Yol.  49.]  METAUORPHIC  HOCKS  AROEND  THE  SHAP  GRANITE.  363 
often  present  one  or  more  shells  of  compact  appearance  and  pale 
colour,  while  the  interior  is  occupied  by  a  confused  crystalline  ag¬ 
gregate  of  dark  hornblende  with  an  occasional  patch  of  pyrites.  In 
slices  the  paler  portions  are  found  to  consist  chiefly  of  quartz  and 
epidote,  with  a  little  actinolite  or  hornblende,  most  of  the  horn¬ 
blende  occurring  in  the. dense  patches  of  imperfect  crystals  which 
are  so  conspicuous  to  the  naked  eye.  The  augite  already  mentioned 
also  occurs.  But  the  most  interesting  feature  is  the  preservation  in 
the  centre  of  these  large  vesicles  of  some  remnants  of  calcite,  a 
mineral  never  left  undestroyed  in  the  smaller  ones  or  in  the  body 
of  the  rock.  The  calcite  is  in  crystalline  patches  moulding  or  en¬ 
closing  the  other  minerals  mentioned,  and,  in  particular,  it  is  often 
penetrated  by  fringes  and  brushes  of  slender  needles  of  actinolite 
[1614J.  From  this  it  is  evident  that  the  carbonate,  though  not  de¬ 
composed,  has  been  recrystallized  at  the  time  of  the  metamorphism. 
The  most  interesting  specimens  come  from  a  large  block  which 
was  not  found  in  place,  but  must  belong  to  a  locality  very  near  the 
last  mentioned.  This  rock,  besides  smaller  vesicles,  has  others  of 
unusually  large  size,  often  2  inches  in  diameter,  and  the  meta¬ 
morphism  of  their  contents  has  given  rise  to  some  curious  features. 
Hand-specimens  show  the  usual  quartz,  epidote,  and  green  horn¬ 
blende,  the  last  less  abundant  than  is  ordinarily  the  case,  with 
occasional  specks  of  pyrites  and  some  residual  calcite  ;  but  the  most 
prominent  mineral  in  all  the  largest  vesicles  is  a  deep-brown  garnet 
in  crowded  groups  of  small  crystals,  or  in  larger  crystals  sometimes 
|  inch  in  diameter.  The  colour  and  lustre  are  those  proper  to  the 
lime-iron-alumina  garnets.  The  faces  of  the  rhombic  dodecahedron 
are  often  well  developed,  especially  when  the  mineral  is  moulded  by 
calcite,  and  the  broken  crystals  show  the  concentric  shell-structure 
which  is  so  frequent  a  character  in  garnets  produced  in  therm o- 
metamorphism. 
In  thin  slices  [1747-1752]  the  garnets  vary  in  colour  from  a 
rather  light  brown  to  a  very  pale  tint.  Zones  of  growth  are  often 
indicated,  either  by  differences  of  colour  in  successive  layers  or  by 
the  accumulation  of  impurities  along  the  surfaces  which  divide 
them,  and  in  this  way  the  dodecahedral  form  may  be  seen  even 
when  the  crystals  are  so  closely  clustered  as  to  prevent  the  develop¬ 
ment  of  bounding-faces  (see  Plate  XVII.  fig.  3).  The  recognizable 
inclusions  are  calcite,  epidote,  quartz,  hornblende,  augite,  and 
sphene,  but  tkese  are  only  wedged  in  between  garnet-crystals,  not 
really  enclosed.  Most  of  the  garnets  examined  are  strictly  isotropic, 
or  give  but  very  indistinct  glimpses  of  illumination  when  rotated 
between  crossed  nicols.  Some,  however,  show  marked  double 
refraction  [1749],  the  phenomena  being  very  similar  to  those 
described  in  the  idocrase-garnet  rock  of  Wasdale  Head  (op.  jam  cit. 
p.  312).  The  polysynthetic  twinning  on  the  ‘  rhombic-dodeca¬ 
hedron  type’  of  Klein,1  and  the  numerous  concentric  zones  differing 
in  birefringence,  are  well  exhibited.  Here,  however,  the  doubly- 
refracting  crystals  are  found  in  juxtaposition  with  others  of  closely 
1  ‘  Optische  Studien  am  Granat,’  Neues  Jalii’b.  1883,  vol.  i.  pp.  -87  et  seqq. 
2  B  2 
