does  not  seem  to  be  essential  that  the  veins  in  a  banded  biotite- 
1  ‘  Geol.  of  Oxford  and  the  Valley  of  the  Thames,’  1871,.  p.  62. 
