378  ME.  F.  ETJTLEY  ON  THE  DWINDLING  AND  [Aug.  1 893, 
minute  lenticular  body  shown  in  PI.  XVIII.  fig.  c,  of  which  fig.  c 
is  an  enlarged  representation. 
On  comparing  figs,  d  and  c,  it  will  be  seen  that  the  lenticular 
form  becomes  more  marked  in  the  later  stages  of  dissolution.  It  may, 
therefore,  be  inferred  that  the  residual  nodules  of  lenticular  form, 
with  sharp,  thin  edges,  denote  the  stage  which  precedes  the  total 
disappearance  of  a  limestone-bed.  If  this  be  the  case,  it  may,  I 
think,  be  assumed  that  a  band  of  lenticular  nodules  at  the  base  of 
a  series  of  limestone-beds  may  have  been  preceded,  or  at  the  top  of 
a  series  may  have  been  succeeded,  by  one,  by  several,  or  even  by 
many  beds  of  limestone  wdiich  have  wdiolly  disappeared.  Different 
stages  in  the  dissolution  of  such  a  series  of  beds  are  represented 
diagrammatically  in  PI.  XVIII.  figs.  (/,  h,  i,  and  1c. 
Although  the  blocks  of  chalk  figured  in  that  Plate,  figs.  A ,  B , 
and  (7,  were  so  small,  yet  one  may  conclude  that  the  action  of  a 
solvent  upon  equally  homogeneous  limestone-blocks  of  vastly  greater 
dimensions  would  probably  give  approximately  similar  results. 
In  connexion  with  these  simple  experiments,  it  is  interesting  to 
note  that  the  directions  in  which  the  blocks  are  most  rapidly 
dissolved  correspond  with  those  in  which  0.  Lehmann  has  shown 
that  the  most  rapid  growth  of  a  crystallite  takes  place.1  The 
reasoning  upon  which  Lehmann’s  theory  is  based  has,  it  would 
appear,  simply  to  be  reversed.  Instead  of  considering  relative 
areas  in  which  fresh  material  is  supplied  to  the  growing  body,  we 
have  to  deal  with  those  in  which  matter  is  being  removed  from  a 
body  constantly  diminishing  in  size.  In  Pl.  XVIII.  fig.  e  this  is 
diagrammatically  represented.  The  areas  of  solution  are  bounded 
by  lines  drawn  at  right  angles  to  the  surfaces  of  the  successively 
decreasing  solids  and  at  equal  distances  from  one  another,  and  it 
will  be  seen  that  the  greatest  solution-areas  occur  along  the  edges  and 
at  the  solid  angles.  The  rate  of  solution  is  indicated  by  shading, 
which  is  darkest  where  the  solid  becomes  most  rapidly  dissolved, 
i.  e.  where  it  is  acted  upon  by  the  largest  amount  of  the  solvent. 
It  will  be  noted  that  the  dark  bands  in  the  shading  correspond  in 
direction  with  main  ribs  of  chiasmolitic  crystallites  (‘  furculites  ’ 
and  ‘  arculites  ’).  There  can,  of  course,  only  be  a  perfect  corre¬ 
spondence  of  solubility  with  form  in  homogeneous  solids  in  which  no 
differences  of  cohesion  exist.  In  dissolving  a  crystal,  the  solubility 
would  be  more  or  less  influenced  by  differences  of  cohesion  in 
different  directions.  In  the  case  of  a  limestone-block  when  acted 
upon  by  a  solvent,  it  is  the  external  form  of  the  mass  which 
determines  the  directions  in  which  it  is  most  rapidly  dissolved, 
directions  which  vary  with  the  gradually  changing  form  of  the  solid. 
The  solution  of  a  limestone-block  cannot,  therefore,  bear  a  strict 
comparison  with  that  of  a  crystal,  since  in  the  one  case  the  aggregate 
molecular  grouping  is  indefinite,  while  in  the  other  it  is  definite. 
On  referring  to  PI.  XVIII.  fig.  e ,  it  will  be  noted  that  the 
1  ‘IJeber  das  Wachsthum  der  EVystalle,’  Zeitsohr.  fiir  Kryst.  u.  Min.,  1877, 
pt.  i.  pp.  453-496 ;  see  also  Rosenbasch’s  1  Mikroskopische  Physiographie,’ 
3rd  ed.  (1892)  vol.  i.  p.  29. 
