Yol.  49.]  CRYSTALLINE  SCHISTS  OF  THE  MALVERN  HILLS. 
401 
The  rounded  or  lobate  forms  of  crystalline  grains  have  long  been 
recognized  1  as  characteristic  of  gneiss,  as  distinguished  from  granite. 
Corrosion  during  the  primary  consolidation  of  granite  has  been 
fully  described  by  Prof.  Sollas,  who  states  2  that  muscovite  corrodes 
biotite,  albite  corrodes  biotite,  orthoclase  and  quartz  corrode  albite. 
(2)  Reconstruction.— In  the  course  of  my  present  enquiry,  I  have 
often  been  met  by  a  demand  for  signs  of  mechanical  force,  such  as 
broken  or  distorted  crystals,  shear-planes,  decomposition-products, 
or  strain-shadows.  These,  however,  are  the  effects  of  an  early 
stage  of  the  metamorphism,  and  it  would  be  as  reasonable  to  expect 
them  in  the  completed  product  as  to  look  for  petals  and  stamens  in 
a  strawberry. 
Fortunately,  at  Malvern  mechanical  force  has  acted  so  inter¬ 
mittently  that  the  stages  of  metamorphism  can  be  followed  from 
beginning  to  end  in  scores  of  sections.  For  example,  the  change 
from  felsite  or  diorite  into  mica-gneiss  can  be  traced  inch  by  inch, 
the  mineral  decompositions  and  syntheses  can  be  verified  step  by 
step,  and  we  are  thus  forced  to  unite  the  extremes  by  countless 
intermediate  links,  and  to  conclude  that  the  schist  has  been  formed 
out  of  the  massive  rock. 
The  process  of  reconstruction  often  commences  almost  as  soon  as 
the  decomposition.  Take  the  case  of  a  granite-diorite  shear-zone. 
*  While  the  hornblende  of  the  diorite  may  be  passing  into  a  chlorit  e, 
some  of  the  felspar  may  be  replaced  by  quartz  and  muscovite,  and 
these  are  ultimate  products.  When,  within  the  shear-zone,  the 
chlorite  has  passed  into  black  mica,  more  of  the  quartz  and  mus¬ 
covite  will  have  been  generated.  Iron  oxide,  liberated  in  an  early 
stage,  is  either  absorbed  later  on,  as  in  the  change  from  chlorite  to 
biotite,  or  it  aggregates  into  definite  crystalline  forms.  The  acidic 
constituents,  quartz  and  felspar,  fill  up  the  spaces  between  the  micas 
and  any  other  basic  mineral  which  may  remain.  The  rottenness  and 
dirty  appearance  produced  by  the  presence  of  chlorite  and  disseminated 
iron  oxide  disappear,  and  a  sound  clear  gneiss  results. 
The  gneissic  rocks  have  undergone  but  little  mechanical  change 
since  the  secondary  consolidation,  and  their  minerals,  even  the 
biotite  not  excepted,  rarely  show  undulose  extinction. 
IY.  Classification  of  the  Malvern  Schists. 
In  paper  Ho.  I.  the  present  writer  divided  the  Malvern  schists 
and  gneisses  into  two  groups,  viz.  : — Simple  Schists ,  or  those  formed 
from  one  kind  of  rock,  and  Injection-Schists ,  or  those  produced  by 
the  interveining  of  two  varieties.  The  subsequent  discovery  of  the 
infiltration-gneiss  of  the  'Wych  makes  it  necessary  to  subdivide 
the  latter  group  into  (a)  Schists  of  Primary  Injection  and  (6)  Schists 
of  Secondary  Injection. 
1  T.  G-.  Bonney,  Pres.  Address,  Quart.  Journ.  Geol.  Soc.  vol.  xlii.  (1886) 
Proc.  p.  92. 
2  ‘Contributions  to  a  Knowledge  of  the  Granites  of  Leinster,’  Trans.  Roy. 
Irish  Acad.  vol.  xxix.  (1891)  p.  492,  et  alibi. 
