Yol.  49.I  GRANITE  IN  THE  GABBRO  OF  THE  CUILLIN  HILLS.  191 
have  pyrites-crystals,  sometimes  of  considerable  dimensions.  These, 
in  all  probability,  have  been  formed  at  the  expense  of  some  of 
the  magnetite  of  the  rock  and  are  found,  sometimes  attached  to 
comparatively  unaltered  centres  of  the  spherulites,  and  at  other 
times  scattered  promiscuously  through  their  mass.  The  pyrites- 
crystals  often  show  every  stage  of  the  conversion  of  their  substance 
into  limonite.  Backstrom  has  also  found  pyrites  to  be  among  the 
secondary  minerals  developed  in  the  inclusions  of  granite  in  diabase. 
There  are.  moreover,  grounds  for  believing  that  fayalite,  the  iron- 
olivine,  was  a  constituent  of  the  spherulites.  Mr.  Iddings  has 
shown  how  commonly  this  mineral  occurs  in  the  spherulites  of  acid 
rocks.1  In  some  of  the  spherulites  of  the  Cuillin  Hills  there  are 
masses  of  brown  oxide  of  iron,  which  are  evidently  the  product  of 
alteration  of  another  mineral.  In  the  midst  of  these,  particles  may 
sometimes  be  detected  which  under  the  microscope  exhibit  the 
lustre  and  cleavage  of  partially  altered  fayalite,  like  that  of  the 
Mourne  Mountains.  In  spite  of  the  minute  size  of  these  particles, 
they  can  be  extracted  by  a  needle-point,  and,  on  boiling  with  hydro¬ 
chloric  acid,  they  are  seen  to  be  decomposed  with  separation  of 
gelatinous  silica. 
7.  Bearing  of  the  Evidence  afforded  by  these  Inclusions  on 
the  Question  of  the  Belative  Ages  of  the  Igneous  Boces 
in  the  "Western  Isles  of  Scotland. 
There  is  one  important  inference  from  the  phenomena  exhibited 
at  I)ruim-an-Eidhne,  which  is  so  obvious  that  it  would  not  be 
necessary  even  to  call  attention  to  it,  but  for  the  circumstance  that 
opposite  conclusions  have  been  persistently  and  authoritatively  put 
forward.  It  is  perfectly  clear  from  the  study  of  these  inclusions 
that  the  gabbro  of  the  Cuillin  Hills  was  erupted  after  the  granites 
of  the  Bed  Mountains  of  Skye,  and  it  is  further  indisputable  that 
the  acid  rocks  must  have  consolidated  and  acquired  their  existing 
features,  including  the  remarkable  micropegmatitic  structure,  before 
the  gabbros  were  forced  through  them  and  caught  up  these  4  horses  ’ 
of  granite. 
As  long  ago  as  1S19,  Macculloch  discussed  the  relative  ages  of 
these  two  series  of  igneous  rocks,  and,  although  he  hesitates  about 
giving  a  final  and  definite  opinion  on  the  subject,  it  is  clear  that  he 
leans  towards  the  view  that  the  granites  are  the  older  rocks  of  the 
two.2 * 
In  1816  Principal  J.  D.  Eorbes  made  a  very  careful  study  of 
1  ‘  On  the  Occurrence  of  Fayalite  in  the  Lithophyses  of  Obsidian  and  Rhyolite 
in  the  Yellowstone  National  Park,’  Am.  Journ.  Sci.  ser.  3,  vol.  xxx.  (1885) 
p.  58 ;  see  also  Iddings  and  Penfield,  ‘  Fayalite  in  the  Obsidian  of  Lipari,’  ibid. 
toI.  xl.  (1890)  p.  75  ;  ‘  The  Minerals  in  Hollow  Spherulites  of  Rhyolite  from 
Glade  Creek,  Wyoming/  ibid.  vol.  xlii.  (1891)  p.  39. 
2  ‘A  Description  of  the  Western  Islands  of  Scotland,’  1819,  vol.  i.  pp.  362- 
393.  See  the  order  adopted  in  the  legend  to  the  Map  of  Skye.  vol.  iii.,  and  also 
the  legend  to  his  general  Map  of  Scotland  published  in  1832,  where  ‘Diallage 
Rock  ’  (gabbro)  is  placed  last  of  all  the  formations  except  ALluvium. 
