423 
Yol.  49.]  CRYSTALLINE  SCHISTS  OF  THE  MALVERN  HILLS. 
are  the  replacement  of  orthoclase  by  quartz  and  muscovite,  of 
plagioclase  by  quartz  and  muscovite,  of  chlorite  by  biotite  and 
white  mica,  and  of  biotite  by  white  mica.  The  reconstruction 
of  felspar  is  also  a  prominent  feature  in  the  metamorphism. 
8.  The  granite  and  diorite  have  often  transferred  to  each  other 
some  of  their  alkaline  bases,  so  that  the  diorites  often  locally 
show  an  unusual  amount  of  potash,  which  has  been  largely 
used  in  the  production  of  the  micas.  The  liberation  of  potash 
from  the  granite  is  (at  least  sometimes)  accompanied  by  silici- 
fication  of  the  veins  and  the  production  of  garnets. 
9.  In  the  metamorphosis  of  diorite  into  a  muscovite-  (or  sericite-) 
gneiss,  lime  is  removed  readily,  and  magnesia  more  slowly. 
The  persistence  of  alumina  and  the  elimination  of  lime  and 
magnesia  favour  the  formation  of  white  mica,  which  appears  to 
be  one  of  the  ultimate  products  of  the  schist-making  process. 
10.  In  the  conversion  of  diorite  into  a  gnoissoid  quartzite,  alumina 
and  alkalies  are  removed,  as  well  as  lime,  magnesia,  and  iron. 
11.  Chlorite  and  iron  oxide,  produced  by  the  decomposition  of 
diorite,  may  be  intiltrated  into  the  cracks  and  cleavage-  (or 
shear-)  planes  of  crushed  granite,  and  may  migrate  to  a  con¬ 
siderable  distance  from  the  contact  with  the  diorite,  thus 
giving  rise  to  an  interfoliation  of  chlorite  and  biotite  with  the 
quartz,  felspar,  and  muscovite  of  the  resulting  gneiss. 
12.  The  schists  produced  by  the  means  here  described  are  divisible 
into  two  groups : — (1)  Simple  schists,  formed  from  one  kind 
of  rock,  such  as  biotite-gueiss,  mica-schist,  sericite-gneiss, 
muscovite-gneiss,  and  hornblende-gneiss.  (2)  Injection- 
schists,  formed  from  more  than  one  kind  of  rock,  frequently 
with  a  banded  structure,  divided  into  (a)  schists  of  primary 
injection,  the  injected  material  being  an  intrusive  rock — • 
examples  are  duplex  diorite-gneiss,  granite-diorite  gneiss,  and 
biotite-gneiss;  and  (6)  schists  of  secondary  injection,  the 
injected  matter  being  products  of  secondary  decomposition — 
the  chief  varieties  are  chlorite-gneiss,  biotite-gneiss,  and 
biotite-muscovite  gneiss. 
Discussion, 
The  President  said  that  the  Author  was  to  be  congratulated  on 
the  completion  of  his  triple  memoir  on  the  Malvern  Hills.  He  had 
brought  before  the  Society  an  ingenious  and  bold  hypothesis,  which 
involved  metasomatosis  on  a  large  scale.  He  (the  President)  did 
not  feel  qualified  to  express  an  opinion  on  the  intricate  subject  of 
dynamo-metamorphism,  but  he  supposed  that  the  crushing  of  rocks 
in  Nature’s  mortar  might  increase  solubility  and  facilitate  chemical 
change.  There  appeared  to  be  two  schools  of  chemical  geologists  at 
present — one  believing  that  rocks  could  not  be  materially  changed 
in  composition,  the  other  accepting  very  great  modifications.  There 
was  one  very  important  feature  about  this  paper  :  it  was  illustrated 
by  many  chemical  analyses. 
