Yol.  49.]  GRANITE  IN  THE  GABBRO  OF  THE  CUILLIN  HILLS.  183 
granites  and  gabbros.  As  the  granite  weathers  so  much  more 
readily  than  the  gabbro,  these  patches  of  granite  form  depressions 
in  the  great  rusty-brown  mass  of  the  basic  rock,  which  are  usually 
filled  with  debris ;  some  of  the  patches  show  a  section  several 
square  yards  in  area.  Within  these  depressions  the  acid  rock  can 
be  clearly  traced,  while  the  boundary  of  the  surrounding  gabbro 
may  be  followed  without  difficulty,  and  the  complete  isolation  of 
the  patches  of  granite  is  very  obvious.  The  acid  rock  within  the 
depressions  is  at  once  seen  to  present  a  very  different  appearance 
from  the  normal  granite  :  although  the  phenocrysts  of  quartz  and 
felspar  are  often  conspicuous,  the  rocks  exhibit  the  general  aspect  of 
a  rhyolite  rather  than  that  of  a  granite.  They  are  compact  in  texture 
and  splintery  in  fracture,  and  sometimes  almost  glassy  in  appearance. 
The  greatly  weathered  surfaces  reveal  the  peculiar  structures  of 
the  rock  in  a  most  admirable  manner.  Banding  is  very  manifest 
on  these  weathered  surfaces,  and  in  many  instances  the  banding 
can  be  seen  to  be  due  to  a  parallel  disposition  of  spherulites  in 
the  mass  (see  PI.  II.  figs.  1  &  2).  Spherulitic  structures,  of  all 
dimensions  from  a  pin’s  head  to  a  small  orange,  are  everywhere 
conspicuous  (PI.  II.  figs.  3,  I)  ;  they  often  show  the  characters  of 
lithophyses  (hollow  spherulites  ;  see  PI.  II.  fig.  5),  and  not  unfre- 
quently  exhibit  the  crashed  and  distorted  appearances  which  have 
been  so  well  described  in  the  case  of  the  very  fresh  rhyolitic  rocks 
of  the  Western  Territories  of  the  United  States.  On  breaking  open 
the  larger  nodules,  they  are  seen  to  present  the  arborescent  forms 
characteristic  of  the  complex  spherulites  of  the  districts  just  re¬ 
ferred  to.  These  spherulites  often  weather  out  as  perfectly  rounded 
bodies  like  1  marbles,’  which  can,  in  places,  be  collected  in  consider¬ 
able  numbers.  The  general  examination  of  the  enclosed  masses 
indicates  that  they  are  rocks  originally  of  a  glassy  character,  but 
converted  into  4  lithoidite  ’  by  the  development  throughout  the 
mass  of  every  variety  of  spherulitic  structure,  while  scattered 
through  the  whole  may  often  be  seen  the  phenocrysts  of  quartz 
and  felspar.  At  one  or  two  points  I  have  found  the  same  phe¬ 
nomena  displayed,  though  in  a  less  striking  manner,  at  the  junction 
of  the  gabbro  with  the  granite. 
The  general  conclusions  concerning  the  nature  and  origin  of 
these  beautiful  rocks,  which  we  arrive  at  from  their  macroscopic 
study,  are  fully  confirmed  when  we  apply  the  microscope  to  the  ex¬ 
amination  of  thin  sections  prepared  from  them  ;  while  many  new 
and  remarkable  facts  about  their  structure  are  then  revealed. 
The  rocks  are  seen  to  contain  in  many  places  phenocrysts  of  quartz, 
felspar,  and  magnetite,  embedded  in  a  more  or  less  devitrified,  glassy 
base.  These  phenocrysts  exhibit  many  interesting  features,  showing 
that  they  have  been  exposed  to  a  high  temperature.  The  micro- 
pegmatitic  groundmass,  the  last  to  solidify  among  the  constituents 
of  the  original  rock,  has  been  the  first  to  fuse  ;  and  the  liquefaction 
has  often  been  sufficiently  perfect  to  allow  of  the  development  of 
flow-structures  in  the  mass.  It  is  worthy  of  notice,  however,  that 
