Yol.  49.] 
DISAPPEARANCE  OE  LIMESTONES. 
379 
rounding  of  edges  and  solid  angles  is  less  apparent  in  the  early 
than  in  the  later  stages  of  solution,  when  the  solution-areas  of 
greatest  magnitude,  which  began  at  90°,  eventually  fall  little  short 
of  180°,  the  limiting  angle  of  the  solution-area. 
Although,  in  this  paper,  the  belief  is  expressed  that  many  calcar¬ 
eous  nodular  bands  represent,  as  residual  nodules,  what  were 
once  beds  of  limestone,  it  must  by  no  means  be  inferred  that  all 
calcareous  nodular  bands  have  had  a  similar  origin.  Some  are 
distinctly  concretionary.  Both  residual  and  concretionary  nodules 
may  contain  fossils.  In  the  former  the  fossils  are  contemporaneous 
with  the  nodules  which  enclose  them.  Concretionary  nodules 
sometimes  exhibit  a  radiating  or  divergent  crystalline  structure. 
Such  structure  in  a  residual  nodule  would  be  quite  exceptional. 
Both  kinds  of  nodules  may  be  traversed  by  planes  of  lamination  ; 
but,  before  saying  more  about  their  points  of  resemblance  or 
dissimilarity,  it  is  desirable  to  examine  a  large  series  of  typical 
examples,  so  that  some  accurate  means  of  discrimination  may  be 
arrived  at. 
If  the  views  advocated  in  this  paper  be  correct,  the  unqualified 
expression  4  nodular  limestone  5  is  insufficient,  and  its  use  should,  at 
least,  be  restricted  to  observations  in  the  field. 
In  support  of  the  foregoing  statements  it  may  be  mentioned  that 
Mr.  Beeby  Thompson,  F.G.S.  (in  a  paper  on  ‘  The  Upper  Lias  of 
Northamptonshire,’  part  ii.),  describes  nodular  conditions  of  the 
argillaceous  limestone  (Fish  Bed)  and  gives  reasons  for  regarding 
these  nodules  as  portions  of  once  continuous  beds,  which  were  sub¬ 
sequently  traversed  by  transverse  shrinkage -cracks,  the  dissolving 
of  the  limestone  and  subsequent  widening  of  the  cracks  resulting 
in  the  formation  of  bands  of  distinct,  flattened  nodules,  and  these 
nodules  are  stated  not  to  be  concretionary. 
The  following  extract  from  a  paper  on  ‘  The  Northern  Slopes  of 
Cader  Idris,’  by  Prof.  Grenville  A.  J.  Cole  and  Mr.  A.  Y.  Jennings, 
Quart.  Journ.  Geol.  Soc.  vol.  xlv.  (1889)  p.  427,  is  worthy  of  note 
as  also  tending  to  confirm  the  belief  that  our  older  formations  were 
once  richer  in  limestones  than  they  now  are  : — “  If  the  iron  ore  of 
Cader  Idris  was  at  one  time  a  fairly  calcareous  band  among  the 
more  prevalent  muds  and  shales,  it  owes  its  preservation  to  the 
pseudomorphic  action  that  has  gone  on.  The  very  marked  propor¬ 
tion  of  carbonate  of  lime  that  occurs  in  the  cavities  of  the  permeable 
rocks  from  Llyn  Gwernen  to  the  summit  of  the  mountain  may  have 
been  in  part  derived  from  similar  seams  or  shelly  patches,  the 
absence  of  which  we  have  so  frequently  to  deplore  when  examining 
Ordovician  or  Cambrian  strata.  It  is  unnecessary  to  dwell  upon 
the  increased  effect  of  solvent  action  when  occurring  in  a  region 
subjected  to  pressure  and  earth-movement.” 
In  some  localities,  where  limestones  are  interbedded  with  shales, 
it  has  been  noted  that  the  latter  weather  away  more  rapidly,  so 
that  the  limestones  form  projecting  courses.  Such  conditions  may, 
I  think,  be  regarded  as  due  either  to  dry  situation,  or  to  the  crystal- 
20  2 
