554  PK0E.  J.  W.  JUDD  ON  COMPOSITE  DYKES  IN  AEEAN.  [Nov.  1 893, 
we  are  chiefly  concerned  in  this  memoir.  Although  the  actual 
relation  of  these  several  dykes  to  each  other  is  sometimes  difficult 
to  trace,  the  nature  of  the  rock-materials  in  them  is  of  such  a 
character  as  to  warrant  the  belief  that  they  are  all  intimately 
connected,  and  that  all  the  fissures  have  been  filled  from  the  same 
subterranean  reservoirs.  I  will  now  proceed  to  describe  in  detail 
the  characteTs  of  these  four  composite  dykes. 
The  Great  North-and-South  Dyke  (I  in  Map). — This  great  igneous 
intrusion  can  be  traced,  at  intervals,  where  not  covered  by  shingle, 
for  a  distance  of  over  600  yards.  It  is  indicated  on  Jameson’s 
plan  by  the  letters  A,  B,  C,  D.  It  should  perhaps  be  called  in  some 
places  a  sheet  rather  than  a  dyke,  for  it  is  occasionally  almost 
horizontal,  but  the  changes  in  its  dip  appear  to  be  numerous  and 
sometimes  sudden.  At  the  most  northerly  point  where  it  can  be 
seen  rising  from  under  the  shingle,  it  consists  entirely  of  a  green 
pitchstone  which,  by  microscopic  study,  is  found  to  be  a  ‘  vitro- 
phyric  augite-enstatite-andesite,’  without  quartz.  For  a  distance 
from  north  to  south  of  about  450  yards  it  seems  to  follow  pretty 
closely  the  direction  of  the  shore  ;  but,  at  its  southern  extremity, 
it  bends  from  this  direction  at  an  angle  of  40°  towards  the  west, 
and  runs  out  seawards.  Here,  too,  the  mass  loses  its  vertical 
position  and  in  places  has  a  dip  of  less  than  60°.  The  dyke,  where 
it  can  be  measured,  appears  to  have  a  width  of  10  to  12  feet,  and 
a  marked  alteration  of  the  red  sandstone,  forming  its  walls,  is  seen 
to  be  produced  by  it. 
This  intrusion  constitutes  an  interesting  example  of  a  composite 
dyke.  While  at  its  northern  extremity  it  consists  entirely  of  green 
pitchstone,  at  its  southern  extremity  the  glassy  rock  passes  near 
the  sides  into  a  4  lithoidite 9  or  stony  andesite ;  and  on  its  western 
Fig.  2. — Diagrammatic  Plan  of  Nortk-and-South 
Composite  Dyke ,  Tormore. 
12 
a  =  Augite-andesite.  c  =  Pitchstone  (Andesite). 
b  —  Banded  and  Spherulitic  Felsite  (Andesite).  s  =  Sandstone. 
side  it  gives  off  a  vein  of  hornstone-like  rock  (E  of  J ameson)  which 
runs  out  seaward.  On  its  eastern  side,  however,  a  band  of  dark- 
coloured  augite-andesite,  weathering  into  balls,  is  seen  coming  in 
between  the  more  acid  rock  and  the  sandstone-walls  of  the  dyke. 
This  dark-coloured  rock  precisely  resembles  that  forming  the  sides 
of  the  Cir  Mhor  dyke.  While  there  may  be  some  doubt  on  the 
