﻿Spencer 
  — 
  Devonian 
  Strata 
  in 
  Colorado. 
  129 
  

  

  The 
  Ouray 
  limestone 
  is 
  a 
  massive 
  limestone 
  varying 
  in 
  

   thickness 
  from 
  100 
  to 
  300 
  feet. 
  In 
  places 
  the 
  limestone 
  is 
  in 
  

   one 
  massive 
  layer, 
  while 
  elsewhere 
  it 
  is 
  in 
  two 
  or 
  three 
  heavy 
  

   bands 
  separated 
  by 
  greenish 
  crumbling 
  marls. 
  The 
  limestone 
  

   is 
  usually 
  white, 
  but 
  is 
  sometimes 
  stained 
  red 
  or 
  pink. 
  Certain 
  

   strata 
  are 
  somewhat 
  coarsely 
  crystalline, 
  but 
  as 
  a 
  rule 
  the 
  lime- 
  

   stone 
  is 
  fine-grained. 
  The 
  chemical 
  composition 
  is 
  variable, 
  as 
  

   is 
  shown 
  by 
  the 
  careful 
  selection 
  of 
  certain 
  portions 
  for 
  use 
  in 
  

   the 
  smelter 
  at 
  Durango. 
  The 
  discarded 
  portion 
  is 
  said 
  to 
  be 
  

   too 
  siliceous 
  for 
  metallurgical 
  use. 
  

  

  The 
  total 
  thickness 
  of 
  the 
  Devonian 
  in 
  the 
  Animas 
  region 
  

   varies 
  from 
  200 
  to 
  nearly 
  500 
  feet, 
  the 
  former 
  figure 
  repre- 
  

   senting 
  the 
  usual 
  development. 
  At 
  Ouray 
  the 
  thickness 
  is 
  not 
  

   far 
  from 
  200 
  feet. 
  

  

  The 
  fossils 
  which 
  have 
  enabled 
  a 
  determination 
  of 
  the 
  age 
  

   of 
  this 
  series 
  are 
  found 
  in 
  the 
  limestone 
  a 
  short 
  distance 
  below 
  

   the 
  top. 
  Neither 
  the 
  extreme 
  upper 
  or 
  lower 
  layers 
  have 
  thus 
  

   far 
  yielded 
  organic 
  remains. 
  In 
  the 
  zone 
  in 
  which 
  the 
  shells 
  

   occur 
  there 
  are 
  several 
  thin 
  layers 
  which 
  are 
  more 
  coarsely 
  

   crystalline 
  than 
  the 
  mass 
  of 
  the 
  limestone, 
  and 
  these 
  are 
  very 
  

   frequently 
  fossiliferous. 
  The 
  fossils 
  are 
  never 
  prominent 
  to 
  

   casual 
  observation, 
  but 
  when 
  their 
  horizon 
  is 
  once 
  located 
  and 
  

   patiently 
  searched, 
  it 
  usually 
  proves 
  to 
  be 
  quite 
  productive. 
  

   In 
  some 
  places, 
  however, 
  where 
  many 
  fossils 
  are 
  present, 
  col- 
  

   lecting 
  proves 
  unsatisfactory 
  because 
  of 
  minute 
  jointing 
  which 
  

   makes 
  it 
  impossible 
  to 
  secure 
  perfect 
  specimens. 
  A 
  list 
  of 
  the 
  

   forms 
  which 
  have 
  been 
  recognized 
  is 
  here 
  given 
  : 
  

  

  Streptela&ma 
  sp. 
  Jlytilarca? 
  sp. 
  

  

  Schizopfioria 
  stria 
  tula. 
  Paracyclas 
  sp. 
  

  

  Orthothetes 
  chemungensis 
  var. 
  Modiomorpha 
  sp. 
  

  

  Productella 
  semiglobosa. 
  Allorisma 
  sp. 
  

  

  Productella 
  subalata? 
  Euomphalus 
  clymenioides. 
  

  

  Spirifer 
  dixjunctus 
  var. 
  Ani- 
  Euomphalus 
  Decewyi. 
  

  

  masensis. 
  Pleurotomaria 
  sp. 
  

  

  Spirifer 
  coniculus. 
  Naticopsis 
  ? 
  humilis. 
  

  

  Athyris 
  color 
  adoznsis. 
  Natieopsis 
  gigantea. 
  

  

  Athyris 
  vittata 
  var. 
  Bellerophon 
  sp. 
  

  

  Camarotcechia 
  {JRhynconella) 
  Orthoceras 
  sp. 
  a. 
  

  

  Endlichi. 
  Orthoceras 
  sp. 
  b. 
  

  

  Camarotcechia 
  cont/acta 
  f 
  Orthoceras 
  sp. 
  c. 
  

  

  Devonian 
  Strata 
  in 
  Central 
  Colorado. 
  — 
  In 
  the 
  course 
  of 
  

   the 
  geological 
  studies 
  which 
  have 
  been 
  made 
  from 
  time 
  to 
  

   time 
  in 
  Central 
  Colorado, 
  under 
  the 
  direction 
  of 
  Mr. 
  S. 
  F. 
  

   Emmons 
  of 
  the 
  U. 
  S. 
  Geological 
  Survey, 
  the 
  determination 
  of 
  

   the 
  presence 
  or 
  absence 
  of 
  Devonian 
  rocks 
  was 
  a 
  problem 
  most 
  

   difficult 
  of 
  satisfactory 
  solution. 
  Only 
  the 
  recent 
  work 
  at 
  

  

  