384  DWINDLING  AND  DISAPPEARANCE  OP  LIMESTONES.  [Aug.  1893? 
seen  when  thinning  out.  In  the  case  of  the  Bala,  Wenlock,  and 
Aymestry  Limestones,  for  instance,  though  the  main  mass  was  often 
uniformly  nodular  throughout,  they  died  out  into  bands  and  streaks 
of  less  calcareous  rock,  not  into  rounded  relics  of  similar  limestones, 
and  were  frequently  represented  at  last  by  lines  and  single  specimens 
of  the  corals  or  RlujnchoneV.ee  or  other  organisms,  to  the  greater 
abundance  of  which  in  the  adjoining  area  the  limestone  owed  its 
existence.  Moreover,  when  such  limestones  as  these,  or  the  Car¬ 
boniferous  limestones,  were  dissolved  away  in  clays,  they  weathered 
into  fretted  forms,  not  into  more  or  less  flattened  and  irregular 
spheroids.  Subsequent  movements  occasionally  produced  greater 
compression  in  the  shale  than  in  the  hard  concretions,  and  these 
showed  evidence  of  such  movement  in  their  slickensided  surface,  but 
through  them  the  lines  of  stratification  could  often  be  traced.  On  the 
other  hand,  where  a  single  band  of  solid  limestone  passed  into  a  series 
of  rounded  masses,  as  in  the  case  of  the  limestones  in  the  Kimeridge 
Clay  of  Littleport,  for  example,  these  rounded  masses  were  obviously 
concretions  (often  determined  by  bones  or  shells)  whose  vertical 
diameter  was  the  same  as  the  thickness  of  the  adjoining  ban  a 
of  limestone,  and  whose  shrinkage-cracks,  radiating  from  the 
centre,  had  a  distinct  relation  to  the  form  of  the  septarian  nodule 
and  not  to  the  joints  of  the  adjoining  band  of  limestone. 
Mr.  H.  W.  Monckton  and  Dr.  GL  J.  Hinde  also  spoke.  f 
The  Author,  in  reply,  stated  that  the  thicknesses  given  were 
derived  from  various  sources  which  he  believed  to  be,  on  the  whole, 
trustworthy.  In  many  cases,  however,  the  thickness  of  a  limestone 
series  was  quoted,  and  it  was  then  difficult  to  ascertain  how  much 
of  that  series  consisted  of  rocks  other  than  limestone.  He  did  nob 
attach  much  importance  to  the  estimates  given,  and  had  indicated 
at  the  outset  how  different  might  be  not  only  the  relative  thicknesses, 
but  also  the  distribution  of  rocks  in  different  areas.  With  regard 
to  the  diverse  opinions  expressed  concerning  the  origin  of  certain 
nodular  conditions,  there  were,  as  he  had  already  pointed  out, 
two  distinct  types  of  nodule — the  one  residual,  the  other  concre¬ 
tionary  ;  and  he  considered  that  the  one  type  might,  in  most  cases, 
be  distinguished  from  the  other  by  structural  peculiarities.  He 
thought  that  the  type  of  weathering  alluded  to  by  Prof.  Hughes 
was  due  to  the  presence  of  fossils.  Had  the  limestone  been 
perfectly  homogeneous,  it  would  probably  have  weathered  into 
smooth-surfaced  nodules. 
Owing  to  the  lateness  of  the  hour,  he  could  not  adequately  reply 
to  the  criticisms  of  each  speaker.  He  admitted  that  in  some  of  the 
remarks  there  was  much  justice,  and  he  thanked  the  President  and 
Fellows  of  the  Society  for  the  kindly  manner  in  which  the  paper 
had  been  received.  The  discussion,  which  embodied  many  different 
shades  of  opinion,  had  been  highly  interesting. 
