543 
Vol.  49.]  PROF.  J.  W.  JUDD  ON  COMPOSITE  DYKES  IN  ARRAN. 
recognized  in  the  name  given  to  the  rocks.  The  glassy  forms  are 
known  as  4  pitchstones,’  but  the  devitrified  varieties  are  usually  called 
in  this  country  by  the  name  of  4  felsites.’  We  find  every  gradation 
from  one  type  to  the  other,  while  occasionally  the  rock  has  the  per¬ 
fectly  vitreous  lustre  of  an  obsidian.  I  propose  in  this  paper  to 
speak  of  the  stony  varieties  as  simply  ‘felsites’  or  4  quartz-felsites/ 
according  as  quartz-crystals  are  absent  or  present,  while  the  glassy 
forms  will  be  referred  to  by  the  old  names  of  4  pitchstone  ’  and 
4  pitchstone-porphyry.’ 
That  the  two  very  strongly  contrasted  classes  of  rocks  are 
present  in  the  dykes  of  the  Noith  of  Ireland,  as  well  as  in  Scotland 
and  England,  has  been  recently  shown  in  a  very  interesting  paper 
by  Prof.  W.  J.  Sollas.1 
IV.  The  Cir  Mhob  Dyke — Earlier  Notices. 
One  of  the  most  interesting  examples  of  the  union  of  the  two 
types  of  late  Tertiary  lava — the  basic  augite-andesites  and  the  acid 
4  pitchstone  ’ — in  a  single  dyke,  is  found  in  the  midst  of  the  great 
mass  of  Tertiary  granite  occupying  the  northern  half  of  the  Island 
of  Arran.  This  dyke  can  be  traced  from  the  summit  of  Cir  Mhor, 
which  may  be  regarded  as  the  central  peak  of  this  granitic  mass, 
and  runs  almost  due  eastwards  for  about  one-third  of  a  mile  till  it  is 
lost  near  the  north-western  spur  of  Goatfell.  Along  nearly  the 
whole  of  this  line  the  dyke  is  traceable,  and  at  many  points  its 
characters  may  be  very  distinctly  made  out. 
The  earliest  notice  of  this  dyke  that  I  have  been  able 
to  discover  is  in  Jameson’s  account  of  the  Island  of  Arran, 
published  in  1798,  in  which  the  author  speaks  of  it  as  a  basalt- 
vein,  rising  perpendicularly  through  the  granite,  while  near  it  were 
several  masses  of  green  pitchstone.2  In  his  later  work  the 
dyke  is  referred  to  in  almost  the  same  terms,  and  he  adds  that, 
44  after  considerable  labour,”  he  was  unable  to  discover  the  source 
of  the  44  columns  of  leek-green  coloured  pitchstone  ’  which  were 
lying  near  among  the  granite-debris.3 
Macculloch  in  his  account  of  Arran,  published  in  1819, 4  while 
referring  to  the  existence  of  both  basalt  and  pitchstone-dykes  tra¬ 
versing  the  granite,  does  not  directly  mention  this  composite  dyke. 
The  first  geologist  to  give  a  clear  account  of  this  remarkable 
dyke  was  the  late  Sir  Andrew  Eamsay.  He  pointed  out  its 
exact  position  on  the  southern  side  of  Cir  Mhor,  and  described 
it  as  crossing  the  ridge  dividing  Glen  Hosa  from  Glen  Sannox. 
He  stated  that  at  one  point  five  distinct  bands  of  rock  can 
1  ‘  On  Pitchstone  and  Andesite  from  Tertiary  Dykes  in  Donegal,’  Scient. 
Proc.  "Roy.  Dubl.  Soc.  vol.  viii.  (n.  s.)  1893,  p.  87. 
2  ‘  Outline  of  the  Mineralogy  of  the  Shetland  Islands  and  of  the  Island  of 
Arran,’  1798,  pp.  72-3. 
3  ‘  Mineralogy  of  the  Scottish  Isles,’  vol.  i.  (1800)  p.  40. 
4  4  Western  Isles  of  Scotland,’  vol.  ii.  p.  416. 
Q.  J.G.S.  No.  196.  2  b 
