﻿ENDLicH.] 
  SAN 
  JUAN 
  REGION 
  DAKOTA 
  GEOUP. 
  181 
  

  

  related 
  to 
  those 
  given 
  from 
  the 
  Rio 
  Animas 
  region 
  in 
  the 
  report 
  of 
  1874. 
  

   They 
  differ, 
  however, 
  from 
  those 
  described 
  as 
  occurring 
  along 
  the 
  base 
  

   of 
  the 
  Front 
  Range. 
  It 
  has 
  been 
  impossible 
  to 
  retain 
  the 
  systematic 
  

   division 
  of 
  the 
  Cretaceous 
  here 
  that 
  is 
  applicable 
  to 
  the 
  same 
  formation 
  

   farther 
  east 
  and 
  northeast. 
  Three 
  groups 
  have 
  eventually 
  been 
  decided 
  

   upon 
  : 
  Dakota, 
  Colorado, 
  and 
  Fox 
  Hills. 
  The 
  first 
  of 
  these 
  includes 
  

   the 
  heavy 
  beds 
  of 
  white 
  and 
  yellowish 
  sandstones, 
  containing 
  narrow 
  

   interstrata 
  of 
  shales 
  and 
  thin 
  seams 
  of 
  a 
  hard 
  coal 
  in 
  its 
  upper 
  members. 
  

   To 
  the 
  Colorado 
  group 
  we 
  have 
  assigned 
  the 
  heavy 
  strata 
  of 
  dark 
  gray 
  

   shales 
  with 
  characteristic 
  fossils. 
  The 
  Fox 
  Hills 
  group 
  comprises 
  the 
  

   series 
  of 
  yellow 
  and 
  greyish 
  shales 
  interbedded 
  with 
  sandstones 
  and 
  

   showing 
  a 
  number 
  of 
  good 
  coal-beds.* 
  Above 
  these 
  we 
  find 
  in 
  a 
  con- 
  

   tinuous 
  section 
  the 
  lower 
  members 
  of 
  the 
  Tertiary 
  formation, 
  which, 
  

   farther 
  south 
  reach 
  a 
  very 
  considerable 
  development, 
  and 
  become 
  of 
  the 
  

   greatest 
  interest 
  to 
  the 
  palaeontologist. 
  In 
  the 
  district 
  assigned 
  to 
  us 
  

   we 
  were 
  not 
  fortunate 
  enough 
  to 
  find 
  any 
  vertebrate 
  remains 
  in 
  these 
  

   formations, 
  probably 
  being 
  still 
  too 
  low 
  down 
  in 
  the 
  succession 
  of 
  strata. 
  

   A 
  number 
  of 
  interesting 
  stratigraphical 
  relations 
  were 
  noticed 
  in 
  the 
  

   Csetaceous 
  beds 
  under 
  discussion, 
  and 
  they 
  will 
  be 
  referred 
  to 
  at 
  the 
  

   proper 
  place 
  in 
  the 
  subjoined 
  pages. 
  

  

  Baliota 
  (jrowp. 
  — 
  Outcrops 
  belonging 
  to 
  this 
  group 
  were 
  first 
  noticed 
  

   after 
  having 
  crossed 
  Weeminuche 
  Pass. 
  In 
  the 
  district 
  of 
  1874 
  the 
  

   sandstones 
  belonging 
  to 
  it 
  were 
  overlying 
  the 
  Carboniferous 
  strata, 
  

   crowding 
  out 
  the 
  latter 
  more 
  and 
  more 
  as 
  we 
  proceeded 
  northward 
  

   along 
  their 
  western 
  edge. 
  After 
  leaving 
  the 
  metamorphic 
  granite, 
  

   which 
  contains 
  the 
  headwaters 
  of 
  Weeminuche 
  Creek, 
  we 
  found 
  a 
  

   series 
  of 
  thinly-bedded 
  yellow 
  sandstones 
  exposed 
  in 
  a 
  narrow 
  ravine. 
  

