•559 
Yol.  49.I  PROP.  J.  W.  JCDD  OX  COMPOSITE  DYKES  IN'  ARRAH. 
IT.  is  an  analysis  of  a  pitchstone  from  Tormore  ;  although,  it  is 
not  expressly  stated  to  he  from  the  north-and-south  dyke,  there  can 
be  little  doubt  on  the  subject. 
Y.  is  said  to  be  from  a  4  claystone  ’  from  the  same  locality.  It 
is  probably  the  porphyritic  felsite  of  the  great  southern  composite 
dyke. 
YI.  and  YU.  are  analyses  made  by  John  Arthur  Phillips,  and 
published  by  Yu.  Allport  (Geol.  YLag.  for  1872,  p.  540).  The 
results  given  are  the  mean  of  two  closely  concordant  analyses.  I. 
is  a  red  4  hornstone-like  ’  felsite,  and  A  II.  represents  the  nodules 
contained  in  the  same  rock. 
X.  Microscopical  Characters  or  the  Bocks  composixg-  the 
Toehore  Dykes. 
Although  there  are  minor  differences  to  be  recognized  between 
the  rocks  of  Cir  Mhor  and  those  of  Tormore,  their  essential  identity 
is  sufficiently  obvious.  At  both  localities  we  find  in  association  in 
the  same  dyke  a  rock  of  essentially  basic  composition  (augite- 
andesite)  and  one  distinctly  acid,  with  a  large  amount  of  glassy  base 
which  mav  be  more  or  less  devitrified. 
The  augite- andesite  is  of  the  ordinary  character  in  most  of  the  dykes 
at  Tormore.  Bounded  gas-cavities  filled  up  with  glass,  now,  wholly 
or  in  part,  converted  into  chlorites  and  other  secondary  products,  are 
by  no  means  unfrequent.  In  the  middle  transverse  composite  dyke 
the  rock  is  of  considerable  thickness,  and  the  slow  cooling  of  such  a 
large  mass  of  material  has  led  to  the  formation  of  an  ophitic  dolerite 
or  diabase,  which  passes  by  insensible  gradations  into  an  inter- 
sertal  rock  (4  tholeite  ’),  and  thence  into  the  ordinary  andesite  with 
its  felted  base  of  felspar-microlites  and  its  porphyritic  crystals  of 
zoned  plagioclase. 
The  acid  rocks,  however,  show  a  much  greater  variety  in  compo¬ 
sition  and  character,  though  they  always  reveal  clear  traces  of  their 
close  family  relationships. 
The  great  north-and-south  dyke  is  an  example  of  the  most  basic 
type  of  these  pitchstones.  It  contains  no  quartz-crystals  and  only 
somewhat  sparsely  scattered  plagioclase,  with  possibly  a  few  crystals 
of  anorthoclase,  but  no  true  orthoclase.  Porphyritic  crystals  ot 
enstatite  and  augite  occur,  the  former  mineral  being  not  unfr equently 
enclosed  in  or  zoned  by  the  latter ;  the  microlites  of  the  ground- 
mass  are  as  usual,  however,  hornblende.  The  4  claystone  ’  and 
4  hornstone  ’  into  which  this  rock  passes  show  beautiful  examples  of 
the  transition  of  a  glassy  to  a  stony  rock,  the  groundmass  becoming 
first  spherulitic,  and  then  taking  the  characters  assigned  to  what  is 
known  as  the  4  felsitic  substance.’  It  is  very  interesting  to  note  how 
in  this  process  the  hornblende-microlites  appear  to  be  re-absorbed, 
while  the  glassy  groundmass  breaks  up  into  an  aggregate  of  quartz- 
and  felspar-microlites,  with  or  without  spherulitic  arrangement. 
This  rock  must  be  classed  as  an  augite-enstatite-andesite  with  much 
Q.  J.  G.  S.  Xo.  196,  ~  2  p 
