110 
PROF.  T.  G.  BONNET  ON  BIOT1TE  AND 
[Feb.  1893, 
of  augite.  But  by  this  change  the  stratulte  of  opacite,  whether 
straight  or  bent,  are  not  affected,  except  that  possibly  a  little  of  the 
most  minute  dust  disappears. 
That  this  rock  was  originally  sedimentary,  but  afterwards  became 
crystalline,  seems  highly  probable  ;  that  it  was  theu  crushed  seems 
certain,  and  that  afterwards  these  large  crystals  of  hornblende, 
the  tiakes  of  mica  (in  some  cases  at  their  expense),  and  some 
imperfect  grains  of  felspar  were  developed  seems  the  most  natural 
inference.  If  this  be  so,  those  molecules  at  any  rate  which  have 
built  up  the  hornblende  must  have  travelled  about  the  rock  for 
some  distance,  because  even  if  we  suppose  that  an  elongated  crystal 
existed  at  an  earlier  stage,  it  would  be  more  or  less  crushed  down  if 
a  statical  pressure  acted  in  the  direction  of  its  length,  and  it  would  be 
‘  dragged  out  ’  and  more  or  less  distributed  in  a  transverse  direction 
if  affected  by  a  shearing  force,1  so  that  iu  either  case  a  considerable 
displacement  of  some  of  the  molecules  is  necessary  to  bring  the 
crystal  back  into  a  position  at  right  angles  to  the  cleavage-foliation. 
The  process  of  crystal-building  described  above  has  evidently  left 
the  structure  of  the  rock  practically  undisturbed,  and  resembles 
some  forms  of  pseudomorphism.  Hence  the  direct  agent  was 
probably  water.  As  an  hypothesis — but  nothing  more — 1  will 
venture  on  the  following  sketch  of  the  process.  After  the  rock  had 
been  crushed  up,  much  of  the  material  thus  produced  (in  the  first 
case)  corresponded  in  composition  very  nearly  with  biotite.  A  here 
this  correspondence  was  most  close,  crystal-building  began  in  the 
presence  of  water.  A  nucleus  once  formed,  further  constituents 
would  be  derived  from  the  water  itself,  which,  when  the  pressure 
■was  greater,  would  have  dissolved  the  finer  dust,  and  would  still 
continue  to  act  on  some  of  the  materials.  Thus  the  crystal  would 
continue  to  grow,  developing  the  more  easily  because  the  matrix 
was  so  nearly  of  the  right  composition.  This  would  explain  the 
peculiar  development;  of  the  biotite  described  above,  in  parts  of 
which  the  border  of  the  crystal  can  hardly  be  said  to  be  defined, 
and  the  process  of  development  seems  to  have  failed  rather  than  to 
have  been  arrested.  Quiet  growth  is  indicated  even  more  strongly 
b}’  the  large  actinolitic  crystals  in  the  second  case.  Here  we  may 
suppose  that  there  was  a  slight  excess  of  magnesia  and  a  defect  of 
alkali;  so  hornblende  began  to  form  instead  of  biotite,  and  con¬ 
tinued  to  grow  as  before  until  the  conditions  became  more  favourable 
to  the  production  of  the  latter  mineral,  probably  owing  to  the  water 
becoming  more  alkaline,  when  it  was  at  last  even  able  to  attack  some 
of  the  hornblende  already  formed.  Bocks  very  often  illustrate  the 
old  proverb,  4  robbing  Peter  to  pay  Paul.’ 
I  have  described  these  two  rocks  at  considerable  length  because, 
so  far  as  my  experience  goes,  molecular  movements  on  so  large  a 
scale  are  very  exceptional  after  crushing  or  shearing.  My  collection 
contains  over  three  hundred  slides  of  Alpine  rocks,  and  perhaps  a 
1  We  hear  just  at  present  so  much  about  ‘  shearing  ’  that  one  fancies  some 
geologists  suppose  rocks  to  be  sheep.  Nature  also  subjects  them  to  a  treatment 
like  that  of  a  Bramah-press.  This  probably  is  the  usual  result  ot  simple 
folding;  while  shearing  results  from  overthrusting. 
