198  TERTIARY  GRANITE  IN  GABBRO  OF  THE  CTJILXIN  HILLS.  [May  1 893, 
Tertiary  diabase  erupted  even  after  the  Tertiary  gabbro,  and  that 
the  Tertiary  granite  erupted  after  the  diabase,  and  was  thus  the  last 
eruptive  member  of  all  in  that  island. 
Mr.  Butley  commented  on  the  close  resemblance  between  the 
microscopic  structures  described  by  the  Author  and  those  occurring 
in  vitreous  lavas. 
Mr.  W.  W.  Beaumont  and  Mr.  Geo.  Barrow  also  spoke.  % 
Prof.  Bonney  being  unavoidably  absent,  the  Author  of  the  paper 
asked  permission  (in  accordance  with  a  precedent  that  had  recently 
been  established)  to  read  the  following  statement  which  had  been 
received  from  him  : — 
“  The  specimens  which  you  showed  me  recalled  to  memory  certain 
felstones  which  had  been  affected  by  the  heat  of  later  intrusive 
rocks  ;  they  differed  from  felstones  which  had  cooled  in  the  usual 
way.  The  differences  became  more  marked  on  microscopic  exami¬ 
nation.  It  seems  impossible  to  deny  that  these  rocks  represent  a 
variety  of  granite  which  has  been  partially  melted  down  again  in 
situ.  Portions  of  the  granitic  original  remain  here  and  there,  and 
form  nuclei  in  the  spherulites.  This  does  not  resemble  the  ordinary 
granites  of  Scotland,  such  as  that  of  Ballachulish,  but  appears  to  be 
identical  with  the  Tertiary  granite  (often  called  4  granophyre  ’)  of  the 
Western  Isles.  So  the  latter  must  be  earlier  in  date  than  the  gabbro. 
The  Author,  replying  to  Sir  Archibald  Geikie’s  suggestion  that  the 
masses  of  altered  rock  might  be  veins  and  not  inclusions ,  pointed  out 
that  not  only  were  the  forms  of  the  masses  lying  enclosed  in  the 
gabbro  quite  different  from  those  of  veins,  but  the  petrographical 
evidence — such  as  the  remarkable  alteration  of  the  minerals  and 
the  occurrence  of  fragments  of  micropegmatitic  granite  as  nuclei  in 
the  spherulites — were  altogether  irreconcilable  with  such  an  idea. 
To  the  assertion  that  the  occurrence  of  these  inclusions  at  one 
locality  is  not  sufficient  to  prove  the  relative  ages  of  these  two  great 
rock-masses,  he  rejoined  that  a  single  and  indisputable  case  of  one 
rock  enclosed  in  another  did  prove,  beyond  all  controversy,  that 
the  enclosing  rock  is  younger  than  the  enclosed  masses.  In  this 
case  the  inclusions  occur  near  to  the  line  of  junction  of  the  largest 
and  most  typical  areas  of  granite  and  gabbro  in  the  whole  region — 
those  of  the  Bed  Mountains  and  the  Cuillin  Hills  of  Skye,  and  at  a 
locality  selected  by  Sir  Archibald  Geikie  himself  as  a  most  perfectly 
representative  one.  With  respect  to  the  assertion  that,  at  this  place, 
veins  proceed  from  the  granite  into  the  gabbro,  earlier  writers  on 
the  district  had  looked  for  and  failed  to  discover  such  evidence.  He 
had  .himself  gone  again  and  again  to  the  very  spots  indicated  by  the 
Director-General  of  the  Survey,  and  could  find  nothing  of  the  kind, 
while  he  did  obtain  abundant  evidence  opposed  to  this  view.  He 
maintained  that  loose  fragments,  like  those  picked  up  on  the  shore 
at  St.  Hilda,  and  now  exhibited  by  Sir  Archibald  Geikie,  do  not 
support  his  views.  They  show  a  dark  rock  traversed  by  veins  of  a 
light  one,  but  the  dark  rock  is  not  a  gabbro,  and  the  light  rock 
is  not  a  granite. 
