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DISCUSSION — LN CL  USIOXS  OP  TEBTIABY 
[May  1893. 
Discrcssiox. 
The  Peestdestt  remarked  that  those  who  had  followed  the  subject 
attentively  were  well  aware  that  the  Author  had  made  the  "Western 
Islands  of  Scotland  his  own  particular  study,  and  the  Society  was 
glad  to  be  a?ain  favoured  with  an  interesting  memoir  from  him. 
The  paper  was  distinctly  separable  into  two  portions,  the  first  of 
which,  purely  petrological,  afforded  a  useful  definition  of  the 
processes  of  fusion  of  acid  by  basic  rocks.  The  second  portion 
was  peculiarly  interesting,  since  it  conveyed  a  direct  challenge  to 
those  vrho  took  opposite  views  ( to  those  of  the  Author)  as  regards 
the  relative  age  of  acid  and  basic  rocks.  The  geology  of  the 
Highlands  had  ever  been  fruitful  of  controversy.  It  was,  however, 
impossible  for  any  one  not  thoroughly  acquainted  with  the  district 
to  say  how  matters  stood.  He  was  sure  that  all  present  were  now 
anxious  to  hear  the  other  side  of  the  question. 
Sir  Abchtbxld  Geixie  observed  that  a  doubt  might  well  be 
entertained  whether  the  so-called  4  inclusions  ?  of  the  Author  were 
really  such :  for  the  petrographical  description  of  them  was 
suggestive  rather  of  obsidian  and  rhyolite-veins.  Yo  account 
had  been  given  of  the  structure  of  the  surrounding  rock  with 
relation  to  the  4  inclusions '  and  their  supposed  metamorphism. 
He  vras  willing,  however,  for  the  sake  of  argument,  to  allow  them 
to  be  actual  blocks  enclosed  within  the  gabbro.  But  surely  the 
sweeping  generalization  drawn  from  them  by  Prof.  Judd,  as  to 
the  age  of  the  great  acid  protrusions  of  the  Western  Isles,  was  not 
warranted.  Geologists  have  been  in  the  habit  of  believing  that 
all  that  can  be  demonstrated  from  such  evidence  is  that  an  enclosed 
block  must  be  older  than  the  rock  immediately  enclosing  it. 
Beyond  that  point  all  attempt  to  fix  more  precisely  their  relative 
dates  is  inference,  which  may  be  probable  or  not  according  to  the 
circumstances  of  the  case.  But  the  speaker  was  again  ready,  for 
the  sake  of  argument,  to  waive  this  objection  and  to  concede  that 
the  •'  inclusions  ‘  were  not  ancient  rocks,  but  portions  of  some  of 
the  acid  masses  of  the  Tertiary  volcanic  series  in  the  Hebrides. 
Yet.  even  with  this  admission,  was  the  Author  entitled  to  say  that 
he  had  brought  forward  any  valid  proof  of  the  relative  ages  of  the 
basic  and  acid  bosses  of  the  West  of  Scotland?  Most  assuredly 
not.  Sir  Archibald  Geikie  stated  that  he  had  shown  that  there 
were  traces  of  acid  protrusions  at  an  early  part  of  the  volcanic 
history  of  that  region,  and  that  blocks  of  various  acid  rocks  were 
abundant  in  the  volcanic  agglomerates  and  in  some  of  the  tuffs 
intercalated  among  the  plateau-basalts.  The  alleged  4  inclusions J 
of  the  Author,  if  truly  such,  would  thus  be  most  naturally  assigned 
to  this  early  series  of  acid  rocks,  of  which  only  ejected  fragments 
had  yet  been  found.  Hence,  even  on  the  Author’s  own  ground,  the 
supposed  demonstration  that  the  granitic  bosses  are  older  than  the 
gabbro  masses  entirely  failed. 
But  the  true  state  of  the  case  had  not  been  put  before  the 
Society.  Quoting  from  his  memoir  in  the  Transactions  of  the 
