﻿328 
  CONTRIBUTIONS 
  TO 
  THE 
  GEOLOGY 
  OF 
  

  

  - 
  

  

  formerly 
  described. 
  At 
  the 
  foot 
  of 
  this 
  cliff 
  appears 
  the 
  underlying 
  

   stratum 
  of 
  clay. 
  

  

  18. 
  Description 
  of 
  the 
  cliffs 
  at 
  Yorktown, 
  on 
  the 
  York 
  river. 
  

  

  The 
  elevation, 
  abrupt 
  form, 
  and 
  peculiar 
  structure 
  of 
  the 
  cliffs 
  at 
  

   this 
  point, 
  and 
  for 
  some 
  distance 
  both 
  above 
  and 
  below, 
  render 
  it 
  an 
  

   interesting 
  spot 
  to 
  the 
  geologist. 
  A 
  dry 
  and 
  ample 
  beach, 
  uninter- 
  

   rupted 
  by 
  creeks 
  or 
  inlets 
  for 
  several 
  miles, 
  affords 
  a 
  ready 
  access 
  to 
  

   the 
  banks 
  ; 
  while 
  the 
  river's 
  edge, 
  strewed 
  with 
  fossils 
  which 
  have 
  

   fallen 
  from 
  the 
  cliff, 
  exposes 
  a 
  considerable 
  variety 
  of 
  interesting 
  spe- 
  

   cimens. 
  Immediately 
  at 
  York, 
  the 
  river 
  is 
  only 
  three-eighths 
  of 
  a 
  

   mile 
  in 
  width, 
  but 
  both 
  above 
  and 
  below, 
  it 
  expands 
  to 
  a 
  breadth 
  six 
  or 
  

   seven 
  times 
  as 
  great. 
  

  

  At 
  Wormley's 
  creek, 
  about 
  two 
  miles 
  below 
  the 
  town, 
  the 
  cliff 
  

   about 
  to 
  be 
  described 
  begins 
  ; 
  but 
  from 
  this 
  point 
  down 
  to 
  the 
  extre- 
  

   mity 
  of 
  the 
  peninsula, 
  the 
  banks 
  are 
  uniformly 
  flat 
  and 
  low. 
  The 
  

   cliff 
  here 
  consists 
  at 
  bottom 
  of 
  a 
  bluish 
  sandy 
  clay, 
  containing 
  immense 
  

   numbers 
  of 
  Turritella 
  alticosta, 
  Cytherea 
  Sayana, 
  and 
  many 
  small 
  uni- 
  

   valves, 
  over 
  which 
  lies 
  a 
  layer 
  of 
  brownish 
  yellow 
  sand, 
  with 
  very 
  few 
  

   shells, 
  and 
  those 
  chiefly 
  Nucula 
  limatula 
  and 
  a 
  few 
  other 
  species. 
  To 
  

   this 
  succeeds 
  a 
  stratum 
  composed 
  almost 
  entirely 
  of 
  Crepidula 
  costata, 
  

   so 
  closely 
  packed 
  together 
  as 
  to 
  leave 
  little 
  space 
  for 
  sand 
  or 
  other 
  earthy 
  

   matter. 
  The 
  whole 
  is 
  covered 
  to 
  a 
  variable 
  depth 
  by 
  a 
  stratum 
  of 
  coarse 
  

   sand 
  of 
  various 
  strong 
  tints, 
  and 
  evidently 
  highly 
  ferruginous. 
  The 
  ele- 
  

   vation 
  of 
  the 
  cliff 
  increases, 
  and 
  the 
  nature 
  of 
  its 
  contents 
  gradually 
  

   changes, 
  in 
  approaching 
  York. 
  The 
  lower 
  stratum 
  disappears 
  entirely 
  

   after 
  continuing 
  for 
  something 
  less 
  than 
  half 
  a 
  mile, 
  previous 
  to 
  

   which, 
  however, 
  its 
  fossil 
  contents 
  are 
  changed 
  ; 
  the 
  layer 
  of 
  the 
  

   Turritellae 
  being 
  replaced 
  by 
  Crepidula 
  closely 
  packed 
  together. 
  Cre- 
  

   pidula 
  still 
  runs 
  on 
  horizontally 
  above, 
  and 
  the 
  intermediate 
  stratum 
  is 
  

   now 
  densely 
  filled 
  with 
  Pectens, 
  Venus 
  deformis, 
  Ostrea, 
  and 
  a 
  great 
  

   variety 
  of 
  small 
  shells 
  frequently 
  connected 
  together 
  so 
  as 
  to 
  form 
  

   hard 
  masses 
  of 
  considerable 
  size. 
  Still 
  higher 
  up 
  the 
  river 
  the 
  deposit 
  

   assumes 
  the 
  character 
  of 
  successive 
  layers 
  composed 
  of 
  comminuted 
  

   shells, 
  connected 
  together 
  so 
  as 
  to 
  form 
  a 
  porous 
  rock. 
  These 
  frag- 
  

   ments 
  are 
  generally 
  so 
  small 
  and 
  so 
  much 
  rubbed 
  and 
  water-worn, 
  as 
  

   to 
  render 
  it 
  impossible 
  to 
  ascertain 
  the 
  species 
  of 
  shell 
  of 
  which 
  they 
  

  

  