﻿Spencer 
  — 
  Devonian 
  Strata 
  in 
  Colorado. 
  127 
  

  

  be 
  quite 
  analogous 
  to 
  the 
  relations 
  observed 
  between 
  the 
  two 
  

   formations 
  in 
  other 
  places. 
  

  

  The 
  most 
  important 
  areas 
  of 
  Devonian 
  lie 
  in 
  the 
  region 
  

   south 
  and 
  west 
  of 
  the 
  pre-Paleozoic 
  core 
  of 
  the 
  Needle 
  

   Mountains. 
  (Quartzite 
  Mountains 
  of 
  the 
  Hayden 
  map.) 
  The 
  

   Ouray 
  limestone 
  has 
  been 
  noted 
  as 
  far 
  east 
  as 
  Los 
  Pinos 
  river, 
  

   and 
  may 
  be 
  traced 
  across 
  the 
  Florida 
  river, 
  and 
  still 
  westward 
  

   to 
  the 
  Animas, 
  where 
  it 
  was 
  first 
  observed 
  by 
  the 
  writer. 
  

   Detailed 
  studies 
  have 
  not 
  been 
  made 
  throughout 
  this 
  region, 
  

   but 
  the 
  two 
  lower 
  members 
  of 
  the 
  section 
  are 
  known 
  to 
  be 
  

   locally 
  absent. 
  All 
  of 
  them 
  are 
  present 
  along 
  the 
  west 
  side 
  of 
  

   the 
  Animas 
  river 
  for 
  about 
  twenty-five 
  miles, 
  from 
  their 
  

   southernmost 
  outcrop 
  about 
  thirteen 
  miles 
  north 
  of 
  Durango, 
  

   at 
  Pinkerton 
  Springs, 
  to 
  where 
  they 
  are 
  covered 
  by 
  volcanic 
  

   rocks 
  of 
  Tertiary 
  age, 
  just 
  south 
  of 
  Silverton. 
  In 
  the 
  vicinity 
  

   of 
  Ouray 
  this 
  formation 
  rests 
  upon 
  the 
  Algonkian 
  rocks, 
  the 
  

   lower 
  beds 
  being 
  absent. 
  Southeast 
  of 
  Silverton, 
  in 
  Deer 
  

   Park, 
  and 
  along 
  the 
  Bare 
  creek 
  trail, 
  the 
  quartzite 
  is 
  locally 
  

   missing, 
  while 
  the 
  shaly 
  series 
  is 
  always 
  present 
  underneath 
  the 
  

   limestone. 
  Still 
  farther 
  east 
  and 
  more 
  to 
  the 
  north, 
  upon 
  the 
  

   headwaters 
  of 
  the 
  Lake 
  Fork 
  of 
  the 
  Gunnison 
  river, 
  none 
  of 
  

   the 
  Devonian 
  formations 
  are 
  found 
  in 
  place, 
  but 
  several 
  huge 
  

   blocks 
  of 
  quartzite 
  and 
  limestone 
  like 
  those 
  of 
  the 
  Devonian 
  

   were 
  observed 
  in 
  the 
  lower 
  part 
  of 
  the 
  great 
  complex 
  of 
  effu- 
  

   sive 
  rocks 
  which 
  covers 
  that 
  region. 
  Apparently 
  these 
  were 
  

   wrenched 
  off 
  during 
  some 
  eruption 
  and 
  floated 
  up 
  by 
  the 
  

   fluid 
  or 
  viscous 
  rhyolite 
  lava, 
  to 
  be 
  subsequently 
  covered 
  by 
  

   streams 
  of 
  andesite 
  and 
  similar 
  rhyolite. 
  The 
  Ouray 
  lime- 
  

   stone 
  has 
  also 
  been 
  identified 
  in 
  the 
  marbleized 
  limestones 
  

   which 
  appear 
  near 
  the 
  center 
  of 
  the 
  remarkable 
  structural 
  

   dome 
  at 
  Pico, 
  some 
  fifteen 
  miles 
  west 
  of 
  the 
  Animas 
  valley, 
  

   opposite 
  the 
  Needle 
  mountains. 
  The 
  known 
  occurrences 
  of 
  

   the 
  Devonian 
  thus 
  indicate 
  its 
  distribution 
  over 
  an 
  area 
  of 
  

   approximately 
  six 
  hundred 
  square 
  miles 
  in 
  the 
  San 
  Juan 
  

   region, 
  with 
  every 
  reason 
  for 
  supposing 
  an 
  actual 
  extent 
  very 
  

   much 
  in 
  excess 
  of 
  this. 
  

  

  The 
  relations 
  of 
  the 
  formations 
  indicate 
  an 
  overlap, 
  and, 
  

   therefore, 
  an 
  advancing 
  sea 
  during 
  the 
  period 
  of 
  their 
  deposi- 
  

   tion 
  ; 
  however, 
  the 
  vertical 
  element 
  of 
  the 
  overlap 
  was, 
  at 
  the 
  

   most, 
  not 
  more 
  than 
  200 
  feet. 
  Thus, 
  the 
  relations 
  of 
  the 
  sedi- 
  

   ments 
  to 
  their 
  basement 
  is 
  indicative 
  of 
  a 
  surface 
  of 
  very 
  low 
  

   relief 
  in 
  the 
  pre-Devonian 
  topography. 
  

  

  The 
  quartzite 
  which 
  forms 
  the 
  basal 
  member 
  of 
  the 
  series 
  

   here 
  considered 
  varies 
  in 
  thickness 
  from 
  zero 
  to 
  perhaps 
  100 
  

   feet. 
  It 
  is 
  typically 
  a 
  massive 
  white 
  or 
  reddish 
  quartzite 
  in 
  

   which 
  a 
  well 
  marked 
  jointing 
  is 
  usually 
  developed. 
  It 
  is 
  com- 
  

   posed 
  almost 
  entirely 
  of 
  quartz 
  and 
  its 
  grain 
  is 
  rather 
  fine 
  and 
  

  

  