﻿Twenhofel 
  — 
  Wrefprd 
  and 
  Foraker 
  Limestones. 
  415 
  

  

  to 
  have 
  developed 
  as 
  secondary 
  centers 
  of 
  secretion. 
  

   Tangential 
  sections 
  across 
  these 
  protuberances 
  indicate 
  

   that 
  at 
  these 
  places 
  there 
  is 
  a 
  bulging 
  outward 
  of 
  the 
  con- 
  

   centric 
  bands. 
  The 
  interior 
  in 
  every 
  specimen 
  studied 
  

   is 
  of 
  banded 
  concentric 
  structure, 
  the 
  structural 
  bands 
  

   being 
  commonly 
  wider 
  on 
  the 
  top 
  of 
  a 
  nodule 
  than 
  on 
  the 
  

   bottom 
  side, 
  and 
  laterally 
  generally 
  wider 
  than 
  on 
  top. 
  

   The 
  bands 
  are 
  not 
  always 
  the 
  same 
  widths 
  on 
  horizontally 
  

   opposite 
  sides 
  ; 
  one 
  nodule 
  which 
  was 
  studied 
  having 
  six- 
  

   teen 
  bands 
  on 
  one 
  side 
  extending 
  through 
  three 
  and 
  one- 
  

   half 
  inches 
  of 
  radius. 
  On 
  the 
  other 
  side 
  these 
  merge 
  and 
  

  

  Fig. 
  1. 
  

  

  Fig. 
  1. 
  The. 
  specimen 
  on 
  the 
  right 
  is 
  from 
  the 
  second 
  chert 
  zone 
  of 
  the 
  

   Wreford 
  limestone. 
  The 
  irregular 
  mass 
  crossing 
  the 
  center 
  is 
  limestone, 
  

   the 
  rest 
  is 
  chert. 
  Two 
  small 
  nodules 
  of 
  chert 
  are 
  shown 
  in 
  the 
  limestone. 
  

   The 
  specimen 
  on 
  the 
  left 
  is 
  from 
  the 
  third 
  chert 
  of 
  the 
  Wreford. 
  The 
  

   dark 
  blocks 
  are 
  chert, 
  one 
  with 
  its 
  longest 
  diameter 
  parallel 
  to 
  the 
  bedding, 
  

   the 
  other 
  with 
  its 
  longest 
  diameter 
  transverse 
  thereto. 
  Each 
  specimen 
  is 
  

   adjusted 
  so 
  that 
  the 
  bedding 
  planes 
  are 
  in 
  horizontal 
  position. 
  These 
  two 
  

   chert 
  zones 
  are 
  known 
  to 
  extend 
  over 
  more 
  than 
  a 
  hundred 
  square 
  miles 
  

   and 
  are 
  not 
  more 
  than 
  5 
  feet 
  from 
  each 
  other 
  vertically. 
  Except 
  that 
  each 
  

   is 
  composed 
  of 
  silica, 
  they 
  have 
  no 
  other 
  common 
  feature. 
  The 
  dimensions 
  

   are 
  shown 
  by 
  the 
  6-inch 
  rule. 
  

  

  extend 
  through 
  only 
  five-eighths 
  of 
  an 
  inch 
  of 
  radius. 
  

   In 
  any 
  radial 
  section 
  the 
  bands 
  are 
  not 
  the 
  same 
  width, 
  

   varying 
  in 
  one 
  specimen 
  from 
  three-fourths 
  to 
  one-fourth 
  

   of 
  an 
  inch. 
  The 
  exteriors 
  of 
  exposed 
  nodules 
  are 
  of 
  a 
  

   brown 
  color 
  due 
  to 
  oxidation. 
  The 
  color 
  of 
  the 
  weathered 
  

   interiors 
  is 
  a 
  striped 
  blue 
  and 
  gray, 
  the 
  shades 
  varying, 
  

   depending 
  on 
  degree 
  of 
  exposure. 
  Interiors 
  of 
  freshly 
  

   broken 
  specimens 
  do 
  not 
  have 
  the 
  banding 
  so 
  plainly 
  

   shown 
  and 
  are 
  commonly 
  of 
  dark 
  gray-blue 
  color. 
  

   Weathered 
  nodules 
  are 
  porous, 
  the 
  gray 
  areas 
  more 
  than 
  

  

  