﻿ENDLicH.] 
  CONCLUSION-^VOLCANICS. 
  213 
  

  

  an 
  alteration 
  of 
  tlie 
  former 
  has 
  generally 
  been 
  observed. 
  This, 
  however, 
  

   is 
  invariably 
  local, 
  confined 
  to 
  narrow 
  limits 
  vertically. 
  Were 
  it 
  possi- 
  

   ble 
  to 
  make 
  examination 
  below 
  the 
  present 
  surface 
  of 
  the 
  trachorheites, 
  

   we 
  could, 
  no 
  doubt, 
  reproduce 
  a 
  comparatively 
  accurate 
  picture 
  of 
  the 
  

   configuration 
  of 
  the 
  country 
  before 
  the 
  period 
  of 
  eruption. 
  Natural 
  

   agents 
  have 
  furnished 
  us 
  with 
  too 
  limited 
  a 
  number 
  of 
  outcrops 
  to 
  

   form 
  any 
  but 
  the 
  most 
  general 
  idea 
  as 
  to 
  the 
  distribution 
  of 
  ranges 
  and 
  

   valleys 
  at 
  that 
  time. 
  It 
  can 
  scarcely 
  even 
  be 
  made 
  out 
  with 
  any 
  degree 
  

   of 
  accuracy 
  where 
  the 
  largest 
  masses 
  of 
  mountains 
  or 
  hills, 
  now 
  covered, 
  

   were 
  to 
  be 
  found. 
  So 
  much 
  is 
  certain, 
  however, 
  that 
  the 
  Quartzite 
  

   Mountains 
  were 
  then 
  already 
  too 
  high 
  to 
  be 
  invaded 
  by 
  the 
  flowing 
  

   lavas, 
  and 
  that 
  they 
  were 
  the 
  highest 
  group 
  of 
  the 
  entire 
  region. 
  To- 
  

   ward 
  the 
  north 
  they 
  sloped 
  oif 
  in 
  ridges, 
  that 
  now 
  appear 
  as 
  isolated 
  

   outcrops 
  of 
  metamorphic 
  rocks. 
  At 
  the 
  southern 
  end 
  of 
  the 
  Sawatch 
  

   Eange 
  we 
  find 
  another 
  locality 
  that 
  must 
  have 
  been 
  too 
  high 
  (station 
  

   94) 
  for 
  the 
  trachorheites 
  to 
  cover 
  with 
  any 
  heavy 
  beds. 
  Possibly 
  that 
  

   and 
  the 
  Quartzite 
  Mountains 
  were 
  in 
  connection 
  at 
  the 
  time; 
  if 
  so, 
  

   there 
  existed 
  a 
  low 
  depression 
  in 
  the 
  region 
  where 
  now 
  Pagosa 
  Peak 
  

   rises 
  to 
  an 
  elevation 
  of 
  more 
  than 
  13,000 
  feet. 
  

  

  A 
  very 
  interesting 
  feature 
  of 
  this 
  region 
  is 
  the 
  trachytic 
  conglom- 
  

   erate 
  occurring 
  with 
  such 
  great 
  regularity 
  along 
  the 
  north 
  and 
  south- 
  

   western 
  edge 
  of 
  the 
  volcanic 
  area. 
  It 
  is 
  directly 
  included 
  between 
  two 
  

   series 
  of 
  trachorhe'itic 
  flows, 
  and 
  is 
  composed 
  entirely 
  of 
  material 
  orig- 
  

   inating 
  from 
  the 
  lower. 
  Evidently 
  the 
  conglomerate 
  was 
  deposited 
  by 
  

   water 
  that 
  flowed 
  over 
  the 
  trachorheites, 
  as 
  its 
  composition 
  fully 
  proves. 
  

