551 
Yol.  49.]  PROF.  J.  W.  JUDD  OH  COMPOSITE  DYKES  IK  ARRAH. 
place  in  tlie  groundmass.  The  feathery  outgrowths  of  the  horn¬ 
blende  skeleton- crystals  are  wanting  and  only  the  shafts  remain ; 
while,  besides  the  ordinary  spheruiites,  numerous  centres  of  incipient 
spherulitic  devitrification  have  been  set  up,  and  thus  the  hyaline 
character  of  the  materials  is  almost  completely  destroyed. 
Inethe  felsites  (‘  claystones  ’  of  the  older  geologists)  the  spherulitic 
structure  gradually  disappears,  and  with  it  the  hornblende-crystals, 
which  appear  to  be  completely  absorbed  (if  they  were  ever  developed), 
until  finally  we  have  a  cryptocrystalliue  mass  of  quartz  and  felspar 
(‘felsite’  or  ‘petrosilex’  of  some  authors)  in  which  the  porphyritie 
crystals  of  quartz,  felspar,  augite,  and  magnetite  are  scattered. 
Some  doubt  may  exist  as  to  the  period  when  the  devitrification- 
process  took  place  in  this  rock.  Allport  regards  the  stony  rock  as 
resulting  from  secondary  devitrification ;  my  own  studies,  however, 
lead  me  to  regard  it  as  formed  by  primary  devitrification,  taking 
place  probably  during  its  original  consolidation. 
Alike  in  its  glassy  groundmass  and  in  the  crystallized  minerals  it 
contains,  the  ‘  quartz-pantellerite  ’  forming  the  centre  of  the  Cir 
Mhor  dyke  is  absolutely  dissimilar  from  the  augite-andesite  forming 
the  sides  of  the  dyke.  In  the  one  rock  the  glass  resembles  an 
obsidian,  in  the  other  a  tachylyte ;  while  the  free  quartz,  the  alkali- 
felspars,  the  green  augite,  the  enstatite,  and  the  hornblendes  of 
the  one  rock  have  nothing  in  common  with  the  lime-felspars  and 
the  brown  augite  of  the  other.  By  no  conceivable  process  of 
selective  crystallization  and  liquation  could  these  two  rocks  be 
produced  from  the  same  magma. 
VII.  The  Toemore  Plexus  of  Composite  Dykes — 
Earlier  Notices. 
At  several  points  in  Arran,  and  in  other  parts  of  the  Western 
Highlands,  examples  of  composite  dykes,  presenting  a  greater  or 
less  similarity  with  that  of  Cir  Mhor,  may  be  found.  One  of 
the  most  remarkable  localities  at  wffiich  illustrations  of  these 
phenomena  can  be  seen  is  that  part  of  the  west  coast  of  Arran 
stretching  northward  from  Drumadoon  Point,  about  two  miles, 
to  Tormore.  There  is  reason  to  believe  that  the  whole  of  the 
dykes  and  intrusive  masses  along  this  portion  of  the  coast  have  a 
very  close  connexion  one  with  another,  and  that  they  all  stand 
in  intimate  relation  to  the  great  sheet  of  columnar  quartz-felsite 
rock  forming  Drumadoon  Point.  It  will  be  sufficient  for  my  present 
purpose,  however,  to  direct  attention  to  the  northern  part  of  this 
line  of  coast — from  a  little  north  of  the  King’s  Cave  to  a  point 
south  of  the  village  of  Tormore.  (See  Map,  p.  552.) 
We  have  in  this  section  of  the  coast,  about  half  a  mile  in  length, 
a  very  remarkable  plexus  of  dykes,  admirably  exposed  on  the  shore 
between  high-  and  low-water  marks,  and  also  seen  in  the  adjoining 
cliffs,  below  the  hill  of  Torr  High  Beag ;  these  dykes  present  clear 
evidence  of  the  successive  injection  of  the  same  fissure  with  rocks 
of  different  chemical  composition. 
