Yol.  49.] 
HORNBLENDE  FROM  THE  BTNNENTHAL. 
109 
were  embedded  in  one  irregularly  defined  grain  of  some  mineral. 
Further  examination  shows  that  in  one  of  the  more  definite  instances 
this  intervening  material  exhibits  an  oscillatory  twinning,  one  set 
of  bands  extinguishing  at  2^°,  the  other  at  271°  with  the  com¬ 
bination-plane,  which  indicates  one  of  the  more  basic  felspars. 
Two  hypotheses  may  be  advanced  in  explanation  of  this  structure. 
One,  the  partial  dissociation  of  an  original  felspar-grain  into 
granular  quartz  and  white  mica,  as  described  in  some  cases  from  near 
Sudbury  (Canada)  1 ;  the  other,  the  recombination  of  felspar  granules 
in  a  mixture  (due  to  crushing)  of  quartz,  felspar,  opacite,  etc.  The 
latter  seems  to  me  more  probable,  for  the  amount  of  white  mica  is 
much  smaller  than  it  should  be  on  the  former  hypothesis,  and 
in  parts  of  the  felspar  which  are  most  free  from  quartz  a  granular 
structure  can  be  detected,  sometimes  by  shadowy  outlines,  visible 
only  in  certain  positions  with  crossed  nicols,  sometimes  by  the 
presence  of  opacite,  as  if  in  a  pencil  sketch  of  the  structure  the 
lines  had  been  made  with  dots.  The  larger  flakes  of  brown  mica 
are  both  smaller  in  size  and  more  irregular  in  outline  than  in  the 
specimen  already  described,  but  here  also,  once  or  twice,  the 
stratulse  of  opacite  are  carried  through  them  unbroken. 
The  hornblende  is  the  most  noteworthy  mineral  in  this  specimen. 
The  outline  of  the  crystals,  as  we  might  expect,  is  very  ragged,  but  the 
usual  cleavage  is  sometimes  distinct.  The  colour  is  olive-green,  the 
polychroism  fairly  strong :  for  vibrations  parallel  with  a,  pale 
greenish  straw-colour  ;  with  b,  strong  sap-green  ;  c,  indigo  green. 
The  mineral  contains,  like  the  St.  Gfothard  actinolite,  not  infrequent 
granules  of  quartz ;  possibly  also  some  of  felspar.  The  stratulse 
of  opacite  pass  through  the  crystals  practically  unaffected;  even  the 
ivaves  and  sharp  flexures  are  preserved?  In  one  case  (see  fig,  -2,  p.  108) 
a  crystal  of  hornblende  full  half  an  inch  long3  has  formed  without 
in  any  way  disturbing  this  structure ;  biotite  is  occasionally  en¬ 
closed  by,  and  not  seldom  fringes,  these  crystals  of  hornblende. 
On  a  closer  examination  of  the  biotite  and  its  mode  of  occurrence, 
we  notice  that  the  flakes  often  border  the  hornblende,  and  lie  within 
a  line  which  seems  to  be  its  natural  boundary  as  if  they  cut 
into  the  mineral :  groups  of  them  occasionally  occupying  deep 
bays,  as  they  may  be  called,  in  the  grain.  The  most  simple 
explanation  of  this  peculiar  association  is  that  portions  of  an  original 
crystalline  grain  of  hornblende  have  been  replaced  by  biotite,4  just 
as  small  grains  of  the  former  mineral  replace  part  of  a  large  grain 
1  Quart.  Journ.  Gfeol.  Soc.  vol.  xliv.  (1888)  p.  36. 
2  It  would  have  been  easy  to  find  instances  where  the  flexures  are  much 
more  pronounced  than  in  fig.  2,  but  in  some  other  respects  this  crystal  is  more 
instructive. 
3  It  goes  right  across  the  slice  and  is  broken  off  by  the  edges,  which  are 
defined  by  cleavage-foliation. 
4  This,  however,  would  imply  the  breaking  up  of  some  felspar,  because  even 
if  the  hornblende  originally  contained  alumina,  more  must  be  added,  and  potash 
obtained  (if  not  wholly,  in  great  part)  from  such  a  mineral  as  orthoclase;  silica 
would  then  be  liberated,  and  lime  must  be  removed.  In  some  cases  the  last 
might  be  used  to  form  epidote  or  saussurite,  in  some  a  carbonate,  which  might 
or  might  not  pass  away  in  solution.  ■ 
