﻿4:16 
  Twenhofel 
  — 
  Wreford 
  and 
  Foraker 
  Limestones. 
  

  

  the 
  blue, 
  and 
  on 
  much 
  exposure 
  to 
  the 
  weather 
  the 
  gray 
  

   portions 
  become 
  yellow 
  or 
  some 
  shade 
  of 
  brown. 
  Little 
  

   distinction 
  can 
  be 
  made 
  between 
  the 
  peripheral 
  portions 
  

   and 
  the 
  interior. 
  No 
  parts 
  of 
  these 
  nodules 
  have 
  a 
  dis- 
  

   tinct 
  conchoidal 
  fracture. 
  

  

  Fossils 
  are 
  scattered 
  throughout 
  the 
  chert 
  and 
  occur 
  

   on 
  the 
  surface, 
  many 
  being 
  represented 
  by 
  cavities. 
  A 
  

   form 
  quite 
  common 
  in 
  some 
  nodules 
  is 
  a 
  fenestelloid 
  

   bryozoan. 
  The 
  fossils 
  in 
  the 
  chert 
  are 
  as 
  good 
  as, 
  and 
  

   apparently 
  a 
  little 
  better 
  than, 
  those 
  in 
  the 
  limestone. 
  

   The 
  chert 
  on 
  weathered 
  surfaces 
  is 
  sharply 
  separated 
  

   from 
  the 
  limestone, 
  but 
  fresh 
  fractures 
  across 
  both 
  chert 
  

   and 
  limestone 
  show 
  that 
  each 
  is 
  intergrown 
  with 
  the 
  

   other. 
  Nodules 
  were 
  observed 
  with 
  masses 
  of 
  limestone 
  

   almost 
  completely 
  surrounded 
  by 
  chert, 
  so 
  that 
  it 
  is 
  quite 
  

   probable 
  that 
  nodules 
  exist 
  in 
  which 
  masses 
  of 
  limestone 
  

   are 
  completely 
  enclosed. 
  Quite 
  fresh 
  chert 
  contains 
  a 
  

   great 
  deal 
  of 
  calcite 
  — 
  at 
  least 
  twenty-five 
  per 
  cent 
  — 
  

   scattered 
  throughout, 
  and 
  the 
  solution 
  of 
  this 
  is 
  believed 
  

   to 
  be 
  largely 
  responsible 
  for 
  the 
  cavities 
  in 
  weathered 
  

   nodules, 
  these 
  in 
  places 
  making 
  from 
  twenty-five 
  to 
  fifty 
  

   per 
  cent 
  of 
  the 
  surface 
  of 
  a 
  thin 
  section, 
  the 
  lighter 
  

   colored 
  bands 
  of 
  the 
  chert 
  appearing 
  to 
  be 
  largely 
  due 
  to 
  

   their 
  presence; 
  since, 
  except 
  for 
  the 
  cavities, 
  there 
  is 
  

   little 
  microscopic 
  difference 
  between 
  the 
  lighter 
  and 
  

   darker 
  bands. 
  

  

  The 
  groundmass 
  consists 
  of 
  an 
  aggregate 
  of 
  micro- 
  

   scopic 
  grains 
  of 
  quartz, 
  chalcedony, 
  calcite 
  and 
  limonite. 
  

   The 
  last 
  occurs 
  both 
  as 
  stains 
  and 
  little 
  particles, 
  and 
  in 
  

   places 
  is 
  so 
  abundant 
  as 
  to 
  render 
  the 
  thin 
  sections 
  nearly 
  

   opaque. 
  It 
  is 
  present 
  both 
  in 
  weathered 
  and 
  unweathered 
  

   chert. 
  A 
  little 
  siderite 
  - 
  also 
  appears 
  to 
  be 
  present, 
  but 
  

   the 
  quantity 
  is 
  not 
  great. 
  Sections 
  from 
  unweathered 
  

   chert 
  contain 
  much 
  calcite. 
  Some 
  of 
  the 
  cavities 
  are 
  

   lined 
  with 
  minute 
  crystals 
  of 
  quartz 
  and 
  chalcedonv, 
  mak- 
  

   ing 
  them 
  microscopic 
  geodes. 
  No 
  amorphous 
  silica 
  was 
  

   observed. 
  

  

  Source 
  of 
  the 
  Silica. 
  

  

  Respecting 
  the 
  source 
  of 
  the 
  silica 
  from 
  which 
  -cherts 
  

   are 
  formed, 
  there 
  are 
  three 
  general 
  theories, 
  namely: 
  (1) 
  

   the 
  silica 
  is 
  directly 
  derived 
  from 
  the 
  shells 
  and 
  support- 
  

   ing 
  structures 
  of 
  organisms, 
  such 
  as 
  spicules 
  of 
  sponges 
  

   and 
  shells 
  of 
  radiolarians 
  and 
  diatoms, 
  these 
  organisms 
  

  

  