   Whether 
  it 
  was 
  deposited 
  into 
  water 
  might 
  seem 
  doubtful, 
  considering 
  

   the 
  absence 
  of 
  all 
  animal 
  remains. 
  Structural 
  character, 
  however, 
  ad- 
  

   mits 
  of 
  no 
  doubt 
  that 
  it 
  was 
  deposited 
  into 
  either 
  gently-moving 
  or 
  still 
  

   water, 
  more 
  probably 
  the 
  former. 
  It 
  seems, 
  judging 
  from 
  the 
  physical 
  

   character 
  of 
  the 
  conglomerate, 
  that 
  its 
  deposition 
  must 
  have 
  taken 
  place 
  

   during 
  a 
  comparatively 
  short 
  period 
  of 
  time. 
  Comparing 
  the 
  eruption 
  

   of 
  all 
  this 
  volcanic 
  material 
  with 
  i^henomena 
  observed 
  in 
  connection 
  with 
  

   active 
  volcanoes 
  of 
  the 
  present 
  time, 
  it 
  is 
  but 
  reasonable 
  to 
  suppose, 
  

   that 
  at 
  certain 
  stages 
  of 
  the 
  expulsion 
  of 
  lava, 
  bowlders, 
  fragments, 
  and 
  

   " 
  ashy 
  " 
  lava 
  were 
  ejected. 
  In 
  reality, 
  we 
  do 
  find 
  deposits 
  that 
  very 
  

   closely 
  resemble 
  the 
  " 
  ash 
  " 
  from 
  existing 
  volcanoes. 
  If 
  this 
  was 
  the 
  

   case, 
  it 
  would 
  have 
  been 
  an 
  easy 
  matter 
  for 
  the 
  waters 
  flowing 
  over 
  the 
  

   hardened 
  lava 
  to 
  remove 
  in 
  a 
  short 
  time 
  a 
  vast 
  amount 
  of 
  material, 
  

   which 
  was 
  then 
  deposited 
  at 
  the 
  places 
  most 
  favorably 
  situated. 
  

  

  As 
  to 
  the 
  origin 
  of 
  this 
  mass 
  of 
  volcanic 
  rocks, 
  I 
  have 
  no 
  occasion 
  to 
  

   change 
  the 
  suggestion 
  made* 
  three 
  years 
  ago. 
  Although 
  I 
  am 
  not 
  at 
  

   present 
  able 
  to 
  prove 
  conclusively 
  the 
  hypothesis 
  that 
  we 
  have 
  in 
  the 
  

   trachorheites 
  nothing 
  but 
  a 
  highly-fused 
  granite, 
  the 
  entiire 
  licibitus 
  of 
  

   the 
  formation, 
  and 
  the 
  constancy 
  of 
  its 
  constituents 
  shown 
  upon 
  ulti- 
  

   mate 
  analysis, 
  are 
  so 
  characteristic, 
  and 
  so 
  closely 
  agree 
  with 
  observa- 
  

   tions 
  on 
  Archaean 
  groups, 
  that 
  I 
  cannot 
  otherwise 
  than 
  regard 
  the 
  view 
  

   formerly 
  expressed 
  as 
  correct 
  in 
  the 
  main. 
  Dr. 
  Oscar 
  Loewt 
  says: 
  

   " 
  Here 
  [Burro 
  Mountains, 
  N'. 
  Ilex.) 
  the 
  rock 
  {rJiyoUte) 
  exhibits 
  a 
  close 
  

   relation 
  to 
  the 
  granite 
  which 
  it 
  overlies, 
  inasmuch 
  as 
  it 
  incloses 
  semi- 
  

   fused 
  fragments 
  of 
  the 
  latter. 
  Moreover, 
  we 
  can 
  trace 
  quite 
  distinctly 
  

   the 
  efl"ects 
  of 
  various 
  degrees 
  of 
  heat 
  upon 
  masses 
  of 
  feldspar, 
  which 
  

   have, 
  in 
  some 
  instances, 
  assumed 
  a 
  glassy 
  appearance; 
  extensive 
  veins 
  

   of 
  quartz 
  also 
  penetrate 
  the 
  rhyolite. 
  From 
  this 
  it 
  would 
  appear 
  that 
  

   we 
  here 
  have 
  a 
  granite 
  with 
  partial 
  transformation 
  into 
  a 
  rhyolite." 
  

  

  * 
  Report 
  United 
  States 
  Geological 
  Survey 
  1873, 
  p. 
  350. 
  

  

  t 
  Explorations 
  and 
  Surveys 
  West 
  of 
  One 
  Hundredth 
  Meridian, 
  vol. 
  iii, 
  1875, 
  p. 
  641. 
  

  

  