﻿Twenhofel 
  — 
  Wreford 
  and 
  Foraker 
  Limestones. 
  417 
  

  

  largely 
  obtaining 
  it 
  from 
  silica 
  dissolved 
  in 
  the 
  waters 
  of 
  

   the 
  ocean 
  in 
  which 
  they 
  live, 
  or 
  from 
  silicates 
  held 
  in 
  sus- 
  

   pension; 
  (2) 
  the 
  silica 
  is 
  precipitated 
  directly 
  from 
  the 
  

   sea 
  water 
  through 
  chemical 
  processes; 
  (3) 
  the 
  silica 
  is 
  

   deposited 
  with 
  the 
  sediments 
  as 
  a 
  mechanical 
  precipitate 
  

   of 
  finely 
  divided 
  silica 
  or 
  silicates 
  carried 
  in 
  suspension 
  

   in 
  the 
  sea 
  water, 
  and 
  is 
  subsequently 
  segregated 
  to 
  form 
  

   nodules. 
  

  

  That 
  organic 
  silica 
  in 
  great 
  abundance 
  is 
  deposited 
  

   with 
  sediments 
  hardly 
  needs 
  statement. 
  The 
  immense 
  

   deposits 
  of 
  diatom 
  and 
  radiolarian 
  ooze 
  show 
  the 
  extent 
  

   to 
  which 
  silica 
  now 
  accumulates 
  and 
  nearly- 
  all 
  bottom 
  

   deposits 
  appear 
  to 
  contain 
  siliceous 
  tests 
  in 
  greater 
  or 
  

   less 
  abundance. 
  

  

  That 
  silica 
  is 
  carried 
  by 
  the 
  rivers 
  to 
  the 
  sea 
  in 
  great 
  

   abundance 
  is 
  now 
  well 
  known. 
  River 
  water 
  that 
  does 
  not 
  

   have 
  some 
  silica 
  in 
  solution 
  is 
  extremely 
  rare 
  and 
  great 
  

   quantities 
  of 
  silica 
  are 
  annually 
  delivered 
  to 
  the 
  ocean. 
  

   Of 
  the 
  solid 
  matter 
  in 
  solution 
  in 
  average 
  river 
  water, 
  

   11-67 
  per 
  cent 
  consists 
  of 
  silica, 
  2 
  and 
  the 
  rivers 
  of 
  the 
  

   world 
  annually 
  deliver 
  to 
  the 
  sea 
  dissolved 
  silica 
  to 
  the 
  

   extent 
  of 
  a 
  little 
  more 
  than 
  223 
  million 
  tons. 
  3 
  As 
  there 
  

   is 
  very 
  little 
  silica 
  solution 
  in 
  the 
  ocean 
  water, 
  4 
  it 
  follows 
  

   that 
  essentially 
  all 
  of 
  this 
  is 
  precipitated, 
  and 
  that, 
  too, 
  

   before 
  going 
  very 
  far 
  from 
  the 
  shore. 
  Some 
  of 
  it 
  is 
  

   taken 
  out 
  by 
  organisms, 
  but 
  it 
  seems 
  quite 
  certain 
  that 
  

   considerable 
  quantities 
  are 
  chemically 
  precipitated. 
  Tarr 
  

   has 
  demonstrated 
  5 
  that 
  this 
  precipitation 
  is 
  accomplished 
  

   in 
  ordinary 
  sea 
  water 
  by 
  the 
  salts 
  which 
  are 
  in 
  solution 
  

   therein. 
  The 
  silica 
  probably 
  reaches 
  the 
  bottom 
  as 
  

   extremely 
  tiny 
  particles 
  and 
  is 
  mingled 
  in 
  that 
  form 
  with 
  

   the 
  sediments. 
  

  

  The 
  abrasion 
  brought 
  about 
  by 
  wind 
  and 
  the 
  corrasive 
  

   work 
  of 
  water 
  and 
  ice 
  certainly 
  produce 
  a 
  s:reat 
  deal 
  of 
  

   finely 
  divided 
  silica. 
  Much 
  of 
  this 
  reaches 
  the 
  sea 
  and 
  is 
  

   deposited 
  with 
  other 
  sediments. 
  Possibly 
  this 
  may 
  be 
  

   dissolved 
  and 
  segregated 
  to 
  form 
  chert. 
  

  

  It 
  is 
  certain, 
  therefore, 
  that 
  there 
  is 
  no 
  dearth 
  of 
  silica 
  

   brought 
  to 
  the 
  sea, 
  and 
  that 
  some 
  of 
  this 
  silica 
  ultimately 
  

   becomes 
  segregated 
  to 
  form 
  the 
  chert 
  nodules 
  is 
  equally 
  

  

  2 
  Clarke, 
  F. 
  W., 
  Data 
  of 
  Geochemistry, 
  IT. 
  S. 
  Geol. 
  Survey, 
  Bull. 
  616, 
  

   116, 
  1916. 
  

  

  3 
  Clarke, 
  ibid., 
  115. 
  

  

  4 
  Clarke, 
  ibid., 
  123-125. 
  

  

  5 
  Tarr, 
  W. 
  A., 
  this 
  Journal, 
  44, 
  409445, 
  1917. 
  

  

  