Yol.  49.]  PEOF.  J.  AY.  JUDD  ON  COMPOSITE  DYKES  IN  AEEAN. 
553 
The  first  author  to  describe  this  very  interesting  locality  was 
Prof.  Robert  Jameson.  In  his  earlier  work  on  Arran  the  occur¬ 
rence  of  a  great  pitchstone-vein  crossed  by  a  series  of  transverse 
dykes  of  basalt  is  mentioned.1  In  his  4  Mineralogy  of  the  Scottish 
Isles,’  published  in  1800,  we  find  not  only  a  very  accurate  descrip¬ 
tion  of  the  composition  and  relations  of  these  various  intrusions, 
but  a  plan  of  the  shore  on  which  the  several  dykes  are  indicated 
by  letters.2  So  accurate  are  this  plan  and  description  that  they 
were  reproduced  by  Ramsay  in  his  4  Geology  of  Arran,’ 3  without 
alteration,  and  Bryce  also  quoted  the  same  description.4  Zirkel 
in  1871 5  gave  a  plan  and  description  of  one  of  the  composite  dykes, 
while  Allport  in  the  same  year 6  referred  to  another  of  them. 
There  are  slight  discrepancies  in  the  accounts  of  these  several 
dykes  given  by  different  authors,  owing  to  the  fact  that  no  two 
observers  probably  have  seen  the  shore  in  quite  the  same  con¬ 
dition.  The  tidal  scour,  sweeping  loose  blocks  and  shingle  into 
hollows,  at  times  obscures  certain  tracts  of  the  shore  or  uncovers 
other  portions  of  it.  The  publication  of  the  excellent  maps  of  the 
Ordnance  Survey  enables  us  to  lay  down  the  positions  of  the  several 
dykes  in  a  more  accurate  manner  than  was  before  possible ;  but 
the  original  plan  and  description  of  Jameson  still  constitute  an 
admirable  guide  to  the  position  and  relations  of  these  rock-masses. 
In  the  following  account  of  these  composite  dykes,  I  shall  give  the 
result  of  observations  and  measurements  which  I  have  made  upon 
several  different  occasions,  adding  references  to  the  accounts  of 
Jameson  and  other  geologists  who  have  stated  what  they  saw  there 
in  previous  years. 
YIII.  Geneeal  Stehctuee  of  the  Toemoee  Dykes. 
The  Tormore  plexus  consists  of  a  great  dyke  or  sheet  running 
nearly  north-and-south  for  a  distance  of  about  600  yards  approxi¬ 
mately  parallel  to  the  shore,  sometimes  in  a  vertical  and  sometimes 
in  an  inclined  position,  and  crossed  at  intervals  by  numerous  other 
dykes  in  a  transverse  or  more  or  less  east-and-west  direction.  The 
principal  intrusion  (I  in  the  Map)  and  three  of  the  transverse 
masses  (II,  III,  and  IY  in  the  Map)  are  beautiful  examples  of 
composite  dykes,  and,  taken  together,  they  illustrate  in  an  admirable 
manner  the  main  varieties  of  that  class  of  phenomena  with  which 
1  4  Outline  of  the  Mineralogy  of  the  Shetland  Isles  and  of  the  Island  of 
Arran,’  1798,  pp.  126-128. 
2  ‘Mineralogy  of  the  Scottish  Isles,’  vol.  i.  pp.  101-106,  with  plate  facing 
p.  103.  In  the  text  of  this  paper  I  have  introduced  the  letters  employed  by 
that  author  to  distinguish  the  several  dykes.  It  will  be  seen  that,  by  an 
inadvertence,  Jameson  sometimes  employs  the  same  letter  twice— but  no  real 
inconvenience  has  arisen  from  this  circumstance. 
3  «  The  Geology  of  the  Island  of  Arran  from  original  Survey,’  1841,  p.  51. 
4  ‘The  Geology  of  Arran  and  the  other  Clyde  Islands,’  4th  ed.  (1872) 
pp.  201-203. 
5  Zeitschr.  d.  Deutsch.  geol.  Gesellsch.  vol.  xxiii.  (1871)  p.  40  pi.  ii. 
6  Geol.  Mag.  for  1872,  p.  5.  ’ 
