128  PROCEEDINGS  OF  THE  GEOLOGICAL  SOCIETY.  [May  1893, 
But  as  these  constitute  an  essentially  transitional  series,  poor  in 
fossils  other  than  Cephalopoda,  I  did  not  pursue  the  subject  further 
at  that  time. 
No  stratigraphical  palaeontologist  can,  I  think,  doubt  that 
Mr.  Buckman  has  at  length  succeeded  in  proving  that  the 
different  ‘  Sands  ’  are  not  on  the  same  horizon.  In  the  above- 
mentioned  paper  he  described  a  series  of  sections  of  the  typical 
exposures  from  the  Cottes wolds  near  Frocester  to  the  neighbourhood 
of  Bridport  Harbour.  Dividing  the  series  into  seven  horizons,  each 
characterized  by  their  distinctive  Ammonites,  viz. — Am.  communis , 
variabilis,  striattjlus,  dispansus ,  the  genus  Dumortieria,  Am. 
Moorei ,  and  opcdinus ,  and  taking  the  sTRiATULus-beds  as  a  fixed 
starting-point,  he  demonstrated  how  the  several  beds  varied  in 
regard  to  that  horizon.  Thus  the  Cotteswold  Sands,  containing  a 
portion  of  the  communis- zone  together  with  the  variabilis-\)Qdi$, 
are  below  the  fixed  datum-line :  the  Midford  Sands,  having 
Am.  striatidus  at  their  base  and  being  mainly  characterized  by 
Ammonites  of  the  dispansus -h.0YYA0'a,  are  above  the  fixed  datum¬ 
line  ;  whilst  the  Yeovil  Sands,  containing  the  Dumortieria  and 
jl/oom-horizons,  overlie  a  bed  containing  Ammonites  of  the 
dispansus- horizon  and  are  consequently  still  later  deposits.  In 
South  Dorset  this  test  is  productive  of  yet  more  surprising  results. 
For  instance,  Mr.  Buckman  alleges  that  the  Blue  Clay  of  Down 
Cliff,  which  lies  below  the  Yeovil  or  Bridport  Sands,  has  yielded 
Ammonites  of  the  genus  Dumortieria  and  notably  D.  radians. 
This  clay  therefore  would  seem  to  he  above  the  fixed  datum-line. 
Yet,  because  it  is  a  clay,  it  has  been  regarded  as  Upper  Lias,  while 
the  undoubtedly  older  Cotteswold  Sands,  extending  partly  into  the 
communis-he&s,  under  recent  arrangements  are  to  be  classed  with  the 
Inferior  Oolite. 
This  is  a  dilemma  from  which  there  is  no  possible  escape,  except 
by  impugning  the  value  of  the  evidence.  Consequently  we  are 
asked  to  consider  what  is  meant  by  ‘  horizons,’  or,  as  some  would 
prefer  to  call  them,  ‘  geological  zones.’  There  are  all  sorts  and 
conditions  of  zones,  we  are  told  by  a  very  competent  authority,1  and 
unless  one  defines  them  simply  as  ‘  palaeontological  horizons  ’  there 
is  no  definition  that  would  apply  to  all  that  have  been  constituted. 
Now,  it  has  been  urged  over  and  over  again  that  the  Cephalopoda, 
especially  in  the  Mesozoic  rocks,  are  the  best  guides  to  life-zones 
because  they  were  less  influenced  by  physical  conditions  and  are 
therefore  more  reliable  indicators  of  time  than  the  other  Mollusca. 
1  H.  B.  Woodward,  Proc.  Geol.  Assoc,  vol.  xii.  pp.  295  et  seqq. 
