The  first  discovery  due  to  the  microscope  was  the  detection  of
Olivine,  Having  observed  two  small  crystals  in  one  of  my  sections
which  were  evidently  different  from  any  of  the  other  minerals,  I
examined  them  with  polarised  light,  and  found  that  they  presented
precisely  the  same  appearances  as  those  seen  in  sections  of  Olivine
which  I  had  previously  made.  After  many  visits  to  the  quarries  in
search  of  larger  specimens  of  the  mineral,  I  succeeded  in  finding  it
in  sufficiently  large  grains  to  be  easily  recognized.
Tlio  green  mineral  with  the  crystalline-form  above  mentioned  re-
mained to  be  determined.  It  was  evident,  in  the  first  place,  from
its  action  on  i>olarised  light  that  it  was  a  pseudomorj^h,  the  appear-
ance of  such  an  aggregate  of  mineral  matter  being  very  different
from  that  of  a  single  uniform  crystal ;  and,  after  making  more  than
thirty  sections,  the  last  two  have  enabled  me  to  arrive  at  the  con-
clusion that  the  mineral  in  question  is  a  pseiidoraorph  after  Olivine.
Tlie  two  sections  contain  many  crystals  of  this  mineral  in  various
stages  of  alteration,  from  the  first  commencement  to  the  completion
of  the  process.  The  change  began  either  at  the  surface,  or  along
the  line  of  cracks  running  through  the  crystal ;  in  the  latter  case,
the  transformation  extended  from  both  sides  of  the  fissure,  forming
a  narrow,  or  wide  green  bemd,  according  as  the  process  continued  or


116    /  JL  Dakyns — Note  on  the  Oreen  and  Skiddaw  Slates.
not  Where  Beveral  oracks  existed,  the  green  bands  soon  united,
and  the  entire  crystal  was  changed.  In  another  instance  the  change
commenced  at  the  surface  and  extended  inwards,  till  only  the
central  portion  was  left  unaltered ;  or  again,  the  change  proceeded  in
both  ways  simultaneously.
