122  PROCEEDINGS  OE  THE  GEOLOGICAL  SOCIETY.  [May  1 893, 
The  chief  interest  of  this  paper  centres  in  the  clays  above  the  Oxford 
Clay,  and  especially  in  the  possibility  of  tracing  a  Corallian  Clay 
between  it  and  the  Kimeridge. 
The  clays  which  extend  to  the  top  of  what  has  been  mapped  as 
Lower  Kimeridge  in  Lincolnshire  may,  he  says,  be  conveniently 
divided  into  four  zones.  The  lowest  of  these  is  a  series  of  black 
selenitiferous  clays,  which  he  regarded  as  Corallian  for  the  following 
reasons (1)  They  occur  between  the  Oxford  Clay  and  what  he 
regards  as  the  basement-bed  of  the  Kimeridge ;  (2)  out  of  23 
species  of  fossils  collected  from  this  zone  22  are  recognized  as 
Corallian ;  (3)  the  two  characteristic  oysters  of  the  Oxford  and 
Kimeridge  Clays  respectively  occur  together  in  this  series  of  clays 
and  in  the  Corallian,  but  in  no  other  formation. 
If  we  endeavour  to  test  the  value  of  the  first  conclusion,  it  would 
seem  that  the  black  clays  are  nowhere  observed  immediately 
overlying  the  Oxford  Clay,  but  they  are  found  in  several  places 
immediately  eastward  of  what  is  regarded  as  the  uppermost  zone  ; 
they  are  easily  distinguished  by  their  darker  colours,  and  from  the 
fact  of  the  fossils  never  being  pyritized.  These  indications  cer¬ 
tainly  seem  to  separate  the  black  selenitiferous  clays  from  the 
uppermost  Oxford  Clay  of  the  district,  though  there  does  not  seem 
to  be  any  adequate  explanation  of  the  more  general  oxidation  of 
the  sulphur  compounds.  The  next  point  is  to  ascertain  how 
far  a  profusion  of  Ostrea  deltoidea  is  evidence  of  the  basement- 
beds  of  the  Kimeridge.  That  such  is  the  case  in  the  Lower 
Kimeridge  of  Ringstead  Bay  there  can  be  no  doubt.  In  Cambridge¬ 
shire  the  Author  remarks  that,  immediately  above  a  layer  of 
phosphatic  nodules,  which  is  regarded  as  constituting  the  actual 
base  of  the  Kimeridge  Clay  in  that  district,  Ostrea  deltoidea  occurs 
in  profusion.  It  is  probable  that  this  is  also  the  case  in  Yorkshire, 
but  good  exposures  of  the  basal  Kimeridge  are  not  very  common 
there.  On  the  whole  we  may  fairly  allow  that  Mr.  Roberts  has 
justified  his  first  conclusion. 
Turning  to  the  evidence  of  the  22  recognized  Corallian  species, 
this  speaks  strongly  in  favour  of  Mr.  Roberts’s  view  as  to  the  age  of 
the  black  clays.  The  greatest  number  of  Corallian  forms  occur  in 
pits  at  North  and  South  Kelsey,  and  on  the  whole  the  fauna  inclines 
rather  to  older  than  to  newer  beds.  The  characteristic  Corallian 
sea-urchin  is  not  quoted  in  the  list,  and  this  may  perhaps  be  due  to 
distance  from  actual  reefs,  since  the  spines  of  Cidaris Jlorigemma 
are  not  unknown  from  the  Ampthill  Clay.  Mr.  Roberts’s  third 
