﻿128 
  Spencer 
  — 
  Devonian 
  Strata 
  in 
  Colorado. 
  

  

  uniform. 
  Local 
  variations 
  are 
  observed 
  where 
  the 
  sandstone 
  

   is 
  flaggy, 
  and 
  conglomerates 
  of 
  medium 
  coarseness 
  are 
  not 
  rare 
  

   in 
  the 
  lower 
  part. 
  Near 
  the 
  southernmost 
  exposures 
  in 
  the 
  

   vicinity 
  of 
  Rockwood, 
  the 
  contact 
  with 
  the 
  basement 
  rock, 
  

   which 
  at 
  that 
  place 
  is 
  granite, 
  is 
  marked 
  by 
  a 
  basal 
  conglom- 
  

   erate 
  some 
  six 
  feet 
  in 
  thickness. 
  The 
  bowlders 
  in 
  this 
  con- 
  

   glomerate, 
  which 
  have 
  a 
  diameter 
  of 
  six 
  inches 
  or 
  more, 
  are 
  

   confined 
  to 
  the 
  slight 
  depressions 
  in 
  the 
  floor. 
  Above 
  the 
  

   conglomerate, 
  and 
  running 
  over 
  the 
  tops 
  of 
  the 
  knolls, 
  there 
  

   comes 
  a 
  fine-grained 
  sandstone 
  or 
  quartzite. 
  At 
  the 
  top 
  of 
  the 
  

   quartzite 
  there 
  is 
  an 
  alteration 
  of 
  red 
  or 
  gray 
  earthy 
  limestone 
  

   with 
  reddish 
  shales 
  for 
  a 
  thickness 
  of 
  a 
  few 
  feet. 
  The 
  lowest 
  

   of 
  the 
  earthy 
  limestones 
  is 
  luted 
  onto 
  the 
  topmost 
  layer 
  of 
  the 
  

   quartzite. 
  The 
  materials 
  of 
  this 
  basal 
  formation 
  are 
  remark- 
  

   able 
  in 
  that 
  they 
  contain 
  no 
  fragments 
  of 
  feldspar 
  though 
  

   they 
  rest 
  directly 
  upon 
  a 
  coarse 
  granite, 
  and 
  that 
  the 
  bowlders 
  

   were 
  not 
  derived 
  from 
  the 
  rocks 
  which 
  immediately 
  underlie 
  

   the 
  conglomerate. 
  Such 
  a 
  formation 
  is 
  not 
  the 
  result 
  of 
  a 
  

   rapidly 
  advancing 
  coast-line, 
  but 
  is 
  rather 
  the 
  effect 
  of 
  activi- 
  

   ties 
  which 
  must 
  have 
  been 
  long 
  continued. 
  

  

  The 
  occurrence 
  of 
  fish 
  scales 
  in 
  the 
  sandstone 
  of 
  the 
  basal 
  

   formation 
  was 
  reported 
  by 
  Endlich, 
  and 
  a 
  single 
  slab 
  exhibit- 
  

   ing 
  fish 
  remains 
  was 
  found 
  by 
  us 
  in 
  a 
  talus 
  pile 
  at 
  the 
  base 
  of 
  

   a 
  quartzite 
  cliff 
  near 
  Rockwood, 
  on 
  the 
  Silverton 
  branch 
  of 
  

   the 
  Denver 
  and 
  Rio 
  Grande 
  Railroad. 
  The 
  only 
  other 
  fossils 
  

   from 
  the 
  quartzite 
  are 
  very 
  imperfect 
  impressions, 
  which 
  Mr. 
  

   Walcott 
  is 
  inclined 
  to 
  refer 
  to 
  Cruziana 
  or 
  Rusophycus, 
  sup- 
  

   posed 
  plants 
  which 
  are 
  respectively 
  of 
  Silurian 
  and 
  Cambrian 
  

   types. 
  

  

  The 
  middle 
  or 
  shaly 
  member 
  varies 
  in 
  thickness 
  from 
  25 
  

   to 
  perhaps 
  100 
  feet 
  in 
  the 
  Animas 
  region. 
  As 
  already 
  stated, 
  

   it 
  is 
  missing 
  in 
  the 
  Uncompahgre 
  region. 
  The 
  shale 
  series 
  is 
  

   very 
  imperfectly 
  exposed 
  upon 
  the 
  west 
  side 
  of 
  the 
  Animas 
  

   river, 
  but 
  upon 
  the 
  east 
  side 
  it 
  may 
  be 
  studied 
  at 
  several 
  places. 
  

   It 
  is 
  especially 
  well 
  exposed 
  in 
  the 
  high 
  basins 
  at 
  the 
  head 
  of 
  

   Deer 
  Park, 
  southeast 
  of 
  Silverton. 
  It 
  consists 
  of 
  calcareous 
  

   marls 
  and 
  flaggy 
  shales 
  containing 
  an 
  occasional 
  band 
  of 
  white 
  

   sandstone. 
  In 
  color 
  it 
  has 
  a 
  somewhat 
  variegated 
  appearance, 
  

   due 
  to 
  alternating 
  bands 
  of 
  red, 
  green 
  and 
  gray 
  color. 
  Wher- 
  

   ever 
  it 
  is 
  exposed, 
  or 
  wherever 
  talus 
  derived 
  from 
  it 
  is 
  found, 
  

   it 
  may 
  be 
  readily 
  recognized 
  from 
  its 
  general 
  appearance. 
  One 
  

   of 
  the 
  most 
  characteristic 
  features 
  is 
  the 
  appearance 
  of 
  slabs 
  

   of 
  sandy 
  shale, 
  which 
  contain 
  the 
  curious 
  markings 
  that 
  are 
  

   known 
  to 
  represent 
  crystals 
  of 
  halite, 
  showing 
  hopper-shaped 
  

   crystals 
  preserved 
  as 
  pseudomorphs 
  in 
  the 
  substance 
  of 
  the 
  

   rock. 
  

  

  