THE  CUILLIN  HILLS.  [May  1893. 
180  TEBTIAEY  GEANITE  IN  GABBKO  OF 
re-fused,  a  conclusion  which  has  also  been  arrived  at  by  Mbit-man 
Cross,1  Backstrom,2  and  other  authors. 
In  the  Cuillin  Hills,  however,  the  phenomena  are  exhibited  on  a 
far  grander  scale  than  in  Saxony  or  in  Arran.  The  enclosures  are 
of  much  larger  dimensions,  and  the  mass  of  molten  basic  rock  m 
which  they  have  been  enveloped  is  of  much  grander  proportions, 
while  the  period  during  which  it  has  retained  its  high  temperature 
must  have  been  far  longer— as  is  shown  by  the  highly  crystalline 
character  of  the  gabbro. 
3.  The  Geanites  and  Gabbeos  of  the  Cuillin  Hills. 
The  Tertiarv  granitic  rocks  of  Skye,  Mull,  Hum,  and  St.  Ivilda, 
with  their  southern  developments  in  Arran  and  the  Mourne  Moun¬ 
tains,  were  carefully  described  by  Zirkel  m  1871,  under  the  names 
of  ‘  quartz-syenite  ’  and  ‘quartz-porphyry,5  and  m  18,4  I  gave 
some  further  account  of  their  nature  and  relations  referring  them 
to  granites  and  quartz-felsites.4  More  recently,  in  1888,  Mr.  lea 
referred  to  one  of  the  most  widely-distributed  types  of  the  rock, 
his  notice  of  it  being  illustrated  by  an  excellent  figure  and  a  full 
petrographical  description ; 5  in  the  same  year,  too,  Sir  Archibald 
Geikie  and  Dr.  Hatch  contributed  interesting  details  concerning 
many  of  the  varieties  assumed  by  these  rocks.  . , 
As  I  hope  shortly  to  lay  before  this  Society  an  account  of  the 
whole  series  of  acid  rocks  of  Tertiary  age  as  developed  m  Scotland, 
the  study  of  which  has  occupied  much  of  my  time  and  attention 
during  the  last  twenty  years,  I  will  content  myself  here  with  a 
short  summary  of  their  principal  characters. 
In  some  of  the  more  central  parts  of  their  mass,  these  rocks  pre¬ 
sent  themselves  as  true  biotit  e-granites,  but  there  is  a  con  stant 
tendency  for  the  hiotite  to  be  replaced  wholly  or  m  part  by  horn¬ 
blende  When,  as  not  unfrequently  happens,  the  quantity  ot  pla- 
gioclase  increases  in  amount,  the  biotite-  or  hornblende-granite 
passes  into  a  common  granitite  ora  hornblende-gramtite.  lowards 
the  peripheral  portions  of  the  intrusive  masses,  however,  a  very 
marked  change  takes  place  in  the  characters  of'  the  rock.  -  icro 
pegmatitic  (‘  granophyric  ’)  intergrowths  of  felspar  and  quartz  make 
their  appearance  between  the  phenocrysts  of  the  rock,  the  mica 
and  hornblende  are  gradually  replaced  by  augite,  magnetite  becomes 
a  prominent  constituent  of  the  rock,  and  a,  remarkable  drusy 
(‘  miarolitic  5)  structure  is  developed  in  it.  It  is  this  variety  e 
rock,  which  is  indeed  the  most  widely  distributed,  that  has  been 
called  by  Mr.  Teall  and  Sir  Archibald  Geikie  (following  the  nomen- 
1  Bull.  PLiil.  Soc.  Washington,  vol.  xi.  (1891)  p.  433. 
2  Op.  supra  cit. 
Geol 
Geologische  Skizzen  von  der  Westkiiste  Sckot Hands,’  Zeitschr.  d.  Deutsch. 
..  Gesellsch.  vol.  xxiii.  (1871)  pp.  1-124. 
Quart.  Journ.  Geol.  Soc.  vol.  xxx.  (18,4)  pp.  234-_3b. 
‘British  Petrography,’  1888,  p.  327,  pi.  xxxiii.  fig.  1. 
Trans.  Bov.  Soc.  Edinb.  vol.  xxxv.  (1888)  p.  148. 
