A  ol.  49*]  GRANITE  IN  THE  GABBRO  OP  THE  CEILL1N  HILLS.  193 
massive  type— granites  and  their  allies— but  he  speaks  of  thick 
bodies  of  felstones,  disposed  in  regular  sheets  and  of  amygdaloidal 
structure,  which  alternate  with  beds  of  scorise,  lapilli,  and  ashes,  that 
lie  upon  the  skirts  of  the  central  bosses  of  granite.  These  he  believes 
to  be  the  remnants  of  a  volcano  formed  mainly  of  acid  lavas,  which 
was  piled  up  and  largely  ruined  by  denudation  before  the  discharge 
of  the  plateau-basalts  began.  The  existence  of  the  granite-bosses 
admits  of  no  doubt,  but  Dr.  Geikie  has  depicted  numerous  sections 
which  leave  no  doubt  that  these  rocks  intrude  into  the  basalts  and 
gabbros,  and  are  therefore  of  later  date  than  them.  Xow  that  all  these 
details  are  before  us,  the  question  of  relative  age  can  admit  of  only  one 
answer,  but  it  is  evidently  a  point  on  which  observers,  who  had  not 
opportunities  of  entering  minutely  into  details,  were  apt  to  go 
wrong.  Both  Principal  J.  D.  Forbes  and  Prof.  Zirkel  seem  to  have 
come  to  the  same  conclusion  a3  Prof.  Judd,  and  Dr.  Geikie  has 
supplied  the  explanation.  ‘  That  there  should  ever  have  been  any 
doubt,’  he  says,  ‘  about  the  relations  of  the  two  eruptive  masses  is 
possibly  explicable  by  the  facility  with  which  their  junction  can  be 
observed.  Their  contrasts  of  form  and  colour  make  their  boundary 
over  crag  and  ridge  so  clear  that  geologists  do  not  seem  to  have 
taken  the  trouble  to  follow  it  out  in  detail.  And  as  the  pale  rock 
(granophyre  or  granite)  underlies  the  dark  (gabbro),  they  have 
assumed  this  infraposition  to  mark  its  earlier  appearance.’  All  this 
is  graphically  brought  out  in  fig.  43  of  Dr.  Geikie’s  memoir.  .  .  . 
Anyone  trusting  to  surface-feature  might  well  fancy  that  the  basalts 
...  lay  upon,  and  were  newer  than,  the  granophyre.  .  .  .  Let  us 
all  take  warning  thereby.”  1 
How  anyone  reading  the  memoirs  of  Principal  Forbes  and  Prof. 
Zirkel  could  possibly  imagine  that  they  had  jumped  to  conclusions 
which  they  published  concerning  the  relations  of  the  two  sets  of 
igneous  rocks  in  question,  without  properly  examining  the  evidence, 
I  am  at  a  loss  to  conceive.  Their  memoirs  exhibit  proofs  of  the 
most  careful  and  painstaking  study  of  the  lines  of  junction,  and  it 
seems  to  me  needless  to  combat  the  idea  that  they  published 
statements  based  only  on  hasty  views  of  the  ground  from  a  distance. 
For  myself,  I  may  add  that,  before  adopting  the  conclusions  of  my 
predecessors,  I  carefully  studied  the  sections  they  described,  as  well 
as  others,  and  satisfied  myself  that  these  two  authors  were  perfectly 
accurate  as  to  their  facts  and  rigidly  logical  in  their  conclusions. 
The  confident  statements  of  our  critic  led  me,  however,  to  re¬ 
examine  the  whole  of  the  junctions  again  with  the  greatest  care. 
It  then  soon  became  evident  how  the  mistake  on  the  part  of  Sir 
Archibald  Geikie — for  that  it  was  a  mistake  no  one  who  examines 
the  evidence  now  adduced  can  for  one  moment  doubt — has  arisen. 
In  his  memoir  in  the  Transactions  of  the  Ptoyal  Society  of  Edinburgh 
there  are  given  a  number  of  sketches — most  of  them  necessarily 
taken  from  a  distance,  as  they  represent  whole  mountain-sides — in 
which  the  granites  are  shown  as  sending  off  veins  into  the  1  bedded 
basalts.’  On  reference  to  the  text,  it  is  seen  that  the  ‘  ba,salts  ’  near 
1  ‘Xature,’  vol,  xxxix.  pp.  132-134  (Dec.  6tb,  1888). 
Q.  J.  G.S.  Xo.  194. 
o 
