104 
PROF.  T.  G.  BONNET  OK  BIOTITE  AND 
[Feb. 1893. 
6.  On  a  Secondary  Development  of  Biotite  and  of  Hornblende  in 
Crystalline  Schists  from  the  Binnenthal.  By  Prof.  T.  G. 
Bonnet,  D.Sc.,  LL.D.,  F.B.S.,  Y.P.G.S.  (Bead  December  7th. 
1892.) 
Pressure  is  one  agent  in  mineral  change.  In  some  cases,  as  when 
a  shale  is  converted  into  a  phyllite,  the  constituents  become  rather 
larger,  but  more  commonly  the  result  of  oppression  is  degradation. 
This  is  especially  true  of  the  crystalline  rocks.  The  effects  of 
earth-movements  on  granites,  gneisses,  marbles,  mica-  and  other 
schists  are  familiar  to  all  workers  in  such  districts  as  the  Alps  or 
the  Scottish  Highlands — we  might  say  in  almost  any  great  mountain- 
chain,  whatever  be  its  date.  Sometimes,  indeed,  an  original  con¬ 
stituent  has  ‘gone  through  the  mill’  comparatively  unscathed. 
Its  weaker  associates  have  been  crushed  to  powder,  which  has  acted 
like  a  packing-material.  But  cases  occur  where  new  minerals  of 
considerable  size  are  developed  here  and  there  from  the  debris 
of  the  original  constituents.1  The  greater  part  of  the  rock  is 
little  more  than  reconsolidated  detritus ;  it  still  retains  the  marks 
of  the  peine  forte  et  dure  to  which  it  has  been  subjected ;  but 
nevertheless  it  has  set  up  a  structure  which  might  be  termed 
porphyritic.  The  process  is  analogous  to  the  formation  of  chias- 
tolite  in  contact-metamorphism.  The  mass  of  the  rock  is  little 
changed,  but  a  large  mineral  has  been  developed :  for  the  one 
millimetres,  for  the  other  centimetres  are  appropriate  standards  of 
measurement. 
Still,  so  far  as  my  experience  goes,  this  secondary  development  on 
an  important  scale  is  very  rare  ;  commonly  wThen  large  minerals 
occur  in  a  catathlastic  rock,2  as  the  garnets  in  certain  Alpine  schists, 
or  the  sahlite  in  the  fine-grained  limestone  of  Tiree,  they  are 
survivors  from  the  original  coarsely  crystalline  rock.  From  time  to 
time,  however,  I  have  noted  instances  of  this  secondary  growth,  but 
it  generally  has  been  on  a  very  moderate  scale.  Hence  it  may  be 
worth  while  to  describe  two  specimens,  which,  curiously  enough, 
were  received  together.  Both  came  from  the  same  neighbourhood, 
the  Binnenthal  (Canton  Valais),  and  from  the  same  group  of  rocks 
— the  dark-mica  schists,  in  which  no  small  part  of  that  valley  is 
1  It  will  be  understood  that  I  am  speaking  throughout  of  the  silicates  usually 
present  in  crystalline  rocks.  Such  minerals  as  quartz  and  calcite  form  so  readily 
as  not  to  count  for  much,  nor  do  I  refer  to  the  ordinary  zeolites,  produced  in 
certain  igneous  rocks.  All  these,  however,  seem  to  require  room  (i.  e.  occupy 
cavities),  while  those  of  which  I  speak  occur  in  the  mass  of  the  rock. 
2  Some  such  word  as  this  is  needed  to  express  detrital  rock  due  to  crushing 
in  situ.  ‘Crushed’  would  do,  but  geologists  are  such  excellent  Greek  scholars 
that  no  word  has  a  chance  of  coming  into  fashion  unless  it  be  derived  from  that 
language.  '  Mylonitic  ’  is  doubtfully  correct,  etymologically  speaking,  and 
suggestive  of  an  erroneous  idea,  for  it  implies  grinding  as  between  revolving 
millstones.  Also  it  has  been  generally  applied  to  rock  affected  by  a  shearing 
crush,  not  by  a  direct  crush,  as  of  a  press. 
