414  DE.  C.  CALLAWAY  ON  THE  OETGIN  OF  THE  [Aug.  1 893, 
gneiss  should  be  granite,  though  this  is  usually  the  case  at 
Malvern.  In  two  or  three  localities  south  of  the  Worcestershire 
Beacon  I  have  observed  a  banded  gneiss  in  which  the  veins  were  (or 
rather  had  been)  the  coarse  grey  diorite.  Yet  the  mica  was  pro¬ 
duced  abundantly,  both  in  the  diorite  encasing  the  veins  and  in  the 
diorite  of  the  veins  themselves.  At  one  inch  from  the  veins  the 
encasing  rock,  a  tine-grained  diorite,  was  highly  altered,  and  con¬ 
sisted  of  hornblende,  white  mica,  epidote,  chlorite,  and  quartz,  but 
not  much  felspar,  the  minerals  being  arranged  with  a  rough  folia¬ 
tion.  Near  the  contact  with  the  veins  black  mica  was  developed  in 
fair  proportion. 
These  apparent  exceptions  to  the  general  principle  admit  of  an 
easy  explanation.  In  the  large  number  of  slides  examined  I  have 
usually  found  biotite  to  be  generated  in  diorite  when  the  rock  is  sliced 
by  planes  of  cleavage  or  shearing,  or  when  traces  at  least  of  such 
planes  can  be  detected.  Of  course,  when  the  diorite  has  been 
transformed  into  gneiss,  planes  of  discontinuity  have  largely  disap¬ 
peared  ;  but  I  am  here  speaking  of  rock  in  which  the  dioritic  struc¬ 
ture  is  still  apparent.  Diorite  is  often  sheared  without  the  pro¬ 
duction  of  mica,  especially  in  Midsummer  Hill,  in  which  there  is 
very  little  granite  :  but  the  mass  of  the  Worcestershire  Beacon  is 
extensively  penetrated  by  dykes  and  veins  of  granite,  and  it  is  here 
that  we  find  so  much  biotite  generated  in  the  diorite. 
Gneiss  with  two  Micas. — I  described  in  No.  II.  (pp.  478—481)  a 
gneiss  in  which  the  dark  bands  (modified  diorite)  contained  both 
black  and  white  mica,  and  the  light  bands  (modified  granite)  white 
mica.  The  muscovite  was  formed  from  both  the  orthoclase  of  the 
granite  and  the  felspar  of  the  diorite,  the  biotite  being  produced  in 
the  usual  way  from  the  hornblende  of  the  latter. 
In  closing  this  account  of  the  origin  of  the  banded  gneisses  of 
Malvern,  it  will  be  well  to  summarize  the  explanations  of  the  banded 
structure  which  have  been  made  public. 
1.  The  hypothesis  of  original  sedimentation. 
2.  That  banding  is  produced  when  inclusions  (say  of  diorite  in 
granite)  are  fused  and  drawn  out  into  parallel  strips  under  earth- 
pressures.  Some  of  these  gneisses  are  described  by  Dr.  A.  C. 
Lawson.1  The  writer  has  seen  similar  structures  near  the  town  of 
Galway,  on  the  east  side. 
3.  By  injection  of  granite  along  the  bedding-planes  of  sedi¬ 
mentary  rocks.  See  Dr.  Ch.  Barrois’s  works,  passim. 
4  By  movements  in  plutonic  rocks  when  in  a  viscid  state  (Prof. 
Bonney  and  Lieut.-General  McMahon,  Quart.  Journ.  Geol.  Soc. 
vol.  xlvii.  1891,  p.  464). 
5.  By  the  deformation  of  veined  complexes  of  igneous  rocks 
under  pressure.  (Mr.  J.  J.  H.  Teall,  Geol.  Mag.  for  1887,  and  the 
Geological  Survey,  Quart.  Journ.  Geol.  Soc.  vol.  xliv.  1888,  p.  388.) 
Mr.  Teall  leaves  the  origin  of  the  veins  an  open  question  ;  while  the 
1  £  Report  on  the  Geology  of  the  Rainy  Lake  Region,’  Geol.  &  Nat.  Hist. 
Surv.  Canada,  1887,  p.  187. 
