560  PROF.  J.  W.  JUDD  OX  COMPOSITE  DYKES  IN  ARRAX.  [Nov.  1 893, 
base  and  few  porphyritic  crystals.  Very  careful  determinations  of 
its  specific  gravity  by  Mr.  T.  Barron  showed  that  it  lies  between 
2*348  and  2*344. 
The  4  felsite  ’  and  4  pitchstone  ’  of  the  northern  composite  dyke 
differ  from  the  acid  rock  of  the  north-and-south  dyke  in  containing 
a  considerable  number  of  quartz-crystals  in  addition  to  the  plagio- 
clase-felspar.  The  rock  is  a  quartz-augite-andesite  (dacite)  with  a 
base  which  is  sometimes  glassy  and  at  other  times  stony.  The 
devitrification  of  the  glass  is  seen  to  be  produced,  first  by  the 
development  of  spherulites  around  the  porphyritic  crystals  or  inde¬ 
pendent  centres,  and  then  by  the  breaking  up  of  these  into  ordinary 
4  felsitic  substance.’ 
In  the  middle  composite  dyke  we  have  also  a  quartz-andesite  or 
dacite,  the  great  majority,  if  not  the  whole,  of  the  felspars  being 
plagioclastic.  The  glassy  form  of  the  rock  can  here  also  be  traced 
graduating  into  the  4  lithoidal  ’  form,  the  first  stage  being  in  this 
case  the  development  of  spherulites  either  independently  or 
surrounding  the  porphyritic  crystals ;  and  the  rock  also  exhibits  a 
beautiful  banding.  This  rock,  as  well  as  some  of  the  other  4  pitch- 
stones  ’  of  Tormore,  exhibits  globules  of  hyalite  like  that  of  the 
4  pitchstone  ’  of  the  Cir  Mhor  dyke,  but  less  distinctly.  It  is  worthy 
of  remark  that  the  phenomenon  has  not  escaped  the  notice  of  that 
acute  observer  Mr.  Allport,  who  speaks  of  clear  continuous  bands 
surrounding  quartz-crystals,  and  giving  a  black  cross  between  crossed 
nicols ;  these  he  apparently  considered  as  incipient  spherulites.1 
The  rock  forming  the  mass  of  the  great  southern  composite  dyke  • 
is  the  most  acid  of  all  these  felsites.  In  all  its  main  features  it  is 
identical  with  the  rock  of  Drumadoon,  and  with  the  other  masses  of 
similar  character  which  cover  so  considerable  an  area  in  the  south  of 
Arran.  A  more  or  less  cryptocrystalline  base  is  studded  with  large, 
but  often  much  corroded,  crystals  of  quartz  and  felspar.  The  latter 
appear  to  be  either  orthoclase  or  anorthoclase,  the  few  plagioclase 
crystals  being  quite  subordinate  to  these.  According  to  the  pre¬ 
dominance  of  the  orthoclase  or  the  anorthoclase,  the  rock  must  be 
called,  in  accordance  with  the  generally  accepted  terminology,  either 
a  rhyolite  or  a  quartz-pantellerite. 
One  very  interesting  fact  about  all  the  rocks  of  these  dykes  is  the 
prevalence  in  them  of  derived  crystals.  The  4  felsites  ’  and  4  pitch- 
stones  ’  not  unfrequently  contain  the  zoned  plagioclases  and  the 
brown  augites  of  the  augite-andesites,  and  these  are  usually  much 
corroded  by  the  acid  magma  in  which  they  have  been  caught  up. 
Tar  more  frequent,  however,  are  the  cases  of  minerals  clearly 
derived  from  the  acid  rocks,  enclosed  in  the  basic  ones.  Some  of  the 
augite-andesites  are  completely  studded  with  crystals  of  quartz,  and 
this  is  especially  the  case  near  the  junctions  with  the  pitchstones  or 
felsites.  The  quartz-crystals  are  surrounded  with  zones  of  pyroxene, 
and  the  rock  thus  closely  resembles  the  well-known  basalt  of  the 
Tetunata  in  Transylvania  and  also  the  4  quartz-basalts  ’  of  the 
Western  Territories  of  North  America  wTich  have  been  so  well 
1  Geol.  Mag.  for  1872,  p.  541. 
