184  PROF.  J.  W.  JUDD  ON  INCLUSIONS  OF  TERTIARY  [May  I  893, 
certain  of  the  larger  micropegmatite  groups  have  in  places  escaped 
complete  fusion,  and  some  of  these  masses  of  micropegmatite  form 
the  nuclei  of  large  composite  spherulites.  Re-solidification,  how¬ 
ever,  has  been  accompanied  by  devitrification,  resulting  in  the 
development  of  a  great  variety  of  the  spherulitic  structures,  often 
on  a  very  grand  scale.  During  and  after  the  fusion,  new  minerals 
(pyrites,  fayalite,  etc.)  have  been  formed,  and  the  whole  rock  has 
undergone  sufficient  change  to  lead  to  the  formation  of  many 
secondary  products,  such  as  chalcedony,  chlorite,  etc. 
5.  The  Alterations  which  the  Minerals  in  the  Inclusions 
HAVE  UNDERGONE. 
Beginning  with  the  characters  of  the  phenocrysts,  we  find  the 
grains  of  quartz,  as  we  might  expect,  the  least  altered  of  all  the 
minerals.  Their  contours,  whether  the  idiomorphic  and  corroded 
forms  of  4  quartz-porphyries  ’  or  the  allotriomorphic  forms  charac¬ 
teristic  of  granite,  are  admirably  preserved,  and  the  crystals  appear 
quite  clear  and  transparent.  Careful  examination,  however,  shows 
that  the  crystals  are  not  unfrequently  traversed  by  a  number  of 
fine  cracks,  though  these  cracks  have  only  occasionally  been  deve¬ 
loped  in  such  numbers  as  to  seriously  impair  the  transparency  of 
the  quartz  (PL  III.  fig.  1).  That  these  cracks  are  new  features 
which  have  been  developed  by  the  heat  to  which  the  crystals  have 
been  exposed  is  unquestionable.  Occasionally  the  crystals  have 
opened  along  these  cracks,  and  the  glassy  matrix  has  penetrated 
through  them,  breaking  up  the  quartz-crystal  into  a  number  of 
isolated  fragments,  in  the  manner  so  well  described  by  Lehmann  in 
his  account  of  the  Eifel  rocks  (PI.  III.  fig.  2;  compare  Lehmann, 
op.  supra  cit.  Tab.  I.  figs.  1  &  2).  When  we  come  to  examine 
the  enclosures,  we  find  that  the  granitic  quartz  sometimes  shows 
cavities  containing  liquids  with  moving  bubbles,  which  occur  in 
streams  and  bands  traversing  the  crystals.  In  some  cases,  how¬ 
ever,  these  cavities  appear  empty  or  filled  with  a  black  opaque 
substance.  The  idiomorphic  quartzes  have  cavities,  sometimes  of 
considerable  size,  which  are  filled,  not  with  stony  matter,  as  in  the 
original  rock,  but  with  glassy  material.  This  glass  sometimes 
occupies  well-marked  negative  crystals,  and  at  other  times  cavities 
of  most  irregular  form,  the  connexion  of  which  with  the  surround¬ 
ing  matrix  is  sometimes  clearly  visible. 
The  glass-cavities  in  the  idiomorphic  quartz- crystals  are  so  big 
and  so  numerous  that  it  is  scarcely  possible  to  avoid  the  conviction 
that  some  of  them  have  been  enlarged  during  the  re-fusion  of  their 
contents,  and  there  is  at  least  a  suspicion  that  new  cavities  may 
have  been  formed,  like  the  secondary  glass  inclusions  described  by 
Chrustschoff  as  occurring  in  the  quartzes  of  verglaster  sandstcin 2 
The  corrosion  of  the  outside  of  these  crystals  by  tbe  surrounding 
glass  may  also  have  been  produced,  or  at  all  events  increased,  during 
the  second  fusion  to  which  the  rock  has  evidently  been  subjected. 
1  Tschermak’s  Min.  u.  Petr.  Mittlieil.  1882,  p.  473,  and  1885,  p.  64. 