   They 
  rested 
  immediately 
  upon 
  the 
  granite, 
  and 
  bore 
  evidence 
  of 
  hav- 
  

   ing 
  been 
  subjected 
  to 
  altering 
  influences. 
  Thin 
  layers 
  of 
  greyish 
  -brown, 
  

   sometimes 
  laminated, 
  shales 
  appear, 
  together 
  with 
  the 
  sandstones, 
  until 
  

   both 
  are 
  lost 
  under 
  the 
  glacial 
  drift 
  of 
  Weeminuche 
  Varlley. 
  On 
  either 
  

   side 
  the 
  lower 
  portions 
  of 
  the 
  bluffs 
  or 
  mountain 
  slopes, 
  inclosing 
  this 
  

   valley, 
  are 
  composed 
  of 
  the 
  same 
  sandstones. 
  From 
  there 
  they 
  con- 
  

   tinue 
  southward, 
  forming 
  the 
  rounded 
  ridge 
  which 
  runs 
  approximately 
  

   parallel 
  with 
  the 
  general 
  course 
  of 
  the 
  Piedra 
  west 
  of 
  this 
  river. 
  

   Densely 
  wooded 
  as 
  it 
  is, 
  the 
  ridge 
  affords 
  but 
  little 
  facility 
  for 
  the 
  study 
  

   of 
  structural 
  geognosy. 
  So 
  far 
  as 
  could 
  be 
  learned, 
  the 
  dip 
  of 
  the 
  sand- 
  

   stone 
  strata 
  is 
  toward 
  the 
  southeast, 
  at 
  an 
  angle 
  of 
  4° 
  to 
  7°. 
  From 
  

   here 
  it 
  connects, 
  turning 
  westward, 
  with 
  the 
  outcrops 
  of 
  the 
  Dakota 
  

   group 
  in 
  the 
  1874 
  district. 
  Newberry 
  t 
  speaks 
  of 
  these 
  sandstone 
  

   strata 
  being 
  on 
  their 
  exposed 
  surface 
  "cut 
  by 
  joints 
  into 
  blocks 
  of 
  

   nearly 
  uniform 
  size." 
  This 
  characteristic 
  feature 
  is 
  certainly 
  noticeable, 
  

   and 
  is 
  the 
  result, 
  probably, 
  of 
  metamorphosing 
  agents 
  primarily, 
  the 
  

   effect 
  of 
  which 
  has 
  been 
  increased 
  by 
  subsequent 
  action 
  of 
  water 
  both 
  

   in 
  a 
  liquid 
  and 
  frozen 
  state. 
  I 
  have 
  been 
  unable 
  to 
  determine 
  any 
  

   definite 
  relation 
  of 
  these 
  "cleavage-planes" 
  with 
  either 
  the 
  dip 
  or 
  strike 
  

   of 
  the 
  sandstone 
  strata. 
  Nor 
  do 
  I 
  regard 
  the 
  phenomenon 
  as 
  essential 
  

   to 
  any 
  one 
  formation 
  in 
  particular. 
  It 
  is 
  merely 
  the 
  expression 
  either 
  

   of 
  metamorphosis, 
  or 
  a 
  certain 
  kind 
  of 
  pressure, 
  or 
  both. 
  

  

  All 
  along 
  this 
  ridge 
  the 
  thickness 
  of 
  the 
  Dakota 
  group 
  reaches 
  over 
  

   1,000 
  feet. 
  As 
  farther 
  west, 
  so 
  here, 
  the 
  upper 
  members 
  show 
  greater 
  

   variety 
  of 
  lithological 
  constitution. 
  Strata 
  of 
  shales, 
  some 
  of 
  them 
  

   showing 
  indications 
  of 
  coal 
  set 
  in, 
  forming, 
  as 
  it 
  were, 
  a 
  transition 
  to 
  

   the 
  heavy 
  beds 
  of 
  shale 
  belonging 
  to 
  the 
  Colorado 
  group. 
  

  

  * 
  Report 
  United 
  States 
  Geological 
  Survey 
  1874, 
  p. 
  2"23. 
  

  

  t 
  Iveport 
  Exploring 
  Expedition 
  Captain 
  Macomb, 
  1859-1876, 
  p. 
  77. 
  

  

  