140  PROCEEDINGS  OE  THE  GEOLOGICAL  SOCIETY.  [May  1 893, 
undefined  on  the  dip  towards  the  south-east.  The  depth  of  the 
salt-bed  ranges  from  about  1650  feet  at  Eston  in  Yorkshire  to 
about  950  feet  at  Greatham  in  Durham,  the  salt  averaging  about 
80  feet  in  thickness.  Subsequently  Mr.  Tate  announced  that  the 
bed,  where  last  touched,  was  thickening  on  the  dip  towards  the 
south-east. 
The  exact  geological  position  of  this  bed  of  rock-salt  has  been  the 
subject  of  much  divergence  of  opinion.  (1)  There  is  the  view, 
held  by  Sir  Andrew  Eamsay,  that  the  principal  bed  of  rock-salt 
belongs  to  the  Keuper,  and  that  the  lower  beds  of  rock-salt,  marl, 
limestone,  and  gypsum  belong  to  the  upper  portion  of  the  Permian 
series.  (2)  There  is  the  view  that  all  the  salt-beds  and  associated 
strata  belong  to  the  Permian,  and  that  the  overlying  series  of  red 
sandstone  and  marls  represents  the  Bunter.  (3)  There  is  the 
view  that  all  the  salt-beds,  and  the  whole  of  the  saliferous  marls, 
sandstones,  and  limestones,  above  the  continuous  strata  of  the 
Magnesian  Limestone,  belong  to  the  Keuper :  that  is  to  say,  to  the 
same  general  series  as  that  which  contains  the  rock-salt  in  the 
other  districts  of  the  British  Isles. 
Mr.  Wilson  is  disposed  to  agree  with  this  last  view,  which  was 
the  one  taken  by  the  older  geologists ;  and  he  is  of  opinion  that  these 
beds  of  rock-salt  lie  near  the  base  of  the  Upper  Keuper,  whereas 
in  Cheshire  the  salt-beds  come  high  up  in  the  Keuper  Marls. 
From  a  careful  examination  of  the  cores  he  concluded  that  there  is 
a  close  resemblance  between  the  beds  which  graduate  down  into 
the  saliferous  marls  of  the  Durham  salt  district  and  the  Keuper 
Waterstones  of  the  Midland  counties,  both  in  general  structure  and 
mineral  characters.  As  regards  the  thin  and  variable  series  below 
the  main  salt-bed,  which  the  presence  of  anhydrite,  marls,  and  a 
kind  of  limestone  had  induced  Sir  Andrew  Eamsay  to  look  upon 
as  Permian,  Mr.  Wilson  assures  us  that  these  show  no  resemblance 
to  any  known  beds  of  the  Magnesian  Limestone  of  Durham. 
Consequently,  he  sees  no  difficulty  in  the  fact  that  beds  towards 
the  base  of  the  Keuper  should  be  possessed  of  a  dolomitic  character 
in  a  region  where  the  rocks  rest  on  a  margin  of  the  Magnesian 
Limestone  Series,  more  especially  when  gypsum  and  anhydrite  are 
found  to  occur  in  intimate  association  with  the  dolomites  of  that 
series.  This  latter  argument  seems  to  cut  both  ways. 
Irrespective  of  debatable  matter  as  to  the  precise  horizon  of  the 
rock-salt,  the  borings  in  the  estuary  of  the  Tees  exhibit  a  consi¬ 
derable  variety  both  in  the  Permian  and  Trias  within  a  limited 
area.  The  Upper  Keuper  Eed  Marls  which  form  the  top  beds. 
