Yol.  49.] 
THE  PETROGRAPHY  OE  THE  ISLAND  OE  CAPRAJA. 
135 
(i)  The  Andesite. — The  typical  au  desite,  when  fresh,  has  a 
texture  varying  from  medium-compact  to  porous,  and  a  dark- 
coloured  groundmass  in  which  felspar  is  porphyritically  developed 
in  great  abundance.  Black  mica  and  olive-green  pyroxene  are  also 
generally  recognizable  with  the  naked  eye  :  the  former  in  hexago¬ 
nal  flakes,  often  in  the  felspar ;  the  latter,  when  monoclinic,  as  well- 
crystallized  prisms  with  the  faces 
00  Poo  {100},  00  £  00  {010},  00  P  {HO},  P  {111}, 
easily  determinable.  In  some  varieties  yellowish  grains  of  olivine 
can  be  detected. 
Where  decomposition  has  reached  a  certain  stage,  tridymite  may 
be  found  in  the  clefts  and  cracks,  and  more  seldom  fine  deposits  of 
opal.  So-called  Szaboite  (hypersthene)  was  observed  in  well-deve¬ 
loped,  bronze-coloured  crystals  at  Punta  della  Fica  in  a  very  porous 
piece  of  rock. 
As  soon  as  decomposition  sets  in — especially  in  the  darker  varie¬ 
ties — fine  streaks  and  patches  of  a  reddish  colour  make  their  appear¬ 
ance  in  the  groundmass,  and  these  spread  as  decomposition  advances 
until  the  whole  groundmass  becomes  red.  This  change  of  colour  is 
due  to  a  passage  from  the  ferrous  to  the  ferric  state,  and  can  be 
traced — firstly  to  the  decomposition  of  fine  grains1  of  Fe203  in  the 
groundmass,  and  secondly  to  the  reddening  of  the  mica,  augite,  and 
hornblende.  It  brings  out  admirably  some  of  the  chemical  and 
physical  differences  in  the  rock.  For  instance,  flow-structure,  which 
in  the  fresh  rock  is  invisible,  makes  itself  conspicuous  by  long 
red  bands,  from  1  to  30  or  more  centimetres  (J-  to  12  inches)  thick, 
running  for  great  distances  through  the  mass.  This  decomposition 
must  proceed  very  rapidly,  for  the  felspar  is  still  fresh  after  the 
whole  groundmass  has  turned  red.  In  the  next  stage  of  decompo¬ 
sition  the  red  colour  fades  to  pink,  and  then  to  yellowish  grey,  with 
kaolinization  of  the  felspar  and  total  bleaching  and  disintegration 
of  the  ferro-magnesian  minerals. 
(ii)  The  Andesite-dijkes. — The  above  remarks  do  not  apply,  how¬ 
ever,  to  the  andesite-dykes.  These  have  almost  universally,  when 
fresh,  a  grey  colour,  compacter  texture,  and  somewhat  smaller  por- 
phyritic  (though  equally  abundant)  felspar.  Biotite  always,  and 
pyroxene  generally,  may  be  detected  without  the  aid  of  a  lens.  As 
one  approaches  the  salbands,  the  rock  becomes  darker;  along  the 
line  of  contact  it  is  almost  black  and  semi-vitreous.  The  same  por- 
phyritic  constituents,  however,  are  visible,  but  in  less  abundance. 
The  pyroxene  is  the  first  to  suffer  from  the  effects  of  weathering, 
fading  to  greenish-grey  superficially ;  then  the  groundmass  becomes 
lighter  in  colour  and  friable,  while  the  mica  and  felspar  are  the 
last  to  disintegrate.  The  dykes  never  turn  red  on  weathering. 
(iii)  The  Anamesite. — The  anamesite  occurs  macroscopically  in  two 
varieties.  The  first  variety,  which  forms  the  Punta  del  Zenobito,  is 
a  grey,  fairly  compact  rock,  mottled  with  black  specks.  Under  a 
1  More  rarely  in  hexagonal  flakelets.  The  source  of  the  grains  is  undoubtedly 
to  a  great  extent  magnetite. 
