544  PROP.  J.  W.  JUDD  ON  COMPOSITE  DYKES  IN  ARE  AN.  [Hov.  1893, 
be  made  out  in  the  dyke.  At  the  sides  are  masses  of  4  trap,’ 
weathering  into  balls  which  exhibit  a  concentric  structure ;  in  the 
middle  is  a  band  of  £  green  pitchstone,’  and,  between  the  4  pitch- 
stone  ’  and  4  trap,’  layers  of  ‘  claystone-porphyry  ’  make  their 
appearance.  Higher  up  the  mountain  the  ‘claystone-porphyry’ 
was  found  to  be  absent,  aud  the  dyke  consists  of  4  pitchstone  ’  in 
the  middle,  with  ‘trap’  at  the  sides.1 
Dr.  James  Bryce  added  some  details  to  those  given  by  Ramsay 
concerning  the  Cir  Mhor  dyke.2  He  referred  to  bands  of  a  material 
like  ‘  hornstone  ’  occurring  between  the  ‘  pitchstone  ’  and  the 
4  claystone-porphyry,’  and  his  measurements  of  the  several  layers 
of  rock  differ  somewhat  from  those  of  his  predecessor. 
In  his  account  of  the  microscopical  characters  of  the  Arran 
pitchstones,  Mr.  Allport  refers  to  the  rock  of  this  dyke  at  Cir  Mhor 
as  having  a  banded  structure  similar  to  that  seen  in  a  dyke  1  mile 
south  of  Tormore.3 4 
It  may  be  that  a  dyke  traversing  the  granite  of  Ceum-na-Caillich 
is,  as  suggested  b}T  several  authors,  connected  with  that  of  Cir  Mhor. 
The  Ceum-na-Caillich  dyke  seems  also,  as  Jameson  pointed  out,  to  be 
an  example  of  a  ‘  composite  dyke,’  or,  as  he  called  it,  a  4  stratified 
vein.’ 4 
V.  General  Structure  and  Chemical  Composition  of  the 
Cir  Mhor  Dyke. 
Although  I  have  visited  this  dyke  on  a  number  of  different 
occasions,  it  is  but  little  that  I  can  add  to  the  excellent  general 
descriptions  given  by  Ramsay  and  Bryce.  The  width  of  the  dyke 
is  found,  when  the  sides  can  be  seen,  to  vary  considerably,  from 
12  or  14  up  to  nearly  30  feet.  Evidently  there  are  two  perfectly 
distinct  rocks  present,  which  appear  to  be  sharply  divided  the  one 
from  the  other.  The  centre  of  the  dyke  is  a  ‘  pitchstone-porphyrv,’ 
which  contains  conspicuous  crystals  of  quartz  and  felspar,  is 
sometimes  banded,  and  not  unfrequently  more  or  less  spherulitic 
in  structure.  This  vitreous  rock  passes  into  a  4  hornstone-like  ’ 
modification,  and  finally  into  a  ‘lithoidal’  rock  or  quartz-felsite, 
the  4  claystone  ’  or  4  claystone-porphyry  ’  of  Ramsay  and  Bryce. 
The  sides  of  the  dyke,  however,  are  composed  of  a  totally 
distinct  rock,  a  porphyritic  augite-andesite  (‘  trap,’  ‘  greenstone,’ 
or  4  basalt’  of  the  older  authors),  which  shows  a  very  marked  ten¬ 
dency  to  weather  into  balls  with  concentric  structure.  The  width 
of  the  felsite-and-pitchstone  centre,  and  of  the  andesite  sides,  varies 
greatly  in  different  parts  of  the  dyke,  but  the  acid  and  basic  rocks 
are  always  completely  distinct  the  one  from  the  other.  The 
relations  of  the  rocks,  as  seen  at  the  place  where  they  are  best 
developed,  are  shown  in  the  diagram  on  the  opposite  page  (fig.  1). 
1  ‘  The  Geology  of  the  Island  of  Arran  from  original  Survey,’  1841,  p.  26. 
2  ‘  The  Geology  of  Arran  and  the  other  Clyde  Islands,’  4th  ed.  (1872)  p.  164. 
3  Geol.  Mag.  for  1872,  p.  6. 
4  ‘Mineralogy  of  the  Scottish  Isles,’  vol.  i.  (1800)  p.  81,  footnote. 
