﻿412 
  Twenhofel 
  — 
  Wreford 
  and 
  Foraker 
  Limestones. 
  

  

  masses 
  and 
  also 
  like 
  that 
  of 
  the 
  third 
  zone 
  of 
  chert. 
  

   These 
  may 
  represent 
  two 
  zones 
  and, 
  as 
  they 
  have 
  not 
  been 
  

   well 
  enough 
  seen 
  to 
  place 
  them 
  in 
  the 
  section 
  with 
  suffi- 
  

   cient 
  definiteness, 
  they 
  will 
  not 
  be 
  further 
  considered. 
  

  

  Description 
  of 
  the 
  Cherts. 
  

  

  The 
  chert 
  of 
  the 
  Foraker 
  limestone 
  is 
  quite 
  porous, 
  

   most 
  of 
  the 
  pores 
  appearing 
  to 
  have 
  resulted 
  from 
  the 
  

   removal 
  of 
  an 
  organic 
  fragment. 
  The 
  color 
  is 
  either 
  

   some 
  shade 
  of 
  gray, 
  a 
  pale 
  gray 
  blue, 
  or 
  a 
  mottled 
  gray 
  

   and 
  blue. 
  Yellowish-gray 
  shades 
  are 
  not 
  uncommon. 
  

   The 
  abundant 
  fossils 
  are 
  commonly 
  of 
  a 
  different 
  color 
  

   from 
  the 
  surrounding 
  material. 
  In 
  most 
  instances 
  they 
  

   are 
  lighter 
  than 
  the 
  matrix; 
  in 
  some 
  instances 
  they 
  are 
  

   darker. 
  

  

  • 
  Little 
  concentric 
  arrangement 
  of 
  material 
  is 
  shown 
  in 
  

   these 
  cherts, 
  but 
  the 
  peripheral 
  portions 
  of 
  each 
  nodule 
  

   have 
  a 
  more 
  porous 
  texture 
  and 
  lighter 
  color 
  than 
  the 
  

   interior 
  portions. 
  The 
  interiors 
  also 
  have 
  a 
  conchoidal 
  

   fracture, 
  which 
  is 
  not 
  true 
  of 
  the 
  peripheral 
  portions. 
  

   These 
  differences 
  are 
  probably 
  due 
  to 
  leaching 
  of 
  the 
  

   exterior 
  portions 
  of 
  the 
  nodules. 
  

  

  Fossils, 
  chiefly 
  Fusulina, 
  are 
  abundantly 
  scattered 
  

   throughout 
  the 
  chert, 
  their 
  cross-sections 
  making 
  from 
  

   ten 
  to 
  fifteen 
  per 
  cent 
  of 
  the 
  areas 
  of 
  the 
  thin 
  sections 
  and 
  

   fracture 
  surfaces. 
  The 
  distribution 
  is 
  quite 
  uniform. 
  

   While 
  there 
  are 
  patches 
  where 
  they 
  are 
  not 
  abundant, 
  

   these 
  relatively 
  barren 
  portions 
  are 
  not 
  confined 
  to 
  any 
  

   particular 
  parts 
  of 
  the 
  nodules, 
  so 
  that 
  as 
  many 
  fossils 
  

   are 
  in 
  the 
  bottom 
  portions 
  of 
  a 
  nodule 
  as 
  in 
  the 
  top. 
  

   There 
  is 
  no 
  arrangement 
  of 
  the 
  fossils 
  into 
  bands, 
  and 
  the 
  

   fusiform 
  shells 
  of 
  Fusulina 
  are 
  oriented 
  so 
  that 
  the 
  axes 
  

   of 
  symmetry 
  are 
  parallel 
  neither 
  to 
  a 
  line, 
  plane, 
  or 
  

   curved 
  surface, 
  the 
  shells 
  being 
  oriented 
  in 
  every 
  possible 
  

   direction. 
  The 
  fossils 
  are 
  also 
  composed 
  of 
  chert. 
  

  

  The 
  preservation 
  of 
  the 
  Fusulina 
  is 
  extremely 
  excel- 
  

   lent, 
  being 
  generally 
  better 
  in 
  the 
  cherts 
  than 
  the 
  enclos- 
  

   ing 
  limestone, 
  and 
  some 
  of 
  the 
  most 
  delicate 
  details 
  of 
  

   structure 
  are 
  shown. 
  The 
  concentric 
  walls 
  of 
  the 
  shells 
  

   are 
  perpendicularly 
  perforated 
  by 
  small 
  threads 
  of 
  limo- 
  

   nite. 
  These 
  thread-like 
  structures 
  are 
  considered 
  to 
  be 
  

   fillings 
  of 
  the 
  original 
  pseudopedial 
  openings 
  of 
  the 
  test. 
  

   They 
  are 
  wanting 
  in 
  the 
  radial 
  portions. 
  The 
  material 
  

   composing 
  the 
  main 
  body 
  of 
  both 
  types 
  of 
  wall 
  is 
  of 
  a 
  

  

  