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PROF.  T.  G-.  BONNET  ON  BIOTITE  AND 
[Feb.  1893, 
destroyed  larger  crystals  of  earlier  date  ?  In  many  cases  un¬ 
doubtedly  they  have  done  so  ;  still,  as  already  said,  there  is,  I 
think,  good  evidence  that  many  of  the  larger  mineral  constituents 
in  the  Alpine  schists  are  of  very  ancient  date  and  have  passed  through 
the  Tertiary  4  mill  ’  with  but  little  injury.  In  this,  as  in  every 
mountain-region,  zones  of  nearly  uninjured  rocks  will  be  found  to 
alternate  with  zones  where  they  have  been  shattered  almost  beyond 
recognition.1  It  is  accordingly  possible  that  these  more  important 
changes  must  be  carried  back  even  to  a  very  remote  geological  epoch. 
To  prevent  misunderstanding,  I  wish  to  add  that  nothing  in  this 
paper  is  intended  as  an  expression  of  doubt  as  to  the  possibility  of 
such  mineral  reconstructions  as  have  been  described  by  Prof.  Judd- 
and  others.  I  fully  believe  that  not  only  these  have  occurred,  but 
also  the  evidence  suggests  that  pressure  has  been  a  chief  agent  in 
producing  the  result.  I  think,  however,  that  due  caution  is  not 
always  exercised  in  speculating  on  the  4  flow  of  solid  rocks  under 
pressure.’  Experiments,  such  as  those  of  Spring,  valuable  and 
suggestive  as  they  are,  do  not  help  us  quite  so  far  as  is  sometimes 
supposed,  for  the  substances  employed  were  homogeneous,  while 
the  crystalline  rocks  (with  which  geologists  are  really  concerned) 
are  heterogeneous.  These  substances  also  were  opaque  metals,  so 
that  the  exact  nature  of  the  molecular  changes  could  not  be  studied 
under  the  microscope.  Hence  it  does  not  follow  that  if  a  piece  of 
iron,  for  example,  can  be  made  to  4  flow  ’  under  pressure,  a  piece  of 
granite  will  do  the  same.  The  strength  of  a  chain  is  that  of  its 
weakest  link,  so  that,  as  it  seems  to  me,  before  such  a  rock  4  flowed  ’ 
by  gradual  mineral  reconstitution,  without  visible  fracture,  one  of 
its  constituents  would  have  given  way,  and  the  4  flowing  ’  would  be 
that  of  a  powdered  mass.  We  have  at  present,  so  far  as  I  know, 
no  evidence  to  show  that  foliation,  in  a  heterogeneous  mass  already 
crystalline ,  is  produced  without  previous  fracture. 
In  other  words,  I  believe  that  mineral  changes  are  due  to  more 
causes  than  one.  Thus  pressures  equal  in  every  direction  are  likely 
to  produce  such  reconstitutions  as  the  replacement  of  labradorite  by 
scapolite,  of  felspar  by  a  mosaic  of  quartz  and  white  mica,  of  augite 
by  granular  hornblende;  but  pressure  definite  in  direction,  as  it 
seems  to  me,  will  lead  to  crushing  or  shearing,  and  so  to  a  foliated 
structure.3  The  presence  of  water  will  undoubtedly  facilitate  change, 
1  Often  this  is  so  conspicuous  that,  until  the  right  key  was  found,  it  was 
interpreted  (very  naturally)  as  an  interstratification  of  granitoid  gneiss  and 
fissile  mica-schist,  or  a  transition  from  schist  through  gneiss  to  granite. 
2  Min.  Mag.  vol.  viii.  (1889)  p.  186  ;  Quart.  Journ.  Geol.  Soc  vol.  xlv.  (1889) 
p.  175  ;  Journ.  Cliem.  Soc.  vol.  lvii.  (1890)  p.  404.  The  second  paper  contains 
a  number  of  valuable  references  to  the  writings  of  other  workers. 
3  My  remarks  are  restricted  to  rocks  already  crystalline.  It  may  be  worth 
mentioning  that  basic  crystalline  rocks,  after  crushing,  recrystallize  more 
readily  than  the  acid,  and  the  latter  than  sediments  derived  from  them.  In 
the  Alps  one  may  find  in  the  same  fold  a  hornblendic  schist,  in  which  mineral 
reconstitution  has  obliterated  the  direct  evidence  of  pressure,  a  gneissoid  rock 
where  this  still  remains  among  partially  reconstituted  minerals,  and  a  detrital 
rock  in  which  only  micromineralc  gical  change  has  occurred,  of  which  the  origin 
is  obvious. 
