362 
MESSRS.  A.  HARKER  AND  J.  E.  MARR  ON  THE  [Aug.  1 893, 
pyroxene,  garnet,  etc.,  and  the  way  in  which  epidote-crystals  en¬ 
close  bundles  of  actinolite-needles  [1750,  1747].  (See  PI.  XVII. 
figs.  2  and  4.) 
Sphene,  in  small  quantity,  is  widely  distributed,  usually  building 
small  crystals  of  the  usual  habit,  but  imperfectly  developed.  It  has 
a  pale  brown  tint  in  slices,  with  feeble  pleochroism,  never  showing, 
so  far  as  our  observations  go,  the  purplish  colour  and  strong  pleo¬ 
chroism  of  the  sphene  in  the  metamorphosed  andesites.  (See 
PI.  XVII.  fig.  4.) 
The  iron  ores  of  these  rocks  are  magnetite  and  probably  ilmenite, 
besides  pyrites.  It  is  not  in  every  case  easy  to  judge  how  far  the 
first  two  can  be  regarded  as  metamorphic  minerals,  but  we  may 
fairly  assume  that,  in  the  most  highly  altered  rocks  at  least,  the 
original  iron  ores  have  undergone  complete  reconstruction. 
In  all  the  most  metamorphosed  of  the  basic  lavas  a  large  part  of 
the  mass  consists  of  a  very  finely  granular  mosaic,  perfectly  clear 
and  colourless,  which  must  represent  in  a  general  way  the  felspathic 
constituent  of  the  original  rock,  in  so  far  as  that  was  not  decom¬ 
posed,  and  its  elements  worked  up  into  silicates  of  different  type. 
In  the  porphyritic  felspars  we  can  trace  the  progress  of  a  recrystalli¬ 
zation,  which  begins  with  the  original  turbid  crystals  and  ends 
with  clear  granular  aggregates  retaining  nothing  of  the  original 
except  its  outline.  The  clear  mosaic  in  the  matrix  is  probably  in  a 
great  measure  similar  to  this.  There  is  little  indication  of  crystal 
form,  and  usually  no  twinning.  It  is  quite  possible  that  quartz 
may  form  part  of  this  mosaic.  At  least,  that  mineral  is  easily 
recognized  in  separate  patches,  which  probably  represent  the  silica 
liberated  by  weathering  and  recrystallized  during  the  metamorphism. 
The  similar  quartz  which,  in  larger  patches,  figures  constantly 
inside  the  vesicles  has  clearly  been  recrystallized  in  the  processes 
of  metamorphism,  as  appears  from  its  relations  to  the  associated 
minerals. 
The  metamorphism  of  the  amygdules  or  infilled  vesicles,  which 
occur  more  or  less  plentifully  in  most  hand-specimens  of  the  lavas, 
requires  special  notice.  The  contents  of  the  smaller  vesicles,  from 
1  inch  in  diameter  down  to  much  less  dimensions,  have  usually  given 
rise  to  a  mixture  of  green  hornblende  and  quartz.  In  the. smallest 
quartz  predominates,  in  those  which  are  a  little  larger  hornblende,  the 
quartz  often  surrounding  it  as  a  border,  although  these  relations  are 
not  constant  [1613,  etc.].  The  phenomena  differ  little  from  those 
of  the  similar  vesicles  in  the  andesites,  except  in  the  occasional 
presence  of  other  minerals,  such  as  granular  sphene  [760],  epidote, 
or  scattered  irregular  granules  of  a  light  brown  garnet  [900,  from 
Howe  Gill]. 
In  some  localities,  however,  as  upon  the  northern  bank  of  Long- 
fell  Gill,  these  basic  lavas  enclose  amygdules  of  exceptional  size,  up 
to  2  inches  or  more  in  length.  In  some  cases  the  elongated  form 
due  to  a  fiowing  movement  of  the  lava  has  been  further  accentuated 
by  deformation,  the  rocks  giving  evidence  of  cleavage  [1614].  As 
seen  on  a  surface  of  the  rock  freshly  broken  these  larger  vesicles 
