It  is  probable  that  many  strata  became  solid  at  the  time  when
they  emerged  from  the  waters  in  which  they  were  deposited,
and  when  they  first  formed  a  part  of  the  dry  land.  A  well-
known  fact  seems  to  confirm  this  idea  :  by  far  the  greater  num-
ber of  the  stones  used  for  building  and  road-making  are  much
softer  when  first  taken  from  the  quarry  than  after  they  have
been  long  exposed  to  the  air  ;  and  these,  when  once  dried,  may
afterwards  be  immersed  for  any  length  of  time  in  water  without
becoming  soft  again.  Hence  it  is  found  desirable  to  shape  the
stones  which  are  to  be  used  in  architecture  while  they  are  yet
soft  and  wet,  and  while  they  contain  their  'quarry-water,'  as  it
is  called  ;  also  to  break  up  stone  intended  for  roads  when  soft,
and  then  leave  it  to  dry  in  the  air  for  months  that  it  may
harden.  Such  induration  may  perhaps  be  accounted  for  by
supposing  the  water,  which  penetrates  the  minutest  pores  of
rocks,  to  deposit,  on  evaporation,  carbonate  of  lime,  iron,  silex,
and  other  minerals  previously  held  in  solution,  and  thereby  to
fill  up  the  pores  partially.  These  particles,  on  crystallising,
would  not  only  be  themselves  deprived  of  freedom  of  motion,
but  would  also  bind  together  other  portions  of  the  rock  which
before  were  loosely  aggregated.  On  the  same  principle  v:et  sand


•i-0  ('oNsnLIDATlnV    <>K    ST1JATA.  [

and  nmcl  become  as  hard  as  stone  Avhen  fro/en  :  iKrausr  <>no
ingredient  of  the  mass,  namely,  the  water,  has  crystallised,  so
as  to  hold  firmly  together  all  the  separate  particles  of  which  the
loose  mud  and  sand  were  composed.
