Yol.  49.]  GRANITE  IN  THE  GABERO  OF  THE  CTJTLLIN  HILLS.  185 
Some  of  the  quartz-crystals  are  surrounded  "by  radiating  filaments 
of  brown  oxide  of  iron  (‘ferrite’)  which  greatly  resemble  the 
products  of  alteration  of  the  pyroxene,  and  it  seems  very  probable 
that  they  indicate  the  former  existence  of  zones  of  augite  and 
magnetite,  like  those  which  surround  the  quartzes  in  the  ‘quartz- 
basalts.’  In  some  cases  I  have  found  distinct  evidence  of  the  for¬ 
mation  of  zones  of  secondary  quartz  around  the  corroded  fragments 
of  the  mineral.  This  is  precisely  similar  to  the  pyrogen-quarz 
which  Lehmann  1  and  Chrustschoff 2  have  shown  to  be  so  charac¬ 
teristic  of  inclusions  (see  PI.  III.  fig.  3).  It  is,  of  course,  not 
necessary  to  suppose  that  the  secondary  quartz  has  been  produced 
by  direct  fusion,  as  the  evidences  of  the  presence  and  action  of 
water  are  sufficiently  obvious  in  these  rocks. 
The  felspars  of  these  included  masses  are  almost  always  kaoli- 
nized  and  more  or  less  opaque,  as  indeed  is  the  case  in  the  original 
rock.  The  most  conspicuous  feature  about  them  is  the  occurrence  of 
cracks,  parallel  to  the  orthopinacoid  (010),  to  the  basal  plane  (001), 
and  perhaps  also  to  the  clinopinacoid  (100),  which  have  opened  and 
been  filled  with  a  secondary  deposit  of  felspar  (see  PI.  III.  fig.  4). 
A  similar  rim  of  secondary  felspar  also  surrounds  the  whole  crystal 
in  some  cases,  the  secondary  felspar  being  in  approximate,  but 
seldom  in  complete,  optical  continuity  with  the  original  and  altered 
material.  Occasionally  the  whole  crystal  is  broken  up  into  a  mul¬ 
titude  of  dice-like  fragments,  and  these  are  in  some  cases  displaced, 
the  circumstance  being  revealed  by  the  mass  behaving  like  a  mosaic 
in  polarized  light.  These  altered  felspars  with  their  infilled  *  con¬ 
traction-rifts  ’  offer  a  close  analogy  with  the  ‘  perthites,’  but  there 
does  not  appear  to  be  a  very  marked  difference  in  character  between 
the  original  and  altered  portions  and  the  new  and  infilling  veins 
which  traverse  the  crystal.  Backstrom  and  other  authors  have  fre¬ 
quently  described  a  similar  alteration  of  the  felspars  in  inclusions. 
The  large  ‘  stone  cavities  ’  which  occasionally  occupy  considerable 
spaces  in  the  original  crystals  have  sometimes  been  fused  and 
partially  or  wholly  devitrified.  In  some  cases  the  whole  of  the 
centre  of  the  crystal  has  been  honeycombed  by  the  surrounding 
matrix,  and  in  these  cases  the  mixture  of  felspar  and  glassy 
material  has  easily  fused  and  then  become  devitrified.  It  is  in 
this  way  that  the  interesting  appearances  illustrated  in  PI.  III. 
fig.  5  have  evidently  been  produced.  The  felspars,  like  the 
quartzes,  are  sometimes  surrounded  with  radiating  trichites,  now 
converted  into  limonite  (PI.  III.  fig.  6). 
With  respect  to  the  optical  properties  of  the  felspars,  I  have  made 
many  attempts  to  determine  the  position  of  the  optic-axial  plane, 
but,  owing  to  the  opacity  of  the  crystals,  have  not  succeeded  in 
obtaining  trustworthy  results. 
The  ferro-magnesian  silicate  in  this  particular  variety  of  the 
1  Yerh.  des  Aaturh.  Yer.  d.  preuss.  Kheinl.  u.  Westf.  1877,  p.  203. 
2  Tschermak's  Min.  u.  Petr.  Mittheil.  1885,  p.  295. 
