﻿Twenhofel 
  — 
  Wreford 
  and 
  Foraker 
  Limestones. 
  413 
  

  

  pale 
  yellow 
  color 
  which 
  between 
  crossed 
  nicols 
  breaks 
  up 
  

   into 
  a 
  mosaic 
  of 
  fine 
  grains 
  of 
  chalcedony 
  and 
  quartz. 
  

   The 
  material 
  filling 
  the 
  cavities 
  of 
  the 
  shells 
  has 
  a 
  radiate 
  

   or 
  dove-tailed 
  structure, 
  and 
  consists 
  of 
  colorless 
  chal- 
  

   cedony 
  or 
  of 
  small 
  units 
  of 
  quartz 
  and 
  chalcedony 
  of 
  

   irregular 
  shapes 
  in 
  irregular 
  arrangement. 
  The 
  mate- 
  

   rial 
  surrounding 
  the 
  shells 
  is 
  an 
  aggregate 
  of 
  tiny 
  units 
  

   of 
  limonite, 
  quartz 
  and 
  chalcedony. 
  No 
  amorphous 
  

   silica 
  was 
  observed. 
  Small 
  crystals 
  of 
  calcite 
  are 
  com- 
  

   monly 
  present. 
  

  

  The 
  lowest 
  chert 
  zone 
  of 
  the 
  Wreford, 
  that 
  in 
  which 
  the 
  

   fossils 
  alone 
  are 
  composed 
  of 
  silica, 
  has 
  the 
  chert 
  only 
  

   as 
  a 
  replacement 
  of 
  the 
  original 
  matter 
  of 
  the 
  shells. 
  

   Many 
  of 
  the 
  fossils 
  have 
  the 
  silica 
  in 
  rosette 
  arrange- 
  

   ment, 
  while 
  in 
  others 
  it 
  duplicates 
  to 
  a 
  greater 
  or 
  less 
  

   extent 
  the 
  original 
  structures 
  of 
  the 
  shells. 
  The 
  chert 
  is 
  

   commonly 
  of 
  a 
  gray 
  color. 
  The 
  interiors 
  of 
  the 
  shells 
  

   are 
  filled 
  with 
  intergrown 
  calcite 
  and 
  glassy 
  quartz 
  which 
  

   in 
  many 
  instances 
  are 
  discolored 
  by 
  iron 
  oxide. 
  Shells 
  

   in 
  which 
  the 
  interiors 
  are 
  composed 
  of 
  chert 
  are 
  quite 
  

   rare. 
  

  

  The 
  zone 
  in 
  which 
  the 
  silicified 
  fossils 
  occur 
  was 
  seen 
  

   in 
  fully 
  fixe 
  hundred 
  exposures 
  over 
  an 
  area 
  of 
  about 
  one 
  

   hundred 
  square 
  miles, 
  so 
  the 
  silicification 
  in 
  this 
  zone 
  is 
  

   quite 
  general. 
  Many 
  of 
  the 
  exposures 
  are 
  in 
  deep 
  

   stream-cuttings 
  and 
  were 
  fresh, 
  and 
  one 
  is 
  in 
  the 
  bottom 
  

   of 
  a 
  recently 
  dug 
  well, 
  the 
  shells 
  coming 
  from 
  about 
  

   twelve 
  feet 
  below 
  the 
  surface 
  of 
  the 
  ground, 
  so 
  that 
  it 
  is 
  

   quite 
  certain 
  that 
  the 
  fossils 
  in 
  this 
  zone 
  are 
  silicified 
  

   beneath 
  the 
  surface. 
  No 
  exposure 
  was 
  found 
  where 
  the 
  

   shells 
  are 
  composed 
  of 
  lime 
  carbonate. 
  

  

  In 
  the 
  second 
  chert 
  zone 
  of 
  the 
  "Wreford, 
  the 
  chert, 
  

   except 
  for 
  peripheral 
  portions, 
  is 
  of 
  gray-blue, 
  yellowish- 
  

   gray, 
  dark 
  gray, 
  and 
  mottled 
  gray 
  and 
  blue 
  color. 
  The 
  

   peripheral 
  portions 
  are 
  more 
  granulated 
  than 
  the 
  inte- 
  

   riors 
  and 
  do 
  not 
  have 
  the 
  conchoidal 
  fracture 
  which 
  is 
  

   characteristic 
  of 
  the 
  latter. 
  The 
  outer 
  portions 
  are 
  also 
  

   minutely 
  porous 
  and 
  the 
  colors 
  are 
  shades 
  of 
  yellow 
  and 
  

   brown. 
  Except 
  for 
  this 
  peripheral 
  portion 
  the 
  chert 
  

   shows 
  no 
  concentric 
  bandin<r. 
  There 
  is, 
  however, 
  a 
  rude 
  

   banding 
  parallel 
  to 
  the 
  bedding 
  of 
  the 
  limestones. 
  The 
  

   inner 
  portions 
  are 
  composed 
  of 
  dense, 
  compact 
  chert 
  which 
  

   in 
  some 
  specimens 
  is 
  minutely 
  porous. 
  Fossils 
  are 
  pres- 
  

   ent 
  in 
  both 
  the 
  limestone 
  and 
  the 
  chert. 
  Thev 
  are 
  

  

  