366  MESSRS.  A.  HARKER  AND  J.  E.  MARE  ON  THE  [Aug.  1893, 
may  probably  regard  them  as  common  garnet.  The  hornblende- 
mineral  noticed  near  the  same  place  is  perhaps  the  same  as  that 
which  we  referred  to  tremolite  in  the  Upper  Coldwell  Beds  at 
Backhouse  Hill. 
Another  mineral  of  considerable  interest  discovered  by  Mr. 
Hutchings  is  sillimanite,1  which  he  finds  in  highly  metamorphosed 
ashes  belonging  to  the  rhyolitic  group  at  a  distance  of  3  feet  from 
the  granite-junction.  It  is  associated  with  andalusite.  The  silli¬ 
manite  does  not  occur  in  matted  patches  as  at  Knocknairling  Hill,' 
but  in  isolated  prisms. 
IV.  Conditions  attending  the  Formation  of  Secondary 
Felspar-crystals. 
In  our  former  paper  (op.  jam  cit.  p.  297,  pi.  si.  fig.  6)  we 
recorded  an  instance  of  felspar-crystals  occurring,  in  association  with 
brown  mica,  inside  the  vesicles  of  one  of  the  metamorphosed 
andesites.  This  we  regarded  as  an  exceptional  effect  of  the  meta¬ 
morphism,  and  were  obliged  to  leave  unexplained.  We  have  since 
learnt  that  clear  crystals  of  felspar,  both  monoclinic  and  tricliuic, 
occur  within  the  vesicles  of  lavas  which  have  never  been  subjected 
to  such  metamorphosing  influences  as  those  due  to  the  Shap  Granite. 
Mr.  Hutchings  3  has  observed  numerous  cases  in  the  Lake  District, 
and  the  same  thing  occurs  in  the  Cross  Fell  Inlier.4  These  instances 
certainly  seem  to  prove  that,  with  proper  conditions,  felspars  may 
be  formed  inside  vesicles  without  the  co-operation  of  either  high 
temperature  or  exceptional  pressure.  The  case  near  the  Shap 
Granite  may  well  fall  under  the  same  head.  The  purity  of  the 
crystals,  and  the  manner  in  which  they  are  moulded  by  the  brown 
mica  accord  with  the  supposition  that  the  felspar  was  crystallized 
in  the  vesicle  prior  to  the  metamorphism.  The  apparent  rarity  of 
the  phenomenon  is  another  confirmation  of  the  idea,  for  while  a 
striking  regularity  is  observable  in  the  undoubted  effects  of  thermal 
metamorphism  around  the  granite,  the  formation  of  felspar  under 
ordinary  conditions  of  temperature  seems  to  be  quite  capricious. 
The  remarkable  freshness  of  the  Shap  metamorphic  rocks  precludes 
the  third  alternative,  that  the  felspar  was  formed  subsequently  to 
the  metamorphism.  It  may  be  pointed  out  that,  the  available  lime 
within  these  vesicles  being  used  to  make  felspar,  the  coloured  silicate 
produced  in  the  metamorphism  was  a  brown  mica  instead  of  the 
usual  hornblende. 
The  formation  of  felspar  in  rocks  quite  independently  of  the 
special  conditions  usually  connoted  by  the  term  ‘  metamorphism  ’  is 
a  subject  which  deserves  further  investigation.  Veins  largely 
composed  of  clear  crystalline  felspar  fill  cracks  in  some  of  the  Dart¬ 
moor  diabases,0  and  among  acid  rocks  we  have  noticed  a  similar 
1  See  also  Mr.  Barrow’s  paper  in  this  number  of  the  Quarterly  Journal, 
especially  pp.  337-338. 
2  Miss  Gardiner,  Quart.  Journ.  Geol.  Soc.  vol.  xlvi.  (1890)  pp.  575  et  seqq. 
3  Geol.  Mag.  for  1892,  p.  224. 
4  Marker,  Quart.  Journ.  Geol.  Soc.  vol.  xlvii.  (1891)  pp.  514,  515. 
3  Id.  Geol.  Mag.  for  1892,  p.  346. 
