﻿ENDLicn] 
  SAN 
  JUAN 
  REGION— 
  DRIFT. 
  191 
  

  

  by 
  heat, 
  and 
  are 
  therefore 
  better 
  able 
  to 
  resist 
  atmospheric 
  agents 
  than 
  

   the 
  unaltered 
  portions 
  adjoining. 
  No 
  features 
  that 
  are 
  either 
  new, 
  or 
  

   even 
  of 
  i)articular 
  interest, 
  were 
  observed 
  in 
  connection 
  with 
  the 
  dikes 
  

   of 
  this 
  region. 
  

  

  DRIFT. 
  

  

  Along 
  the 
  western 
  base 
  of 
  the 
  Sawatch 
  Range, 
  avalanchial 
  and 
  river 
  

   drift 
  obscures 
  the 
  contact 
  between 
  the 
  sedimentary 
  and 
  volcanic 
  beds. 
  

   It 
  is 
  derived 
  entirely 
  from 
  the 
  mountains 
  immediately 
  adjoining, 
  and 
  

   often 
  deposited 
  in 
  bluff-ridges 
  of 
  considerable 
  relative 
  height. 
  

  

  South 
  of 
  the 
  Tierra 
  Aniarilla 
  drift 
  was 
  noticed, 
  on 
  a 
  number 
  of 
  the 
  

   the 
  Fox 
  Hills 
  bluffs, 
  that 
  slope 
  southward 
  and 
  show 
  steep 
  faces 
  toward 
  

   the 
  north. 
  Upon 
  examination, 
  the 
  rounded, 
  well-worn 
  bowlders 
  and 
  

   pebbles 
  proved 
  to 
  be 
  granite, 
  gneiss, 
  quartzite, 
  micaceous, 
  hornblendic, 
  

   and 
  chloritic 
  schists, 
  i. 
  e., 
  all 
  of 
  them 
  metamorphic. 
  Trachyte 
  and 
  

   basalt 
  were 
  represented 
  but 
  sparingly. 
  If 
  we 
  assume 
  that 
  at 
  the 
  time 
  

   posterior 
  to 
  the 
  outflow 
  of 
  basalt 
  in 
  that 
  region, 
  the 
  ancient 
  plateau 
  

   alluded 
  to 
  in 
  speaking 
  of 
  the 
  Cerro 
  del 
  Navajo 
  was 
  still 
  in 
  existence, 
  

   we 
  can 
  satisfactorily 
  explain 
  the 
  presence 
  of 
  this 
  drift. 
  The 
  metamorphic 
  

   group 
  upon 
  which 
  station 
  94 
  was 
  located, 
  near 
  the 
  junction 
  of 
  the 
  Chamo. 
  

   and 
  Brazos, 
  was 
  probably 
  never 
  entirely 
  covered 
  by 
  either 
  trachyte 
  or 
  

   basalt. 
  It 
  remained 
  as 
  an 
  island, 
  surrounded 
  on 
  all 
  sides 
  by 
  volcanic 
  rocks. 
  

   Mention 
  has 
  been 
  made 
  (Chapter 
  III) 
  of 
  glacial 
  evidence 
  observed 
  on 
  and 
  

   near 
  that 
  station. 
  It 
  is 
  possible, 
  therefore, 
  granting 
  the 
  first 
  premise, 
  

   the 
  existence 
  of 
  the 
  southward-sloping 
  plateau, 
  that 
  either 
  flowing 
  

   water 
  or 
  moving 
  ice 
  could 
  have 
  transported 
  the 
  erratic, 
  metamorphic 
  

   material 
  to 
  the 
  localities 
  where 
  we 
  now 
  find 
  it. 
  I 
  am 
  inclined 
  to 
  the 
  

   former 
  view. 
  An 
  absence 
  of 
  all 
  defined 
  moraines, 
  and 
  the 
  comparatively 
  

   small 
  size 
  of 
  the 
  bowlders, 
  lead 
  to 
  the 
  inference 
  that 
  probably 
  water 
  

   effected 
  the 
  transportation, 
  and 
  not 
  ice. 
  It 
  would 
  be 
  utterly 
  impossible 
  

   for 
  any 
  stream 
  of 
  the 
  present 
  drainage-system 
  to 
  carry 
  these 
  erratics 
  to 
  

   the 
  summits 
  of 
  the 
  mesas. 
  Even 
  if 
  we 
  allow 
  all 
  possible 
  latitude 
  for 
  

   changes 
  of 
  niveau, 
  of 
  course 
  and 
  quantity 
  of 
  water, 
  we 
  cannot 
  accept 
  

   such 
  an 
  explanation. 
  Two 
  views 
  only 
  are 
  satisfactory 
  : 
  either 
  the 
  drift 
  

   came 
  from 
  some 
  region 
  farther 
  south, 
  or, 
  if 
  it 
  came 
  from 
  the 
  metamorphic 
  

   group 
  near 
  the 
  Chama 
  and 
  Brazos, 
  the 
  plateau 
  must 
  then 
  have 
  been 
  in 
  

   existence, 
  must 
  have 
  extended 
  to 
  the 
  very 
  base 
  of 
  the 
  mountains, 
  and 
  

   was 
  eroded 
  away 
  along 
  that 
  base 
  during 
  the 
  period 
  of 
  the 
  gradual 
  ele- 
  

   vation 
  of 
  the 
  mountain-range. 
  Of 
  these 
  two 
  views, 
  I 
  hold 
  the 
  latter. 
  It 
  

   is, 
  at 
  the 
  same 
  time, 
  an 
  additional 
  argument 
  for 
  the 
  theory 
  of 
  a 
  Post-Cre- 
  

   taceous 
  elevation 
  of 
  the 
  main 
  Rocky 
  Mountain 
  chain. 
  The 
  geological 
  

   age 
  of 
  our 
  volcanic 
  rocks 
  has 
  been 
  sufficiently 
  well 
  established 
  (at 
  least 
  

   in 
  Colorado) 
  to 
  admit 
  of 
  their 
  being 
  utilized 
  as 
  direct 
  evidence 
  regfarding 
  

   geological 
  age, 
  together 
  with 
  the 
  sedimentary 
  formations. 
  

  

  In 
  the 
  caiious 
  and 
  valleys 
  of 
  some 
  of 
  the 
  rivers, 
  drift, 
  both 
  river-drift 
  

   and 
  alluvial, 
  has 
  accumulated, 
  choosing 
  such 
  places 
  that 
  would 
  furnish 
  

   the 
  greatest 
  local 
  inducements 
  for 
  its 
  deposition. 
  

  

  