Yol.  49.] 
DISAPPEARANCE  OF  LIMESTONES. 
381 
be  very  much  less  than  their  original  thicknesses,  and  that  the 
comparatively  thin  limestones  in  the  older  formations  of  this 
country  were  possibly  once  much  thicker. 
8.  That  the  dwindling  and  disappearance  of  limestones  through 
the  action  of  solvents,  especially  of  water  charged  with  carbonic  acid, 
gives  rise  to  palaeontological  breaks,  which,  though  often  small,  are 
numerous  and  consequently  not  unimportant. 
9.  That  the  absence  of  fossils,  as  pointed  out  by  Sir  Charles  Lyell, 
is  no  proof  that  the  limestones  were  not  originally  fossiliferous. 
10.  That  if  such  non-fossiliferous,  crystalline  limestones  are  of  or¬ 
ganic  origin  and  not  chemical  precipitates,  the  thickness  of  those  of 
Laurentian  age,  found  in  Canada,  militates  against  the  assumption  that 
the  thinness  of  our  older  limestones  in  this  country  is  due  to  the  sparse¬ 
ness  of  life  in  those  remote  periods,  as  suggested  by  some  authors. 
11.  That  a  distinction  should  be  carefully  made  between  nodules 
resulting  from  the  partial  dissolution  of  beds  of  limestone  (residual 
nodules)  and  those  concretions  which  result  from  the  segregation  of 
calcareous  matter  about  detached  nuclei  (concretionary  nodules). 
12.  That  nodular  conditions  are  to  be  looked  for  less  among  thick- 
than  among  thin-bedded  limestones,  since,  by  the  time  that  one  bed 
has  passed  into  a  nodular  condition,  the  nodules  resulting  from  the 
dissolution  of  the  bed  immediately  above,  or,  if  at  the  base  of  a  series, 
immediately  below  it,  will  have  been  completely  dissolved. 
13.  That  the  presence  of  large  fossils  in  a  limestone-block  may 
materially  influence  the  form  of  the  resultant  nodule  or  nodules. 
14.  That,  although  not  dealt  with  in  this  paper,  the  mechanical 
agencies  involved  in  marine  and  subaerial  denudation  must  necessarily 
play  a  very  important  part  in  the  removal  of  limestones,  as  well  as 
of  other  rocks,  and  that  they  are  more  or  less  intimately,  sometimes 
inseparably,  connected  with  that  chemical  action  of  solvents  upon 
which  the  foregoing  conclusions  are  based. 
EXPLANATION  OF  PLATE  XYIII. 
Nodular  forms,  procured  by  partially  dissolving  small  blocks  of  chalk  in 
dilute  hydrochloric  acid. 
Fig.  a.  Fusiform  body,  derived  from  the  block  A. 
a' .  The  same,  magnified  about  8  diameters. 
b.  Irregularly  rounded  body,  derived  from  the  block  B  (a  roughly  broken 
cube). 
b\  The  same,  magnified  about  8  diameters. 
(Had  b  remained  longer  in  the  acid  it  would  have  been  reduced  to 
an  approximately  spherical  form.) 
c.  Lenticular  body,  derived  from  the  block  C. 
c\  The  same,  magnified  about  8  diameters. 
d.  Form  intermediate  between  C  and  c,  observed  when  the  block  C  was 
more  than  half  dissolved.  Magnified  about  8  diameters. 
(Figs.  A,  a,  B,  b,  and  C,  c  are  shown  of  the  natural  size.) 
e.  Diagram  representing  successive  stages  in  the  reduction  of  a  block  of  lime¬ 
stone  to  a  lenticular  nodule.  The  lines  drawn  at  right  angles  to  the 
surfaces  successively  formed  represent  solution-areas,  which  are  largest 
at  the  solid  angles  and  along  the  edges  of  the  blocks.  The  shading 
indicates  the  directions  of  most  rapid  solution.  These  correspond 
with  the  initial  directions  of  growth  in  a  chiasmolitic  crystallite 
