138 
ME.  H.  EMMONS  ON  THE  PETEOGEAPHY  [May  1 893. 
pinaeoid  and  domes  most  often  developed.  Less  frequently  it  occurs 
in  irregular  grains  and  fragments.  Its  optical  properties  call  for 
no  special  comment :  the  pleochroism  is  weak.  Inclusions  of  glass, 
apatite,  magnetite,  and  haematite-flakes  are  somewhat  commoner 
than  in  the  augite.  In  those  andesites  whose  groundmass  has 
already  turned  red,  the  hypersthene  has  developed  a  thin,  superficial 
film  of  haematite. 
(v)  Hornblende  pi  ays  a  very  secondary  part  as  a  rock-forming  mineral 
in  the  rocks  of  Capraja.  In  some  cases  it  belongs  to  the  oldest 
group  of  porphyritic  constituents,  in  others  to  the  youngest.  In  the 
latter  case,  the  crystals  are  always  small,  prismatic  in  habit,  of  a 
light  green  colour,  with  very  little  pleochroism,  and  showing  a  great 
tendency  to  twin  after  go  P^  {100}.  Between  crossed  nicols,  the 
colour  is  light  yellow,  and  the  extinction-angle  a  very  small  one. 
The  only  inclusions  are  glass,  and  these  generally  in  the  form  of  long, 
narrow  hands  or  streaks  (sometimes  with  an  immovable  bubble) 
running  parallel  to  the  prism-faces.  On  the  other  hand,  where  the 
hornblende  belongs  to  the  earlier  crystallization,  it  is  either  entirely 
or  almost  entirely  changed  into  magnetite  and  augite  by  magmatic 
metamorphism,  like  the  mica.  Where  a  central  patch  of  hornblende- 
substance  remains,  it  generally  shows  by  its  strong  red  colour  the 
change  of  iron  from  the  ferrous  to  the  ferric  state  (Vado  del  Albero, 
slopes  of  Monte  Porcone).  Very  occasionally  it  is  to  be  found  quite 
fresh,  as  in  the  grey  andesite  (Punta  della  Civitata,  northern  spur 
of  Monte  Campanile). 
(vi)  Olivine  occurs  both  in  the  massive  andesites  and  in  the  dykes, 
in  grains  and  granules,  more  rarely  with  crystal-boundaries,  which 
are  then  the  pinacoids  and  domes.  It  is  never  entirely  fresh.  The 
exterior  and  cleavage-cracks  are  general^  tinged  yellow  (Poppa  alia 
Nave,  "V  ado  del  Porto,  Monte  Scopa).  [Further  weathering  brings 
on  serpentinization,  and  the  secretion  of  opal  (northern  dykes  in 
Cala  del  Bucciso). 
(vii)  Tridymite  occurs  abundantly,  both  macro-  and  microscopically 
(Monte  Patello,  Punta  della  Teja,  Cala  del  Bucciso).  It  is,  as  usual, 
grouped  in  tile-like  aggregates,  and  calls  for  no  further  remark. 
(viii)  Apatite  is  scattered  throughout  the  andesites  in  small  quan- 
tities,  generally  as  an  inclusion  in  the  older  minerals.  It  often 
carries  the  well-known  dust-like  interpositions  or  inclusions,  and 
then  has  a  light  browm  colour. 
(ix)  Magnetite  in  some  of  the  rocks  is  almost  entirely  wanting 
(Patello,  Punta  della  Civitata,  and  Punta  della  Manza  dyke);  in 
others  it  occurs  sparingly ;  generally,  however,  it  is  found  in  con¬ 
siderable  quantities  both  as  a  constituent  of  the  groundmass  and  as 
a  replacement-mineral  in  mica  and  hornblende. 
(x)  Epidote  is  to  be  met  with  only  in  one  rock,  i.  e.  from  the 
headland  just  south  of  the  Island  of  Prajola.  It  occurs  in  bands 
and  patches,  without  a  trace  of  costal-boundaries.  Its  colour  is 
a  muddy  yellow,  and  pleochroism  is  very  weak.  The  augite  is  its 
chief  source  and  may  be  found  in  all  stages  of  alteration,  from  that 
in  which  a  thin  film  of  epidote  has  just  made  its  appearance  on  the 
