548  PROF.  J.  W.  JUDD  ON  COMPOSITE  DYKES  IN  ARRAN-.  [XOV.  1 893, 
The  groundmass  is  remarkable  for  the  tendency  of  the  vitreous 
material  to  separate  both  from  the  larger  crystals  and  from  the 
mesh  of  lath-shaped  felspars.  This  vitreous  material  often  forms 
irregular  nests  in  the  midst  of  the  rock,  but  when,  as  sometimes 
occurs,  there  are  steam-holes  in  the  rock,  these  become  filled  with 
the  easily  separated  glass  (see  PI.  XIX.  fig.  2). 
This  glass,  studied  in  thin  sections,  is  found  to  present  many 
characters  in  common  with  tachylyte.  It  is  often  full  of  dusty 
particles  and  skeleton-crystals  of  magnetite ;  while  felspar  occurs 
as  doubly-forked  microlites.  In  addition,  there  are  sometimes 
long,  brown,  needle-shaped  crystals  which  have  the  extinction 
and  pleochroism  of  hornblende. 
The  glass  of  this  rock  appears  to  be  somewhat  unstable,  and  the 
vitreous  matter  filling  the  spherical  cavities  (steam-holes)  shows 
every  stage  of  change,  till  it  passes  into  amygdaloids  composed  of 
various  zeolites,  calcite,  chalcedony,  chlorites,  etc. 
Turning  our  attention  to  the  material  forming  the  centre  of  the 
dyke,  we  find  rocks  of  a  totally  different  composition  and  character. 
While  the  vitreous  matter  in  the  rock  at  the  sides  of  the  dyke  is 
subordinate  to  the  crystalline  part  of  the  rock,  exactly  the  opposite  is 
true  of  the  central  part,  in  which  the  more  or  less  perfectly  glassy 
groundmass  forms  the  larger  and,  at  the  same  time,  the  most 
characteristic  part  of  the  mass. 
Taking  the  predominance  of  quartz  and  of  an  alkali-felspar, 
which  appears  to  be  a  soda-orthoclase,  into  account,  we  must  refer 
this  rock,  according  to  Rosenbusch’s  terminology,  to  the  ‘  quartz- 
pantellerites  ’  (see  PI.  XIX.  fig.  1).  It  may  be  sufficient  for 
our  purpose,  however,  considering  the  abundance  and  distinctive 
character  of  the  glassy  base,  to  refer  to  it  by  the  old  name  of 
4  pitchstone-porphyry.’ 
Among  the  porphyritic  constituents,  quartz  in  idiomorphic  crys¬ 
tals,  with  large  glass-cavities,  is  the  most  abundant.  The  crystals 
are  sometimes  much  corroded,  but  not  unfrequently  retain  their 
sharp  angles. 
The  felspar-crystals  nearly  all  show,  under  proper  illumination 
with  polarized  light,  the  fine  lamellae  characteristic  of  anorthoclase ; 
they  are  not  unfrequently  fractured,  and  are  sometimes  corroded. 
True  plagioclase  is  rare  in  the  rock. 
The  augite-crystals,  which  are  not  numerous,  are  of  a  green 
colour  and  exhibit  a  faint  pleochroism ;  they  are  probably  to  be 
referred  to  the  acmites  or  soda-augites.  There  are  also  crystals 
that  appear  by  their  pleochroism,  extinction,  and  double  refraction 
to  be  a  variety  of  enstatite  which  is  not  highly  ferriferous. 
Magnetite  occurs  in  scattered  grains  of  various  sizes. 
In  addition  to  these  ordinary^.porphyritic  crystals,  we  find  grains 
of  zoned  plagioclase,  often  mucl^  corroded,  and  of  a  brown  augite, 
both  of  which  have  evidently  been  derived  from  a  rock  similar  in 
character  to  that  forming  the  sides  of  the  dyke. 
