376 
ME.  F.  ETJTLEY  ON  THE  DWINDLING  AND  [Aug.  1893, 
cause  at  all  periods,  and  throughout  an  indefinite  succession  of 
geological  epochs,  a  preponderance  of  calcareous  matter  in  the 
newer  as  contrasted  with  the  older  formations.’’ 1 2 
That  such  action  has  given  rise  to  a  constant  supply  of  calcareous 
matter  in  solution  no  one  can  doubt,  hut  it  seems  probable  that, 
unless  the  operations  of  nature  have  very  materially  varied,  the 
supply  has,  throughout  all  ages,  been  tolerably  uniform  in  amount. 
In  other  words,  during  equal  periods  of  time,  equal  amounts  of 
soluble  matter  have  been  abstracted  from  rocks  of  one  kind  or 
another.  The  transfer  of  matter,  so  derived,  from  one  formation  to 
another  is,  however,  a  different  question,  since  it  implies  no  difference 
in  output,  but  merely  a  shifting  of  material  already  supplied.  The 
output  question  is  one  which  concerns  the  delivery  of  so  many 
gallons  of  lime-bearing  solution  in  a  minute,  or  of  so  many  tons  of 
limestone  in  a  thousand  years." 
It  has  already  been  argued  by  some  writers  that  the  paucity  of 
limestone  in  the  older  formations  may  have  been  due  to  the  compara¬ 
tively  small  number  of  lime-secreting  organisms  which  tenanted  the 
early  seas.  Bischof  has  stated  that  the  sea  contains  five  times  as 
much  carbonic  acid  as  would  be  necessary  to  hold  all  the  calcium 
carbonate  which  it  contains  in  a  state  of  solution,  and  if,  therefore, 
chemical  precipitation  be  out  of  the  question  and  organic  agency  be 
admitted,  as  it  now  very  generally  is,  to  be  the  medium  through 
which  limestones  have  been  formed,  we  can  only  look  to  a  plentiful 
fauna  as  the  indirect  source  of  limestone-beds.  Admitting  this,  how 
are  the  thick  unfossiliferous  Laurentian  limestones  to  be  accounted 
for?  Were  the  pre-Cambrian  seas  more  populous  than  we  imagine 
them  to  have  been  ?  and,  if  so,  has  some  process  of  metamorphism, 
as  in  the  case  of  the  Carrara  marble,  of  Oolitic  age,  obliterated  all 
traces  of  the  fossils  which  those  old  limestones  once  contained  ? 
Sir  Charles  Lyell  seems  to  have  entertained  this  opinion.3 
Moreover,  the  less  stable  character  of  fossils,  when  composed  of 
aragonite,  should  not  be  lost  sight  of,4  and  this,  in  conjunction  with 
the  obliterating  effects  of  superinduced  crystallization,  indicates,  as 
Sir  Archibald  Geikie  remarks,  that  “the  absence  of  all  trace  of 
organic  structure  in  a  limestone  need  not  invalidate  an  inference 
1  ‘  Principles  of  Geology,’  9th  ed.  (1853)  p.  797. 
2  ‘  Taking  the  case  of  water  delivered  by  springs  in  the  Chalk,  and  which 
has  but  a  moderate  degree  of  hardness,  it  is  found  by  analysis  to  contain  about 
seventeen  grains  of  carbonate  of  lime  to  the  gallon.  Now,  out  of  a  rainfall  of 
say  twenty-six  inches  annually,  it  has  been  found  by  experiment  that  in  a 
Chalk  district  about  nine  inches  would,  in  average  seasons,  find  their  way  down 
to  the  springs;  and  it  may  be  readily  calculated  that,  at  the  rate  of  seventeen 
grains  to  the  gallon,  the  amount  of  dry  chalk  or  carbonate  of  lime  dissolved  by 
this  quantity  of  water,  and  delivered  by  the  springs,  and  thus  carried  away, 
is,  in  each  square  mile  of  such  a  district,  upwards  of  one  hundred  and  forty 
tons  in  each  year,  or  about  a  ton  to  every  four  and  a  half  acres.’ — Sir  John 
Evans,  ‘  Ancient  Stone  Implements  of  Great  Britain,’  p.  429. 
3  ‘  The  Student’s  Elements  of  Geology,’  1st  ed.  (1871)  p.  584. 
4  H.  C.  Sorby,  Pres.  Address,  Quart.  Journ.  Geol.  Soc.  vol.  xxxv.  (1879) 
Proc.  pp.  71-72,  where  the  possibly  chemical  origin  of  portions  of  some  lime¬ 
stones  is  also  alluded  to. 
