﻿THE 
  TERTIARY 
  FORMATIONS 
  OF 
  VIRGINIA. 
  329 
  

  

  once 
  were 
  portions. 
  Many 
  small 
  shells, 
  and 
  occasionally 
  large 
  ones, 
  

   particularly 
  Pectens, 
  are 
  found 
  mingled 
  with 
  the 
  other 
  constituents 
  of 
  

   the 
  rocks 
  ; 
  and 
  in 
  some 
  places 
  thin 
  layers 
  of 
  shells, 
  such 
  as 
  Venus 
  and 
  

   Crepidula, 
  intervene 
  between 
  the 
  adjacent 
  strata. 
  The 
  height 
  of 
  this 
  

   fragmentary 
  rock 
  amounts 
  in 
  some 
  places 
  to 
  forty 
  feet, 
  In 
  most 
  places 
  

   it 
  has 
  a 
  highly 
  ferruginous 
  aspect, 
  though 
  this 
  is 
  not 
  invariably 
  the 
  

   case. 
  Frequently 
  shells 
  of 
  considerable 
  size, 
  such 
  as 
  Lucina 
  anodonta, 
  

   are 
  seen 
  coated 
  with, 
  or 
  entirely 
  changed 
  into, 
  crystalline 
  carbonate 
  

   of 
  lime, 
  firmly 
  cemented 
  in 
  the 
  mass. 
  The 
  texture 
  of 
  the 
  rock 
  is 
  vari- 
  

   ous, 
  at 
  some 
  points 
  admitting 
  of 
  being 
  readily 
  excavated 
  by 
  the 
  pick 
  

   and 
  spade, 
  so 
  as 
  to 
  form 
  caves 
  which 
  have 
  been 
  occasionally 
  used 
  by 
  

   the 
  inhabitants 
  ; 
  in 
  other 
  places 
  exhibiting 
  a 
  hard 
  and 
  semi-crystalline 
  

   structure, 
  and 
  having 
  the 
  compactness 
  of 
  some 
  forms 
  of 
  secondary 
  

   limestone. 
  The 
  lower 
  portion 
  of 
  the 
  cliff, 
  having 
  less 
  cohesion 
  than 
  

   the 
  rest, 
  has 
  been 
  scooped 
  out 
  by 
  the 
  action 
  of 
  water 
  so 
  as 
  to 
  give 
  it, 
  

   occasionally, 
  an 
  impending 
  attitude. 
  

  

  Above 
  the 
  town 
  the 
  stratum 
  of 
  fragmentary 
  rock 
  becomes 
  much 
  

   thinner, 
  being 
  now 
  reduced 
  to 
  about 
  ten 
  or 
  twelve 
  feet. 
  A 
  stratum 
  

   of 
  yellowish 
  argillaceous 
  clay, 
  abounding 
  in 
  Artemis 
  acetabulum, 
  

   Mactras 
  and 
  other 
  large 
  shells, 
  lies 
  immediately 
  beneath 
  the 
  rock 
  : 
  and 
  

   lower 
  still, 
  appears 
  the 
  stratum 
  of 
  bluish 
  clay, 
  filled 
  with 
  Nucula 
  li- 
  

   matula, 
  several 
  species 
  of 
  Fusus, 
  and 
  various 
  other 
  fossils. 
  

  

  A 
  narrow 
  layer 
  of 
  iron 
  ore 
  extends 
  along 
  the 
  cliff, 
  with 
  occasional 
  

   interruptions, 
  at 
  a 
  small 
  distance 
  above 
  the 
  fossiliferous 
  strata. 
  

  

  19. 
  This 
  fragmentary 
  rock 
  continues 
  in 
  a 
  narrow 
  band, 
  with 
  some 
  

   interruptions, 
  for 
  about 
  a 
  mile 
  and 
  a 
  half 
  above 
  York. 
  Beyond 
  this 
  

   point 
  it 
  is 
  met 
  with 
  chiefly 
  in 
  detached 
  masses. 
  Extensive 
  beds 
  of 
  

   shells, 
  similar 
  to 
  those 
  which 
  appear 
  at 
  York, 
  come 
  to 
  view 
  in 
  the 
  

   vicinity 
  of 
  Bellefield, 
  and 
  line 
  the 
  shore 
  for 
  a 
  distance 
  of 
  about 
  three 
  

   miles. 
  These 
  beds 
  rest 
  on 
  the 
  usual 
  stratum 
  of 
  sandy 
  clay, 
  and 
  are 
  in 
  

   some 
  places, 
  as 
  already 
  described, 
  covered 
  by 
  a 
  stratum 
  of 
  the 
  same 
  

   substance. 
  At 
  a 
  still 
  remoter 
  point, 
  about 
  six 
  miles 
  above 
  York, 
  on 
  

   Jones's 
  plantation, 
  a 
  porous 
  rocky 
  mass 
  occurs, 
  overlying 
  the 
  stratum 
  

   of 
  shells 
  in 
  a 
  thin 
  and 
  interrupted 
  layer. 
  Though 
  very 
  similar 
  in 
  ap- 
  

   pearance 
  to 
  the 
  fragmentary 
  mass 
  before 
  described, 
  and 
  evidently 
  at 
  

   one 
  time 
  composed 
  of 
  portions 
  of 
  shells, 
  it 
  is 
  almost 
  devoid 
  of 
  any 
  trace 
  

  

  VOL. 
  V. 
  4 
  H 
  

  

  