374  ME.  F.  EUTLEV  ON  THE  DWINDLING  AND  [Alig.  1 893, 
together  or  be  filled  by  rubble,  sand,  etc.,1  or  by  matter  deposited 
from  infiltered  solutions,  or,  under  certain  conditions,  by  sedimentary 
matter. 
When  a  limestone-bed  is  dissolved,  any  calcareous  fossils  which 
it  contains  will  share  the  same  fate,  and  thus  a  page  in  the  life- 
history  of  the  locality,  as  well  as  in  the  stratigraphical  record,  would 
vanish  for  ever.  How  many  such  pages  are  lost  to  us  it  is  impos¬ 
sible  to  tell,  but  if,  in  reviewing  the  stratigraphical  record  from 
Carboniferous  to  Archaean  times,  as  we  find  it  in  England  and  Wales, 
we  note  the  gradually  decreasing  thickness  of  the  limestone-beds, 
it  is  not  hard  to  believe  that  all  of  them  have  lost  either  in  actual 
thickness,  or  have  parted  with  more  or  less  calcareous  matter  which 
has  been  subsequently  replaced  by  material  of  a  different  nature. 
The  latter  hypothesis  seems  to  be  strengthened  by  the  very  impure 
character  of  some  of  the  older  limestones. 
On  the  erosion  of  a  clierty  limestone,  the  bands  of  chert  would 
remain  after  the  limestone  had  been  removed.  In  such  a  case  the 
life-history  of  that  particular  zone  would  not  be  wholly  lost. 
Considering  how  commonly  chert  is  associated  with  limestone, 
the  independent  occurrence  of  chert-bands  or  nodules  in  the  older 
formations  would,  if  found,  afford  a  possible  clue  to  the  horizons  at 
which  long-vanished  limestone-beds  formerly  occurred.  In  such 
an  enquiry  the  chert  would  be  to  the  limestone  what  a  moraine-stone 
is  to  a  departed  glacier,  a  witness  whose  testimony  cannot  be  con¬ 
troverted,  unless  it  could  be  shown  that  the  chert  was  originally 
developed  in  some  rock  other  than  a  limestone.  A  chertless 
limestone  leaves  no  record,  and  it  can,  therefore,  only  be  by  a  dili¬ 
gent  search  for  cherty  seams  in  the  older  formations  that  we  can 
hope  for  any  information  regarding  such  stratigraphical  losses. 
Here,  however,  we  come  to  a  rather  difficult  question  for  the  field- 
geologist,  namely  the  ready  recognition  of  a  chert  in  formations 
where  felstones  and  compact  indurated  grits  are  prevalent.  So  close 
is  the  outward  resemblance  of  these  rocks  one  to  another  that  the 
name  ‘  hornstone  ’  was  indifferently  used  in  bygone  years  both  for 
cherts  and  felstones,  although  the  former  are  infusible  and  the  latter 
fusible  before  the  blowpipe.  It  should,  however,  be  remarked  that 
Macculloch,  when  speaking  of  chert,  pointed  out  the  difference, 
stating  that  “the  hornstone  of  many  authors  is  compact  felspar.’’ 2 
The  ‘  phthanite  ’  of  Continental  petrologists  is  also  an  elastic  term, 
embracing  not  only  cherts  and  the  siliceous  limestones  into  which 
they  graduate,  but  also  siliceous  slate  (Jcieselsclnefer),  jasper,  and 
chalcedony.3  Chert  is  essentially  cryptocrystalline,  and  occasionally  it 
presents  a  banded  structure.  Like  characters  are  also  met  with  in 
some  felstones,  and  very  similar  ones  in  certain  por  cell  unites  and 
1  A  curious  example  of  the  infilling  of  fissures  is  afforded  by  the  sandstone- 
dykes  described  by  Prof.  J.  S.  Diller,  Bull.  Geol.  Soc.  Amer.  vol.  i.  (1890) 
pp.  411-442. 
2  ‘  Geological  Classification  of  Bocks,’  London,  1821,  p.  171. 
3  A.  Benard,  ‘  Becberches  lithologiques  sur  les  Phthanites  du  Calcaire  Car- 
bonifere  de  Belgique,’  Bull.  Acad.  Boy.  de  Belgique,  2eme  sen  vol.  xlvi.  (1878) 
p.  471. 
