Tol.  49.]  PKOF.  J.  W.  JUDD  ON  COMPOSITE  DYKES  IN  AEEAN.  547 
such  as  to  compel  the  inference  that  no  separation  of  the  crystalline 
and  vitreous  material,  like  that  which  sometimes  takes  place  within 
dykes  themselves,  will  account  for  the  startling  dissimilarity  of  the 
two  rocks. 
Beginning  with  the  rock  of  the  sides  of  the  dyke,  we  at  once  see 
that  we  are  dealing  with  an  undoubted  augite-andesite,  which  has 
for  groundmass  a  4  microlitic  felt’  of  the  typical  andesitic  character. 
(See  PI.  XIX.  fig.  2.)  When  the  crystalline  elements  are  numerous 
and  well  developed  the  glassy  groundmass  is  seen  to  be  caught  up 
between  crystals,  so  as  to  give  rise  to  the  4  intersertal  ’  structure ; 
while  occasionally,  by  the  crystallization  of  augite  around  the  lath¬ 
shaped  felspars,  the  ophitic  structure  is  developed.  We  may  there¬ 
fore  conclude  that  the  typical  augite-andesite  passes  into  a  4  tholeite’ 
or  into  an  ophitic  dolerite  (diabase). 
Among  the  porphyritic  minerals  present,  the  most  abundantly 
represented  species  are  lime-soda-felspars,  the  crystals  of  which  are 
usually  zoned  ;  and  there  is  not  unfrequently  a  considerable  variation 
in  the  extinction-angles  between  the  centre  and  the  concentric  zones, 
indicating  a  change  in  composition  from  more  basic  to  less  basic 
felspars.  Many  of  the  felspar-crystals  are  much  corroded  by  the 
action  upon  them  of  the  surrounding  vitreous  material,  and  there  is 
clear  evidence  that  during  the  development  of  the  crystals  the  work 
of  growth  and  of  corrosion  went  on  alternately.  Many  of  the 
crystals  are  surrounded  by  a  ragged  fringe  of  very  clear  and  fresh 
felspar-substance,  which  often  extinguishes  at  a  markedly  different 
angle  from  that  of  the  main  body  of  the  crystal.  There  can  be 
no  doubt,  I  think,  that  these  ragged  outgrowths  have  been  formed 
at  a  much  later  date  than  the  crystals  which  they  surround,  and 
probably,  long  after  the  consolidation  of  the  rock,  at  the  expense  of 
the  glass  in  the  groundmass.1 
The  lath-shaped  felspars  in  the  groundmass  are  sometimes  plagio- 
clase,  but  in  other  cases  may  be  orthoclase.  They  are  occasionally 
clustered  in  cruciform  and  sheaf-like  groups.  In  addition  to  the 
larger  crystals  of  plagioclase,  there  appear  to  be  some  few  felspars 
derived  from  an  acid  rock.  These  are  always  greatly  corroded,  and 
are  often  reduced  to  mere  skeletons  by  the  action  of  the  magma 
upon  them;  they  appear  to  be  either  orthoclase  or  anorthoclase 
(‘  cryptoperthite  ’). 
The  augite-crystals,  which  are  not  very  abundant,  are  either 
brown  or  almost  colourless.  They  have  all  the  characteristics  of 
the  highly  magnesian  varieties  of  the  mineral. 
Both  magnetite  and  titanoferrite  occur  abundantly  as  crystals, 
sometimes  of  considerable  size,  the  latter  mineral  often  exhibiting 
the  first  stages  of  the  change  into  4  leucoxene.’ 
Apatite  is  often  enclosed  in  the  felspars  and  augites,  while  pyrites 
is  among  the  most  common  of  the  secondary  constituents. 
Quartz-crystals,  greatly  corroded  and  surrounded  by  zones  of 
secondary  augite,  are  occasionally  found  in  the  rock,  but  were 
evidently  derived  from  some  more  acid  rock. 
1  Quart.  Journ.  Geol.  Soc.  vol.  slv.  (1889)  p.  175. 
