Yol.  49.]  LIEUT. -GEN.  C.  A.  McMAHON — NOTES  ON  LAET3IOOE.  391 
In  speaking  of  the  Dartmoor  Granite  I  have  used  the  word 
‘  fusion,’  because  that  is  the  word  employed  by  Mr.  Ussher ;  but  I 
may  say  that  an  examination  of  thin  sections  of  this  rock  under  the 
microscope  leaves  no  doubt  in  my  mind  that  this  rock  was  once  in 
as  heated  a  condition  as  any  typical  granite.  I  am  free  to  admit 
that  the  presence  of  liquid  cavities  containing  bubbles,  especially 
when  the  bubbles  are  small  relatively  to  the  area  of  the  cavities, 
does  not  necessarily  imply  conditions  of  great  heat ;  I  also  admit 
that  liquid  cavities  are  sometimes  the  products  of  secondary  pro¬ 
cesses  ;  but  at  the  same  time  it  is  an  undoubted  fact  that  liquid  and 
gas  cavities  are  very  characteristic  of  certain  original  minerals,  found 
in  granites  under  conditions  that  show  them  to  be  congenital.  Dor 
instance,  in  a  granite-dyke  in  the  Himalayas  that  cuts  across  all  the 
other  rocks,  I  found  large  crystals  of  beryl  embedded  in  all  the 
other  constituents  of  the  granite.  These  minerals,  on  being  sliced, 
proved  to  be  extraordinarily  full  of  liquid  and  gas  inclusions.  Yow 
the  quartz  of  the  Dartmoor  Granite  is  not  only  full  of  liquid  and 
gas  cavities,  but  the  size  of  the  bubbles,  and  the  size  of  the  mineral 
deposits  contained  in  the  cavities  along  with  the  bubbles,  shows 
that  the  liquid  must  have  possessed  a  high  solvent  power  when  it 
was  shut  up  in  the  quartz,  compared  with  what  it  possesses  now. 
For  instance,  when  you  see  a  deposited  mineral  and  a  bubble  taking 
up  together  about  two-thirds  of  the  area  of  the  cavity  (in  some 
cases  those  found  in  the  Dartmoor  Granite  take  up  even  more  room), 
the  natural  inference  is  that  the  liquid  was  in  a  superheated  condi¬ 
tion,  and  had  more  than  ordinary  capacity  for  holding  mineral 
substances,  or  salts,  in  solution.  The  same  inference  seems  inevit¬ 
able  when  you  see,  as  you  may  see  in  these  granites,  a  small  body 
of  liquid  and  a  large  body  of  gas  together  in  the  same  cavity.  One 
cavity  that  I  have  before  me  as  I  write  contains  a  large  bubble  of 
gas  (it  is  not  air),  a  mineral  in  the  form  of  a  cube,  and  a  liquid  with 
the  appearance  of  water  containing  a  small  bubble  which  is  certainly 
not  filled  with  the  gas  which  fills  the  large  bubble.  When  the 
cavity  was  charged  with  its  contents  it  is  clear  that  the  heat  was 
intense  enough  to  cause  these  solid,  liquid,  and  gaseous  substances  to 
intimately  blend  together.  These  facts,  and  others  might  be  cited, 
seem  to  point  to  the  existence  of  great  heat  when  the  granite  was 
in  a  plastic  condition.  In  quoting  the  word  4  fusion,’  therefore, 
from  Mr.  Dssher’s  memoir,  I  am  not  catching  at  what  some  may 
consider  an  unguarded  expression,  but  I  do  it  because  I  think  this 
word  (used  in  a  somewhat  popular  way)  sufficiently  indicates  the 
original  condition  of  the  Dartmoor  Granite. 
I  may  note  here  another  point  which  seems  to  me  worthy  of 
attention.  I  have. not  been  able  to  discover  any  ground  for  suppos¬ 
ing  that  the  epidiorites  of  the  Dartmoor  area  were  erupted  after  the 
consolidation  of  the  granite.  These  epidiorites  are  altered  dolerites. 
That  is  the  verdict  of  previous  observers,  and  a  microscopical 
examination  of  a  good  selection  which  I  made  in  the  field  enables 
me  to  confirm  this  view.  The  alteration  has  been  attributed — 
and,  I  think,  rightly  attributed — to  contact-metamorphism.  These 
