555 
Vol.  49.J  PEOF.  J.  W.  JUDD  OX  COMPOSITE  DYKES  IX  AEEAX. 
subject,  I  think  the  evidence  points  to  the  conclusion  that  the  thin 
band  of  more  basic  rock  was  injected  at  a  later  date  than  the  more 
acid  materials  forming  the  mass  of  the  dyke.  (See  fig.  2,  p.  554.) 
This  great  dyke  is  crossed  by  a  number  of  transverse  dykes,  the 
most  northerly  of  which  is  one  composed  of  pitchstone  (Q  of 
Jameson),  which  is  about  10  feet  wide  and  runs  out  to  sea  in  a 
S.W.-by-W.  direction,  but  does  not  appear  to  present  any  special 
features  of  interest.  Other  composite  dykes  occur,  however,  which 
we  will  proceed  to  notice  in  order  from  north  to  south. 
The  Northern  Transverse  Composite  Dyke  (II  in  Map). — This 
dyke  is  indicated  by  the  letters  X,  0,  P  on  Jameson’s  plate  ;  a  plan 
of  the  vein  taken  on  the  shore  near  low-water  mark  is  given  by 
Zirkel.1  The  width  of  the  dyke  is  about  30  feet ;  its  sides  are  com¬ 
posed  of  an  augite-andesite,  which  weathers  into  rounded  balls,  its 
centre  of  a  quartz-felsite  which  in  places  passes  into  a  4  pitchstone- 
porphyry  ’  or  4  vitrophyric  ’  rock.  The  acid  rock  forms  about  one 
half  of  the  width  of  the  dyke,  and  the  ‘.pitchs  tone-porphyry  ’  occurs 
as  a  band  varying  in  width  from  6  inches  to  2  feet,  sometimes 
forming  part  of  the  quartz-felsite  mass  and  at  other  times  inter¬ 
secting  the  adjoining  masses  of  andesite  (see  fig.  3).  The  augite- 
Fig.  3. — Diagrammatic  Plan  of  Northern  Composite  Dyke ,  Tor  more. 
a — Augite-andesite.  c=Pitchstone-porphyry  (Dacite). 
b  —  Quartz-felsite  (Dacite).  5= Sandstone. 
andesite  is  sometimes  coarse-grained,  with  intersertal  structure,  and 
at  other  times  it  is  fine-grained,  with  much  glass  filling  amygda- 
loidal  hollows.  The  4  pitchstone-porphyry 7  is  somewhat  peculiar 
in  character.  In  colour  it  is  nearly  black,  and  instead  of  the  usual 
feathery  masses  of  hornblende-microlites,  so  common  in  the  Arran 
1  Zeitschr.  d.  Deutsch.  geol.  G-esellsck.  vol.  xxiii.  (1871)  pi.  ii. 
