﻿ECONOMIC 
  GEOLOGY 
  197 
  

  

  nous 
  matter. 
  These 
  foreign 
  materials 
  may 
  enter 
  into 
  their 
  com- 
  

   position 
  to 
  such 
  an 
  extent 
  as 
  to 
  give 
  character 
  to 
  the 
  mass, 
  and 
  

   hence 
  they 
  are 
  said 
  to 
  be 
  silicious, 
  argillaceous, 
  ferruginous, 
  

   magnesian, 
  dolomitic, 
  and 
  bituminous. 
  

  

  The 
  chemical 
  composition 
  is 
  subject 
  to 
  great 
  variation, 
  and 
  

   there 
  is 
  an 
  almost 
  endless 
  series 
  of 
  gradation 
  between 
  these 
  

   various 
  kinds. 
  Thus, 
  the 
  magnesium 
  carbonate 
  may 
  vary 
  in 
  

   quantity 
  from 
  a 
  trace, 
  to 
  the 
  full 
  percentage 
  of 
  a 
  typical 
  dolomite. 
  

   Or, 
  the 
  silica 
  may 
  range 
  from 
  a 
  fractional 
  percentage 
  to 
  the 
  ex- 
  

   treme 
  limit 
  where 
  the 
  stone 
  becomes 
  a 
  calcareous 
  sandstone. 
  

   Crystallized 
  minerals, 
  as 
  mica, 
  quartz, 
  talc, 
  serpentine 
  and 
  

   others, 
  also 
  occur, 
  particularly 
  in 
  the 
  more 
  crystalline 
  limestone. 
  

  

  In 
  color 
  there 
  is 
  a 
  wide 
  variation 
  — 
  from 
  the 
  white 
  of 
  the 
  more 
  

   nearly 
  pure 
  carbonate 
  of 
  lime 
  through 
  gray, 
  blue, 
  yellow, 
  red, 
  

   brown, 
  and 
  to 
  black. 
  The 
  color 
  is 
  dependent 
  upon 
  the 
  impuri- 
  

   ties. 
  

  

  The 
  texture 
  also 
  varies 
  greatly. 
  All 
  limestones 
  exhibit 
  a 
  crys- 
  

   talline 
  structure 
  under 
  the 
  microscope, 
  but 
  to 
  the 
  unaided 
  eye 
  

   there 
  are 
  crystalline 
  and 
  massive 
  varieties. 
  There 
  are 
  coarse 
  

   crystalline, 
  fine 
  crystalline, 
  and 
  sub-crystalline 
  varieties, 
  accord- 
  

   ing 
  as 
  the 
  crystals 
  are 
  larger, 
  smaller, 
  or 
  recognized 
  by 
  the 
  aid 
  

   of 
  a 
  magnifying 
  glass 
  only. 
  The 
  terms 
  coarse-grained 
  and 
  fine- 
  

   grained 
  may 
  apply 
  when 
  there 
  is 
  a 
  resemblance 
  to 
  sandstone 
  in 
  

   the 
  granular 
  state 
  of 
  aggregation. 
  Other 
  terms, 
  as 
  saccharoidal 
  

   (like 
  sugar), 
  oolitic, 
  when 
  the 
  mass 
  resembles 
  the 
  roe 
  of 
  a 
  fish; 
  

   crinoidal, 
  made 
  up 
  of 
  the 
  stems 
  of 
  fossil 
  crinoids, 
  also 
  are 
  in 
  use, 
  

   and 
  are 
  descriptive 
  of 
  texture. 
  The 
  state 
  of 
  aggregation 
  of 
  the 
  

   constituent 
  particles 
  varies 
  greatly, 
  and 
  the 
  stone 
  is 
  hard 
  and 
  

   compact, 
  almost 
  like 
  chert, 
  or 
  is 
  loosely 
  held 
  together 
  and 
  crum- 
  

   bles 
  on 
  slight 
  pressure, 
  or 
  again 
  it 
  is 
  dull 
  and 
  earthy 
  as 
  in 
  chalk. 
  

  

  The 
  crystalline, 
  granular 
  limestones, 
  which 
  are 
  susceptible 
  of 
  

   a 
  fine 
  polish, 
  and 
  which 
  are 
  adapted 
  to 
  decorative 
  work, 
  are 
  

   classed 
  as 
  marliles. 
  Inasmuch 
  as 
  the 
  distinction 
  is 
  in 
  part 
  based 
  

   upon 
  the 
  use, 
  it 
  is 
  not 
  sharply 
  defined 
  and 
  scientific. 
  Generally 
  

   the 
  term 
  is 
  restricted 
  to 
  those 
  limestones 
  in 
  which 
  the 
  sediments 
  

   have 
  been 
  altered 
  and 
  so 
  metamorphosed 
  as 
  to 
  have 
  a 
  more 
  or 
  

  

  