369 
Yol.  49.]  MET  AMORPHIC  ROCKS  AROUND  THE  SHAP  GRANITE. 
different  chemical  substances,  and  we  must  also  expect  to  find  it 
greater  at  higher  temperatures  ;  but  we  may  at  least  arrive  at  some 
idea  of  its  order  of  magnitude  by  considering  some  one  case.  The 
case  which  is  most  easily  discussed  is  the  production  of  lime-silicates 
at  the  expense  of  calcite.  The  carbonate  is  decomposed,  as  we  have 
remarked,  only  in  presence  of  silica  or  of  some  silicate  from  which 
silica  can  be  disengaged,  and,  given  this  condition,  the  process  is 
one  that  can  be  effected  at  a  comparatively  low  temperature.  Now, 
in  the  amygdaloidal  basic  lavas  we  have  pointed  out  that  the  calcite 
in  the  centre  of  the  largest  vesicles  is  never  destroyed,  although  it 
has  recrystallized  daring  the  metamorphism,  and  free  quartz  has 
recrystallized  in  the  surrounding  rock  and  in  the  marginal  parts  of 
the  same  vesicles.  Between  the  residual  calcite  and  the  quartz 
various  lime-bearing  silicates  are  interposed.  The  calcite  is  preserved 
only  in  the  centre  of  the  largest  vesicles,  smaller  ones  in  the  same 
rock  being  occupied  by  silicate  minerals,  quartz,  etc.  In  the  examples 
we  have  more  particularly  studied  calcite  is  never  preserved  within 
less  than  5  inch  of  the  boundary  of  the  vesicle,  and  this  gives  a  first 
approximation  to  the  distance  we  are  seeking.  It  is,  however,  a 
maximum  estimate,  and  is  doubtless  too  great,  for  the  vesicles  before 
metamorphism  were  not  always  occupied  by  calcite  alone.  If  we 
measure  the  distance  between  calcite  and  quartz  which  have  re¬ 
crystallized  within  the  same  vesicle,  we  shall  reduce  our  estimate 
considerably :  perhaps  inch  would  be  nearer  the  mark  for  the 
distance  to  which  the  interchange  of  lime  and  silica  has  demonstrably 
taken  place  in  the  production  of  lime-silicates.  Similar  evidence  is 
furnished  by  certain  flat  calcareous  nodules  noted  above  in  the 
metamorphosed  Silurians  of  Wasdale  Beck,  belonging  probably  to  a 
faulted  lentiele  of  Stockdale  Shales  ( Phacops  glaber- zone).  Here 
the  calcite  often  remains  in  the  centre  of  a  nodule,  the  marginal 
part  being  always  converted  into  an  aggregate  of  pyroxene  and  other 
silicates.  This  border  is  often  |  inch  thick ;  but  if,  to  allow  for 
impurities  in  the  nodules,  we  take  the  least  thickness,  we  obtain  as 
before  about  or  ^  inch.  The  distance  from  the  granite — and 
so  presumably  the  temperature  of  metamorphism — is  not  very 
different  from  that  for  the  former  locality. 
The  same  question  might  be  approached  from  another  side,  by 
examining  the  size  of  the  individual  crystals  or  groups  of  meta- 
morphic  minerals  where  their  production  has  been  dependent  on  the 
heterogeneous  nature  of  the  rocks  metamorphosed.  The  clusters  of 
brown  mica  produced  in  some  of  the  ashes  around  grains  of  original 
magnetite  are  an  example.  Here,  however,  we  have  to  exclude 
such  minerals  as  contain  numerous  inclusions,  either  of  other  meta- 
morphic  products  or  of  unchanged  material.  It  is  not  easy  to  arrive 
at  any  quantitative  results,  but  the  impression  gained  by  a  study  of 
specimens  is  in  general  accord  with  the  foregoing  conclusions. 
We  may  add  that  the  dependence  of  the  range  of  transference  of 
material  upon  the  temperature  attained  during  the  metamorphism  is 
well  illustrated  in  the  calcareous  ashes  of  the  Shap  distinct.  Oo  the 
outer  edge  of  the  metamorphic  aureole  only  the  most  finely  divided 
calcite  has  been  decomposed. 
