Yol.  49.]  PROF.  J.  W.  JUDD  ON  COMPOSITE  DYKES  IN  ARRAN.  561 
described  by  Iddings  and  Diller.  In  some  eases  the  felspars  (ortho- 
clase,  anorthoclase,  or  plagioclase)  of  the  acid  rocks  can  be  clearly 
recognized  in  the  midst  of  the  augite-andesites,  and  are  then  seen 
to  be  surrounded  by  a  zone  of  alteration  produced  by  the  reaction  of 
the  magma  on  the  surfaces  of  the  crystals. 
Mr.  Allport  records  the  finding  of  a  porphyritic  pitchstone-boulder 
in  Arran  which  contained  “  a  considerable  proportion  of  fine-grained 
basalt  included  in  it,”  as  well  as  “  small  isolated  fragments  of  the 
same  rock  ....  and  a  few  grains  of  augite.”  1  It  is  not  improbable 
that  this  pitchstone  was  derived  from  one  of  the  numerous  composite 
dykes  of  the  island. 
XI,  Summary  ;  and  General  Conclusions  derived  from  the 
Study  of  the  Composite  Dykes  of  Arran. 
Any  suggestion  concerning  the  possibly  accidental  association  of 
the  augite-andcsite  and  4  pitchstone  ’  in  the  Cir  Mhor  dyke  is  at 
once  negatived  by  the  study  of  the  remarkable  plexus  of  dykes  at 
Tormore.  Xo  one  can  doubt,  after  the  study  of  this  latter  case, 
that  there  is  a  real  and  not  merely  an  accidental  connexion  between 
the  ejection  of  materials  of  such  very  different  composition  and 
character  :  all  the  facts,  indeed,  point  to  the  conclusion  that  the 
fissures  were  injected  from  the  same  subterranean  reservoir,  but  that 
this  reservoir  contained  two  magmas  of  totally  different  chemical 
composition.  In  the  same  way,  as  is  well  known,  a  single  volcanic 
vent  may  give  rise  at  successive  periods  to  two  totally  distinct  kinds 
of  lava. 
Xor  can  there  be  any  difficulty  in  understanding  how  the  same 
fissure,  while  still  in  connexion  with  a  reservoir  of  liquefied  lava,  may 
be  reopened  and  re-injected  at  successive  periods.  The  plane  of 
weakness,  along  which  the  reopening  of  the  dyke  is  effected,  is  some¬ 
times,  as  in  the  Cir  Mhor  dyke,  in  its  centre  ;  in  other  cases,  as  in 
the  great  north-and-south  dyke  of  Tormore,  and  also  in  the  most 
southerly  of  the  transverse  dykes  of  the  same  district,  planes  of 
■weakness  are  found  along  one  or  both  of  the  lateral  walls  of  the  dyke, 
and  it  is  here  that  the  re-injection  is  effected  ;  in  yet  other  cases,  also 
illustrated  at  Tormore,  the  new  fracture  seems  to  be  quite  irregular  in 
position  and  to  traverse  the  old  dyke-material  in  a  sinuous  line.  In 
one  instance  we  have  evidence  of  three  separate  injections  into  the 
same  fissure. 
In  some  cases  the  more  acid  rock  (quartz-felsite  and  pitchstone) 
was  the  first  ejected  ;  but,  quite  as  frequently,  the  basic  material 
(augite-andesite)  was  the  earliest  to  be  intruded  into  the  opening 
fissure.  The  relative  ages  of  the  two  rocks  in  the  dyke  are  shown, 
not  only  by  the  positions  which  they  occupy,  but  by  the  circum¬ 
stance  that  derived  minerals  from  the  older  rock  are  found  included 
in  the  younger  one.  That  a  very  considerable  interval  of  time  must 
have  elapsed  between  the  two  injections  is  shown  by  the  fact  that 
1  Op.  cit.  p.  537. 
