Vol.  49.]  DISAPPEARANCE  OE  LIMESTONES.  375 
altered  slates.  It  would  therefore  he  an  easy  matter,  in  a  district 
where  such  rocks  abound,  to  overlook  a  narrow  chert-hand,  and  this, 
even  if  found,  might  fail  to  contain  any  organic  remains  which  would 
afford  satisfactory  proof  of  its  origin. 
If  the  dwindling  and  disappearance  of  limestones,  as  suggested  in 
this  paper,  be  a  fact,  another  circumstance  connected  with  it  would 
have  to  be  taken  into  account.  Let  A  and  E  be  two  beds  of  shale 
or  other  rock,  between  which  a  series  of  limestone-beds  occurs,  which 
may  be  lettered  in  ascending  order  B,  C,  D  respectively.  Assume 
also  that  B.  C,  and  D  represent  more  or  less  distinctly  different 
fossiliferous  zones.  "When  erosion  has  so  far  advanced  that  only 
the  middle  member  (C)  of  the  series  remains,  we  should  then  have 
two  organic  breaks  to  consider  :  the  one  between  A  and  0,  and  the 
other  between  C  and  E  ;  and  if,  in  time,  the  whole  of  the  limestone 
disappeared,  there  would  be  a  still  more  marked  organic  break  be¬ 
tween  A  and  E. 
It  may  be  contended  that  the  thinness  and  scarcity  of  limestone- 
beds  in  the  pre-Cambrian  formations  of  Britain  indicate  that  the 
waters  of  those  earlv  times  contained  less  calcium  bicarbonate  than 
those  of  later  date.  Against  this  contention  I  would  urge  (without 
entering  into  the  question  of  whether  there  were  any  pre-existing 
sedimentary  rocks  from  which  calcareous  matter  could  have  been 
derived  )  that  in  the  disintegration  of  an  eruptive  rock  the  first  stage 
necessarily  consists  in  the  partial  decomposition  of  the  more  easily 
attacked  constituents  and  the  removal  of  matter  in  solution.  Entil 
this  is  effected,  the  second  stage  cannot  begin.  The  second  stage 
consists  in  the  mechanical  disintegration  or  crumbling  of  the  rock 
and  the  removal  of  solid  matter  in  suspension. 
Disintegration  once  started,  the  two  processes  continue  in  regular 
sequence,  the  one  being  the  consequence  of  the  other.  The  solid 
detritus,  however,  which  results  from  this  action,  may  undergo 
further  decomposition,  since  in  its  more  or  less  finely  divided  state 
it  offers  a  larger  surface  to  any  solvent. 
Assuming  that  merely  eruptive  rocks  were  acted  upon,  we  may, 
I  think,  infer  from  this  that  the  waters  of  the  early  periods  of  the 
earth’s  history  received  mineral  matter  in  solution  quite  as  soon  as 
they  received  any  solid  detrital  matter. 
Of  the  substances  brought  to  them  in  solution  calcium  bicar- 
bonate  must  assuredly  have  been  one,  and  that  not  the  least 
plentiful,  and  we  can,  therefore,  readily  believe  that  fairly  thick 
beds  of  limestone  were  formed  long  before  the  deposition  of  the 
oldest  fossiliferous  beds  wfith  which  we  are  acquainted.  So  far 
as  the  early  life-history  of  the  globe  is  concerned,  we  should 
probably  have  been  more  largely  indebted  to  such  limestones  than 
to  any  other  rocks. 
The  view  that  there  has  been  a  constant  increase  of  limestone¬ 
making  material  is  advocated  by  Sir  Charles  Lyell  in  the  following 
words  : — The  constant  transfer,  therefore,  of  carbonate  of  lime  from 
the  lower  or  older  portions  of  the  earth’s  crust  to  the -surface,  must 
