Yol.  49.]  METAMORPHIC  ROCKS  AROTJND  THE  SHAP  GRANITE.  367 
occurrence  in  some  old  rhyolites  and  ashes  from  Malvern,  collected 
by  Mr.  J.  F.  Bryant.  If  such  processes  are  possible  within  the 
mass  of  a  rock,  as  well  as  in  vesicles  and  fissures,  some  peculiarities 
which  have  been  ascribed  to  dynamic  metamorphism  may  be  found 
to  admit  of  a  different  explanation.  The  rock  described  by  one  of 
us  from  Wyth waite  Top  in  the  Cross  Pell  Inlier  is  a  case  in  point.1 
Y.  The  Metamorphosed  Coniston  Limestone  Beds  oe 
AYasdale  Head. 
In  describing  the  highly  metamorphosed  members  of  the  Coniston 
Limestone  group  at  Was  dale  Head  we  showed  that,  although  pure 
lime-silicates,  such  as  wollastonite,  are  not  wanting,  the  more 
common  minerals  are  augite,  tremolite,  etc.,  and  an  analysis  yielded 
a  noteworthy  amount  of  magnesia.  Since  the  supposition  that  this 
or  any  other  constituent  was  introduced  during  the  metamorphism 
was  opposed  to  all  our  results,  we  suggested  that  the  rocks  had  been 
partially  dolomitized  before  the  intrusion  of  the  granite  {op.  jam  cit. 
p.  315).  To  put  this  to  the  test,  we  have  re-examined  the  non-meta- 
morphosed  rocks  of  Blea  Beck.  In  the  case  of  the  more  ashy  beds 
it  is  not  easy  to  form  any  conclusion,  but  the  purer  strata  exposed 
in  the  beck  give  clear  evidence  of  partial  conversion  to  dolomite. 
One  bed  in  the  Upper  Coniston  Limestone,  for  example,  consists  of 
a  pale  brown,  finely  granular  matrix  enclosing  pure  white  crystalline 
patches  up  to  |  inch  in  diameter.  The  specific  gravity  2*796  indicates 
the  presence  of  both  calcite  and  dolomite  in  the  rock,  and  (allowing 
for  the  few  quartz-grains  which  are  the  chief  impurity)  the  two  car¬ 
bonate  minerals  must  he  present  in  roughly  equal  quantities.  The 
white  patches  are  unaffected  by  dilute  acid,  and  consist  of  clustered 
rhombohedra  of  unusually  pure  dolomite  [1616].  The  greyish-brown, 
finely  granular  portion  of  the  rock  effervesces  with  dilute  acid,  and 
must  be  chiefly  calcite  :  but,  as  it  makes  up  quite  of  the- total  bulk, 
it  would  appear  that  there  is  some  admixture  of  dolomite  in  it. 
The  various  types  of  lime-silicate  rocks  which  we  have  described 
at  Wasdale  Head  all  belonged  to  the  more  impure  portions  of  the 
Coniston  Limestone  group,  representing  the  metamorphism  of  cal¬ 
careous  ashes  and  shales  rather  than  of  true  limestones  or  dolomites. 
Further  search,  however,  shows  that  the  latter  are  not  entirely 
wanting  at  that  locality,  and  that  they  present  an  instructive  contrast 
with  the  less  purely  calcareous  members  of  the  group.  They  illustrate 
what  seems  to  be  a  general  law,  that  the  carbonates  are  not  decom¬ 
posed  in  thermal  metamorphism  unless  silica  in  some  available  form 
be  at  hand  to  replace  the  carbonic  acid.  A  bed  forming  part  of  the 
Lower  Coniston  Limestone,  and  evidently  representing  one  of  the 
purer  layers  in  that  formation,  is  converted  into  what  is  essentially 
a  grey  saccharoidal  marble,  the  calcite  having  apparently,  for  the 
most  part,  recrystallized  without  further  change.  This  is  within  100 
yards  of  the  granite-junction  and  close  to  the  rocks  formerly  described, 
which  have  been  converted  into  aggregates  of  idocrase,  garnet,  wol¬ 
lastonite,  etc.,  with  total  elimination  of  their  carbonic  acid.  Further, 
1  Harker,  Quart.  Journ.  Greol.  Soc.  yoI.  xlvii.  (1891)  pp.  515,  516. 
