556  PROF.  J.  W.  JUDD  ON  COMPOSITE  DYKES  IN  ARRAN.  [Nov.  189^, 
pitch  stones,  it  contains  great  numbers  of  stellate  groups,  each  sur¬ 
rounded  by  its  4  court  of  crystallization  ’  as  described  by  Allport.1 
This  4  pitchstone-porphyry  ’  contains  a  considerable  quantity  of 
pyrites.  It  also  exhibits  a  phenomenon  which  I  have  already 
described  as  occurring  in  the  glassy  rock  of  Chiaja  di  Luna  in 
Ponza.  When  broken  by  a  hammer,  the  mass  tends  to  split  up 
into  spheroids  with  concentric  structure,  and  each  surface  exposed 
only  retains  its  brightness  momentarily,  being  then  covered  with  a 
whitish  film.  This  white  film  is  evidently  formed  by  a  molecular 
change  following  the  relief  from  a  condition  of  strain.2 
xls  the4  pitchstone-porphyry’  in  this  dyke  is  sometimes  enclosed 
entirely  in  the  quartz-felsite,  and  at  other  times  crosses  the 
boundaries  of  that  rock  and  intersects  the  augite-andesite  (see  fig.  3, 
p.  555),  there  is  reason  for  believing  that  the  infilling  of  the  dyke 
was  accomplished  at  three  distinct  periods. 
A  short  distance  south  of  this  northern  composite  dyke  two  dykes 
of  augite-andesite  are  seen  enclosing  a  mass  of  conglomerate  or  coarse 
sandstone.  These  two  dykes,  which  appear  to  be  branches  of  a 
single  one,  are  designated  by  Jameson  M,  L,  M. 
The  Middle  Transverse  Composite  Dyke  (III  in  Map)  occurs 
about  120  yards  south  of  the  northern  dyke,  and  is  marked  in 
Jameson’s  plan  by  the  letters  L,  K,  H,  G,  P.  It  strikes  N.W.- 
and-S.E.  and  is  "from  40  to  50  feet  wide.  The  dyke  is  mainly 
composed  of  augite-andesite,  but  in  its  midst  there  occurs  a  band  of 
quartz-felsite  4  or  5  feet  wide,  passing  at  its  centre,  for  a  width  of 
2  feet,  into  the  4  vitrophyric  ’  form  (‘pitchstone-porphyry’).  The 
band  of  acid  rock  is  usually  much  nearer  to  the  north-eastern  than 
to  the  south-western  side  of  the  dyke,  but  is  very  variable,  and, 
when  followed  up  into  the  cliff,  is  found  to  expand  to  a  width  of  10 
Fig.  4. — Diagrammatic  Plan  of  Middle  Composite  Dyke ,  Tormore. 
s 
iXWiMlii 
wmm 
mmmmm 
■j 
wmm 
a = Augite-andesite.  c = Pitchstone  (Dacite). 
b  —  Quartz-felsite  (Dacite).  s= Sandstone. 
G-eol.  Mag.  for  1872,  p.  5. 
2  Ibid.  1886,  pp,  245-246. 
