VoL  49-]  LIECT.-GEN.  C.  A.  l£ci£ASON - NOTES  ON  DAETltOOE,  393 
may  be  seen.  Masses  of  porphyritic  rock  containing  rectangular 
crystals  of  felspar,  from  to  3  inches  in  length,  are  included  in 
the  fine-grained  variety.  They  have  not  the  appearance  of  blocks 
of  a  coarse-grained  granite  included  in  another  eruptive  roc-k,  but 
look  like  aggregations  of  porphyritic  crystals  in  a  fine-grained  non- 
porphyritic  base.  The  whole  suggests  the  idea  of  an  imperfectly- 
stirred  plum-pudding,  in  which  the  plums  have  got  together  in  a  lump. 
AVe  have  here,  I  take  it,  evidence  of  the  imperfect  mixing  of  two 
portions  of  the  granitic  magma  in  different  conditions  of  fluidity. 
Students  of  quartz-porphyries,  and  similar  rocks,  are  well  aware 
that  when  a  relief  of  pressure  takes  place,  and  a  partially  crystal¬ 
lized  deep-seated  rock  is  moved  towards  the  surface,  a  partial 
remelting  of  the  already-formed  crystals  takes  place.  The  relief  of 
pressure 1  in  this  case  is  believed  to  give  increased  potency  to  the 
solvent  action  of  the  heated  liquid  in  which  the  crystals  are  sus¬ 
pended.  It  seems  to  me  that  a  similar  result  would  be  produced 
if  the  pressure  remained  constant  and  the  heat  were  locally  increased. 
And  the  explanation  I  would  suggest  is  that,  as  the  partially  crystal¬ 
lized  granite  was  moved  upwards,  the  traction  and  friction  against  the 
sides  of  the  vent  broke  up  the  larger  crystals  and  increased  the  heat,  and 
consequent  fluidity,  of  the  marginal  portions  of  the  mass,  so  that  we 
have  a  margin  of  fine-grained  granite  around  the  normal  porphyritic 
rock,  and  an  imperfect  blending  of  the  two  along  the  line  of  junction. 
The  loss  of  heat  during  cooling  would  have  been  more  rapid  along 
the  margin  than  in  the  central  portions,  and  although  it  evidently 
was  sufficiently  slow  to  enable  both  portions  to  set  up  a  holo- 
crystalline  structure,  it  was  not  slow  enough  to  enable  the  marginal 
portion  to  develop  porphyritic  crystals. 
The  explanation  offered  above  was  formed  in  the  field  in  sight  of 
the  rocks,  and  has  been  confirmed  by  an  examination  of  my  speci¬ 
mens  under  the  microscope.  It  is  evident  that  a  strong  solvent 
action  has  been  set  up  by  the  acid  magma  on  the  crystals  of  first 
consolidation.  The  biotites  have  been  eaten  into  internally  and 
around  their  edges.  They  suggest  the  idea  of  slices  of  cheese  that 
have  been  half  nibbled  away  by  mice.  Similarly  the  felspars  are 
eaten  into,  and  riddled  with  granules  of  quartz.  Some  are  dappled 
over  with  numerous  microscopic  crystals  of  a  colourless  mica,  such 
as  one  often  sees  in  quartz-porphyries  and  granites.  There  is  nothing 
whatever  to  suggest  that  any  of  the  above  peculiarities  have  been 
1  By  the  operation  of  Thomson’s  Law,  which  is  to  the  effect  that  in  ordinary 
cases  an  increase  of  pressure  raises,  and  a  decrease  of  pressure  lowers  the 
melting-point.  See  Teall’s  ‘  Brit.  Petrography,’  pp.  407,  408,  and  Lagorio,  Min. 
Mag.  yoL  vii.  (1887)  pp.  224,  225.  Pressure  increases  the  action  of  solvents  in 
some  cases  by  increasing  the  heat.  It  also,  independently  of  a  rise  in  tempera¬ 
ture,  facilitates  melting  and  solution  in  cases  in  which  substances  on  melting 
(as  ice),  or  on  solution  (as  sodium  chloride),  contract  in  volume  (£  Solutions,’ 
by  Ostwald).  In  cases  in  which  melting,  or  solution,  involves  an  increase  of 
volume,  pressure  retards  the  change  of  state,  or,  in  other  words,  raises  the 
melting-point.  The  subject  is  a  complex  one,  but  the  corrosion  of  porphyritic 
crystals  formed  under  plutonic  conditions,  on  the  magma  being  moved  towards 
the  surface,  seems  sufficiently  accounted  for  by  the  relief  of  pressure  lowering 
the  melting-point. 
Q.  J.  G.  S.  No.  195.  2  d 
