536  PROF.  J.  W.  JUDD  ON  COMPOSITE  DYKES  IN  ARRAN.  [Nov.  1893, 
40.  On  Composite  Dykes  in  Arran.  By  J.  W.  Judd,  Esq.,  F.R.S., 
V.P.G.S.,  Professor  of  Geology  in  the  Royal  College  of  Science, 
London.  (Read  June  21st,  1893.) 
[Plate  XIX.] 
Contents. 
I.  Definition  and  Classification  of  Composite  Dykes  .  536 
II.  The  Latest  Volcanic  Rocks  of  the  British  Isles  .  539 
III.  The  Chief  Types  of  Volcanic  Rocks  represented  in  these  late 
Ej  ections  . 540 
IV.  The  Cir  Mhor  Dyke — Earlier  Notices  .  543 
V.  General  Structure  and  Chemical  Composition  of  the  Cir  Mhor  Dyke  544 
VI.  Microscopical  Characters  of  the  Rocks  composing  the  Cir  Mhor 
Dyke  . 546 
VII.  The  Tormore  Plexus  of  Composite  Dykes — Earlier  Notices  .  551 
VIII.  General  Structure  of  the  Tormore  Dykes  .  553 
IX.  Chemical  Composition  of  the  Tormore  Dykes  .  558 
X.  Microscopical  Characters  of  the  Rocks  composing  the  Tormore 
Dykes  .  559 
XI.  Summary  ;  and  General  Conclusions  derived  from  the  Study  of  the 
Composite  Dykes  of  Arran  . 561 
I.  Definition  and  Classification  of  Composite  Dykes. 
Those  dykes  which  are  filled  with  materials  differing  one  from 
another  in  chemical  composition  or  mineralogical  constitution  may 
he  conveniently  spoken  of  as  ‘  composite  dykes.’  In  some  cases 
the  diverse  rocks  found  in  the  same  dyke  may  be  only  varieties  of 
a  single  type,  exhibiting  dissimilarities  in  the  proportions  or 
arrangement  of  their  several  mineral  constituents ;  but  in  other 
cases  two  or  more  rock-species,  absolutely  distinct,  may  be  found 
to  enter  into  the  constitution  of  a  single  dyke. 
A  careful  study  of  these  composite  dykes  shows  that  they  fall 
naturally  into  two  classes,  as  follows : — 
(A)  Composite  dykes  in  which  a  differentiation  has  gone  on  in  the 
material  that  has  jilled  the  dyke. — This  differentiation  usually 
results  in  the  separation,  or  partial  separation,  of  the  vitreous 
base  of  the  rock  from  the  crystals  scattered  through  it ;  but  in 
some  cases  the  crystallizing  process,  acting  selectively  within 
the  dyke,  has  led  to  the  formation  of  rocks  which,  in  different 
parts  of  the  dyke,  show  marked  variations  in  chemical  com¬ 
position  or  texture,  and  even  in  mineralogical  constitution. 
The  separation  of  the  glassy  from  the  crystalline  materials  in 
this  variety  of  composite  dykes  takes  place  in  various  ways ; 
the  vitreous  material  may  form  lateral  bands  (‘  selvages  ’)  or 
irregular  patches,  or  it  may  fill  ‘  amygdaloidal  ’  cavities.  But 
in  all  cases  there  is  found  to  be  a  more  or  less  perfect  gradation 
from  the  one  type  of  rock  to  the  other. 
