194  TERTIARY  GRANITE  IN  GABBRO  OE  THE  COTLLIN  HILLS.  [May  1893. 
these  igneous  intrusions  are  said  to  be  strangely  altered,  and  con¬ 
verted  into  a  splintery  rock  which  weathers  with  white  surfaces. 
These  rocks,  it  is  obvious,  are  the  lavas  which  I  described  under  the 
name  of  ‘  felstones  ’  in  1874,  and  into  which,  as  I  then  showed,  the 
granites  send  numerous  veins.  Two  years  ago  I  gave  full  petro- 
graphical  details  concerning  this  remarkable  series  of  rocks,  and  I 
showed  that,  so  far  from  their  being  basalts,  they  are  for  the  most 
part  andesites,  many  of  them  with  a  very  high  percentage  of  silica, 
but  usually  converted  by  solfataric  action — probably  connected  with 
the  intrusion  into  them  of  the  granites — into  the  ‘  propylitic  ’ 
modification.1 
The  description  of  the  beautiful  and  unmistakable  inclusions  of 
granite  (‘  granophyre  ’  of  some  authors),  lying  within  the  gabbro  in 
the  area  of  its  best  development,  now  completes  the  evidence  as  to 
the  younger  age  of  the  latter  rocks  which  has  been  disputed  by 
Sir  Archibald  Geikie.  I  may  mention,  however,  that,  when  I 
describe  in  detail  the  acid  rocks,  I  shall  be  able  to  add  many  other 
facts  which  are  scarcely  less  decisive  than  this  in  their  bearing  on 
the  question. 
8.  Postscript. 
[In  the  discussion  which  followed  the  reading  of  this  paper 
complaint  was  made  that  no  notice  had  been  taken  of  the  alleged 
existence— at  the  locality  described — of  veins  proceeding  from  the 
granite  into  the  gabbro.  It  was  a  desire  not  to  complicate  the  very 
definite  issue  raised  in  the  title  of  the  paper,  with  questions  upon 
which  a  remarkable  conflict  of  testimony  exists,  that  led  to  my 
avoidance  of  the  subject ;  but,  as  my  motive  has  been  misunderstood, 
I  will  as  briefly  as  possible  state  what  that  testimony  is. 
Macculloch  and  Porbes  alike  called  attention  to  the  profusion  of 
pale-coloured,  contemporaneous  or  segregation-veins  in  the  gabbro 
of  the  Cuillin  Hills  ;  they  both  assert,  however,  that  veins  are  never 
found  passing  from  the  granite  into  the  gabbro.  The  results  of  my 
own  observations,  at  this  and  other  localities  in  the  Western  Isles 
of  Scotland,  were,  as  I  stated  in  1874,  in  complete  accord  with 
those  of  my  predecessors.  Pragments  of  basic  rock  can  of  course  be 
easily  found  which  are  traversed  by  white  veins ;  but  there  is  no 
difficulty,  especially  under  the  microscope,  in  distinguishing  be'tween 
the  white  segregation-veins  of  the  gabbro,  composed  as  they  are  of 
lime-felspars,  and  apophyses  of  the  granite,  containing  alkali- 
felspars.  It  was  not  until  1888  that  Sir  Archibald  Geikie  made  the 
statement  that  veins  of  granite  do  cut  through  the  gabbro  at 
Meall  Dearg  and  other  points.  I  can  only  add  that  since  this  assertion 
was  made  I  have  revisited  all  the  localities  referred  to,  but  have 
never  succeeded  in  finding  true  granite- veins  penetrating  the  gabbro. 
It  was,  in  fact,  while  vainly  engaged  in  searching  for  such  veins 
that  I  discovered  the  very  conclusive  evidence  of  the  inclusions 
described  in  this  paper.  It  appears  to  me  that  the  existence  of 
these  inclusions  of  granite  in  the  gabbro  is  absolutely  irreconcilable 
with  the  occurrence  of  veins  of  the  same  granite  cutting  through 
the  gabbro. — March  17th,  1893.] 
1  Quart.  Journ.  Geol.  Soc.  vol.  xlvi.  (1890)  pp.  311-384. 
