Yol.  49.]  CRYSTALLINE  SCHISTS  OF  THE  MALVERN  HILLS.  421 
IX. 
X. 
XI. 
Silica . 
.  604 
65-7 
771 
Alumina  . 
.  18-6 
17-2 
10-9 
Ferric  oxide . 
.  3-5 
•9 
•7 
Ferrous  oxide  . 
.  5-3 
4-5 
2-3 
Lime  . 
.  1-6 
1-2 
1-3 
Magnesia  . . . . 
.  2-8 
1-9 
1-4 
Soda  . 
.  41 
39 
1-6 
Potash  . 
.  30 
2-6 
1-8 
Loss  . . 
23 
1-5 
993  100-2  98-6 
IX.  Biotite-gneiss.  Swinyard’s  Hill. 
X.  Like  IX.,  and  from  the  same  block. 
XI.  Grneissoid  quartzite,  passing  gradually  into  X. 
This  fall  in  the  percentage  of  alumina  must  arise  either  from  its 
elimination,  or  from  the  introduction  of  silica.  The  microscopic 
evidence  points  towards  the  former  alternative.  In  tracing  the 
changes  of  the  different  minerals  through  a  series  of  slides  in  which 
diorite  and  gneissoid  quartzite  are  the  two  extremes,  we  observe, 
not  the  coming  in  of  quartz  between  the  other  minerals,  but  the 
progressive  destruction  of  these  minerals  themselves.  The  biotite 
produced  out  of  the  hornblende  loses  its  idiomorphic  form,  and 
is  reduced  to  shreds.  The  felspar  is  invaded  by  fingers  and  lobes 
of  quartz,  and  is  often  entirely  replaced  by  it,  the  sheath  of  mica 
which  often  surrounds  felspar-crystals  continuing  to  envelop  the 
quartz-replacement.  Of  course,  where  the  mica  has  nearly  disap¬ 
peared,  and  many  of  the  felspars  are  deeply  corroded  or  replaced  by 
quartz,  a  considerable  proportion  of  the  alumina  must  have  been 
removed.  In  some  slides  of  the  gneissoid  quartzite  there  are  a  few 
crystals  of  garnet ;  but  the  bulk  of  the  alumina  cannot  be  accounted 
for  in  this  way. 
In  the  felspar  of  the  granite-veins  there  is  also  corrosion  and 
replacement  by  quartz,  so  that,  especially  in  the  small  veins  of 
shear-zones,  the  granite  is  converted  into  quartz.  Alumina  must 
have  been  removed  in  this  case  also. 
What  becomes  of  the  eliminated  alumina  ?  I  am  not  prepared 
to  answer  this  question  ;  but  I  have  noted  many  facts  at  Malvern 
and  elsewhere  which  seem  to  indicate  that,  during  advanced 
dynamic  deformation,  there  is  set  up  a  tendency  towards  mineral 
aggregation.  In  the  Malvern  district,  the  diorite  which  has  under¬ 
gone  extreme  alteration  sometimes  occurs  as  a  schist,  in  which 
seams  which  are  mainly  mica  alternate  with  seams  which  are 
almost  pure  quartz.  The  conclusion  appears  inevitable  that  alumina 
has  migrated  from  one  place  to  another,  probably  as  silicate. 
However,  I  merely  suggest  this  point,  with  a  view  to  further  enquiry. 
Elimination  of  the  Oxides  of  Iron . — A  glance  at  the  above 
analyses  reveals  a  reduction  of  iron  oxide  in  the  process  of  schist¬ 
making.  This  fact  may  have  some  bearing  on  the  origin  of  iron  ores. 
