412 
DR.  C.  CALLAWAY  ON  THE  ORIGIN  OE  THE 
[Aug.  1893, 
become  agglutinated  where  they  are  in  contact.  The  pressure  pro¬ 
duces  elongation  of  the  fragments  and  aggregations,  which  thus 
assume  a  linear  or  spindle-like  form.  The  iron  oxide,  liberated  as 
dust,  passes  through  the  form  of  granules  of  various  sizes  and  shapes 
into  regular  crystals  of  magnetite,  and  sometimes  perhaps  haematite. 
It  is  common  in  these  rocks  for  contortion  to  set  in  when  the 
schistosity  becomes  very  marked  and  the  mica  abundant.  The  de¬ 
velopment  of  mica  and  the  contortion  of  the  folia  appear  to  proceed 
concurrently. 
Biotite-gneiss  and  Biotite-grcinite  (from  Diorite)  —  These,  rocks 
are  formed  from  the  diorite  which  yields  the  sericite-gneiss  just 
described.  The  pressure  has  been  comparatively  slight,  and  the 
resulting  changes  are  proportionately  small.  There  is  no  sharp  line 
of  demarcation  between  the  gneiss  and  the  granite,  the  former  being 
merely  a  more  strongly-compressed  form  of  the  other. 
There  can  be  no  question  that  this  rock,  whether  gneissic  or 
granitoid,  has  been  formed  from  the  diorite.  Gradations  between 
the  two  are  numberless.  They  are  best  seen  at  the  northern  end 
of  the  range,  especially  in  the  North  Hill  and  on  the  southern  and 
eastern  slopes  of  the  Worcestershire  Beacon,  where  there  are  many 
granitic  intrusions,  but  only  moderate  pressures.  At  many  spots 
the  passage  from  a  hornblendic  to  a  micaceous  rock  is  clearly  seen 
to  be  connected  with  the  intrusion  of  granite-veins.^ 
Muscovite-gneiss  (from  Granite). — Described  in  No.  I.,  p.  528. 
(B)  Injection-Schists. 
In  No.  I.  (p.  532)  I  stated  that  at  Malvern  a  banded  gneiss  was 
produced  by  the  compression  of  masses  of  veined  igneous  rocks, 
duplex  diorite-gneiss  being  formed  from  diorite  veined  by  diorite, 
and  granite-diorite  gneiss  from  diorite  veined  by  granite.  This  ex¬ 
planation  received  some  support  from  Mr.  J.  J.  H.  Teall,  who  has 
since  applied  a  similar  interpretation1  to  certain  banded  gneisses  at 
the  Lizard.  The  cases  which  I  then  cited,  are  comparatively  simple, 
since  the  rocks  described  had  undergone  little  chemical  change ;  but 
it  will  be  necessary  in  the  present  communication  to  add  certain 
varieties  of  gneiss  which  are  formed  in  the  same  way,  but  in  which 
the  transformation  has  been  carried  much  farther.  I  he  gneisses  in 
which  infiltration  has  played  a  prominent  part  will  come  under  the 
same  head.  The  intrusion  of  rock  in  the  original  veining-process 
may  be  called  primarA  injection ,  while  we  may  describe  as  secondary 
injection  the  infiltration  of  mineral  matter  into  rock  crushed  after 
consolidation. 
The  production  of  gneisses  by  primary  injection  has  been  noticed, 
among  others,  by  Dr.  Ch.  Barrois,2  but  in  the  cases  described  by 
him  the  intrusion  of  the  rock  was  into  sedimentary  strata..  The 
changes  thus  caused  differ  in  degree  from  those  described  in  this 
1  Geol.  Mag.  for  1887,  p.  484.  ^ 
2  ‘  Granulites  du  Morbihan,’  Ann.  Soc.  Geol.  Kord,  vol.  xv.  (188/ )  p.  1. 
