Vol.  49.] 
DISAPPEARANCE  OF  LIMESTONES. 
377 
from  other  evidence  that  the  rock  has  been  formed  from  the  remains 
of  organisms.”  1 
The  nodular  character  of  limestones  appears  to  be  a  matter  deserv¬ 
ing  especial  attention,  since  this  condition  may  frequently  denote  the 
later  stages  of  the  process  of  dissolution  of  a  limestone-bed.  Such 
nodules,  which  I  propose  to  term  residual,  should  be  carefully 
distinguished  from  concretionary  calcareous  nodules. 
The  conversion  of  a  bed  of  jointed  limestone  into  a  band  of  residual 
nodules  is  a  destructive  process,  while  the  formation  of  a  band  of 
concretionary  calcareous  nodules  is  a  constructive  one. 
Nodular  conditions  may  be  most  commonly  met  with  at  the  top 
and  at  the  base  of  a  series  of  limestone-beds  :  that  is,  if  the  lime¬ 
stone-beds  form  an  unbroken  series  ;  but  if  they  alternate  with  beds 
of  shale,  sandstone,  etc.,  they  then  result  in  a  series  of  calcareous 
nodular  bands  with  partings  of  shale,  etc. 
In  considering  the  conversion  of  a  rectangular  block  of  limestone 
into  a  nodule  it  seemed  desirable  to  show  by  experiment  that,  if 
the  block  were  tolerably  homogeneous,  a  solvent  process  would 
produce,  from  a  cube,  a  sphere  ;  from  a  long  square  prism,  a 
fusiform  body  ;  and  from  a  prism  bounded  by  three  dissimilar  pairs 
of  rectangular  faces,  a  lenticular  nodule. 
This  was  most  readily  effected,  on  a  very  small  scale,  by  sub¬ 
mitting  little  prisms  of  ordinary  white  drawing-chalk  of  the  forms 
respectively  shown  in  PI.  XVIII.,  figs.  A,  B ,  and  C ,  to  the  action 
of  dilute  hydrochloric  acid.  These  were  carefully  watched  while 
dissolving.  The  stream  of  ascending  gas-bubbles,  when  viewed 
against  the  light,  appeared  as  darker  and  denser  columns  where  they 
arose  from  the  solid  angles  of  the  chalk-blocks.  This  phenomenon 
was  most  marked  in  tho  earlier  stages  of  dissolution,  and  became 
less  perceptible  as  the  process  advanced. 
A  considerable  time  was  found  to  elapse  before  the  edges  and 
solid  angles  of  the  block  B ,  which  was  a  roughly-broken  cube, 
became  appreciably  rounded,  and  even  when  more  than  three-fourths 
of  the  mass  had  been  dissolved,  some  of  the  edges  remained  tolerably 
sharp.  At  this  stage  the  process  was  stopped,  and  the  result  is 
shown  in  PL  XVIII.  fig.  h,  of  which  fig.  b'  is  an  enlarged  repre¬ 
sentation.  Had  the  process  been  continued,  the  penultimate  result 
would  have  been  an  approximately  spherical  body. 
Another  chalk-block,  of  the  size  and  form  shown  in  PL  XVIII. 
fig.  A,  was  also  immersed  for  some  hours  in  dilute  hydrochloric  acid. 
This  finally  became  reduced  to  a  very  small,  sharply- pointed,  fusiform 
body,  of  which  fig.  a  is  a  representation  of  the  actual  size  and  fig.  a 
an  enlarged  version.  Owing  to  its  extreme  delicacy,  it  broke  when 
being  removed  from  the  acid. 
Pig.  C ,  PL  XVIII.,  represents  yet  another  small  block  of  chalk 
which  was  treated  in  a  similar  manner.  When  reduced  to  less  than 
half  its  original  bulk,  it  assumed  the  form  shown  in  Pl.  XVIII. 
fig.  d  (magnified  8  diameters).  Pinally,  this  was  reduced  to  the 
1  ‘  Text-book  of  Geology,’  2nd  ed.  1885,  p.  113.  , 
Q.J-.G.S.  No.  195.  2  c 
