﻿ESDLicH.] 
  CONCLUSION 
  MINES. 
  215 
  

  

  continues, 
  we 
  will 
  find 
  that, 
  althoup^h 
  rapidly 
  becoming 
  rigid, 
  it 
  is 
  still 
  

   able 
  to 
  form 
  either 
  comparatively 
  high 
  mountains 
  or 
  cover 
  a 
  consider- 
  

   able 
  area, 
  or 
  both. 
  

  

  Regarding 
  the 
  relative 
  age 
  of 
  these 
  volcanic 
  groups, 
  we 
  once 
  more 
  

   have 
  occasion 
  to 
  admire 
  the 
  accuracy 
  of 
  Eichthofen's 
  excellent 
  classifi- 
  

   cation.* 
  At 
  all 
  places 
  where 
  they 
  were 
  observed 
  in 
  our 
  district 
  of 
  1875, 
  

   the 
  succession 
  as 
  given 
  by 
  him 
  was 
  found 
  to 
  hold 
  good. 
  From 
  evidence 
  

   collected, 
  we 
  find 
  that 
  the 
  porphyritic 
  trachyte 
  is 
  younger 
  than 
  the 
  

   trachorheites, 
  older 
  than 
  the 
  basalt. 
  The 
  entire 
  series 
  of 
  volcanic 
  

   eruptions 
  in 
  Southern 
  Colorado 
  falls 
  into 
  a 
  geological 
  period 
  that 
  is 
  

   subsequent 
  to 
  the 
  deposition 
  of 
  the 
  Lignitic 
  group. 
  Each 
  of 
  the 
  three 
  

   eras 
  of 
  volcanic 
  activity 
  has 
  no 
  doubt 
  occupied 
  a 
  long 
  space 
  of 
  time, 
  

   less, 
  ijerhaps, 
  the 
  second 
  one 
  than 
  any 
  other. 
  In 
  discussing 
  the 
  glacial 
  

   phenomena, 
  mention 
  has 
  been 
  made 
  of 
  a 
  conglomeritic 
  deposit, 
  probably 
  

   belonging 
  to 
  that 
  age, 
  which 
  is 
  covered 
  by 
  basalt, 
  and 
  this 
  latter, 
  there- 
  

   fore, 
  must 
  be 
  regarded 
  as 
  the 
  youngest 
  of 
  the 
  volcanic 
  rocks 
  in 
  that 
  section 
  

   of 
  country. 
  Its 
  last 
  eruptions 
  have 
  taken 
  place 
  at 
  a 
  time 
  when 
  the 
  

   configuration 
  of 
  the 
  surface 
  already 
  closely 
  resembled 
  that 
  of 
  to-day 
  in 
  

   its 
  general 
  outlines. 
  All 
  the 
  volcanic 
  rocks 
  that 
  wo 
  find 
  in 
  Southern 
  

   Colorado 
  must 
  be 
  classed 
  as 
  being 
  Post-Cretaceous. 
  

  

  Glacial 
  jjlienomena. 
  — 
  These 
  have 
  been 
  treated 
  of 
  in 
  the 
  appendix, 
  and 
  

   the 
  results 
  derived 
  from 
  observations 
  during 
  two 
  seasons, 
  more 
  partic- 
  

   ularly, 
  have 
  been 
  given. 
  

  

  Drift. 
  — 
  Of 
  drift 
  we 
  find 
  a 
  number 
  of 
  varieties 
  — 
  glacial, 
  avalanchial, 
  

   river-drift, 
  lake- 
  drift, 
  and 
  alluvial 
  drift. 
  All 
  of 
  them 
  occur 
  at 
  places 
  

   where 
  we 
  would 
  most 
  naturally 
  expect 
  to 
  find 
  them. 
  The 
  amount 
  of 
  

   redeposited 
  material 
  in 
  Southern 
  Colorado 
  is 
  enormous, 
  but 
  we 
  can 
  

   readily 
  understand 
  that 
  this 
  must 
  be 
  so, 
  when 
  we 
  see 
  the 
  canons 
  and 
  

   eroded 
  mountain-slopes 
  that 
  have 
  furnished 
  it. 
  For 
  the 
  first 
  time 
  have 
  

   I 
  used 
  the 
  term 
  "avalancial" 
  drift, 
  denoting 
  that 
  species 
  of 
  more 
  or 
  less 
  

   heterogeneous 
  secondary 
  deposit 
  that 
  owes 
  its 
  removal 
  from 
  the 
  origi- 
  

   nal 
  position 
  to 
  earth- 
  and 
  rock-slides, 
  or 
  to 
  subsidences 
  that 
  are 
  some- 
  

   times 
  accompanied 
  by 
  movements 
  resulting 
  in 
  a 
  general 
  breaking 
  up 
  of 
  

   the 
  rocks. 
  For 
  the 
  regions 
  under 
  consideration, 
  the 
  application 
  of 
  this 
  

   name 
  is 
  characteristic, 
  and 
  expresses 
  a 
  typical 
  occurrence 
  of 
  drift. 
  

  

  Mines. 
  — 
  In 
  the 
  district 
  of 
  1875 
  gold 
  and 
  coal 
  are 
  mined. 
  Of 
  these 
  the 
  

   former 
  occurs 
  in 
  the 
  Summit 
  district, 
  southwest 
  of 
  Del 
  Norte, 
  and 
  in 
  the 
  

   southern 
  portion 
  of 
  the 
  Sangre 
  de 
  Cristo 
  Range, 
  the 
  Trinidad 
  gold-min- 
  

   ing 
  district. 
  At 
  the 
  former 
  locality 
  it 
  is 
  found 
  in 
  the 
  trachorheitic, 
  in 
  

   the 
  latter 
  in 
  metamorphic 
  rocks. 
  More 
  time 
  and 
  work 
  will 
  be 
  required 
  

   before 
  a 
  final 
  decision 
  as 
  to 
  merit 
  can 
  be 
  formed, 
  though 
  at 
  present 
  the 
  

   indications 
  looking 
  toward 
  eventual 
  remuneration 
  are 
  favorable. 
  Coal- 
  

   mines 
  have 
  been 
  started, 
  and 
  are 
  worked 
  with 
  varying 
  industry 
  in 
  the 
  

   Trinidad 
  region. 
  In 
  Chapter 
  Y 
  they 
  are 
  described, 
  and 
  analyses 
  of 
  the 
  

   coal 
  obtained 
  there 
  are 
  given. 
  This 
  coal 
  is 
  of 
  a 
  fair 
  quality, 
  applicable 
  

   both 
  for 
  smelting 
  purposes 
  and 
  for 
  the 
  manufacture 
  of 
  gas. 
  Increasing 
  

   population 
  and 
  connection 
  of 
  the 
  settlements 
  by 
  railroads 
  will 
  go 
  far 
  

   toward 
  developing 
  these 
  mines, 
  as 
  with 
  such 
  advantages 
  a 
  greater 
  de- 
  

   mand 
  will 
  establish 
  itself, 
  and 
  enterprising 
  miners 
  will 
  be 
  able 
  to 
  find 
  a 
  

   ready 
  market 
  for 
  their 
  coal. 
  

  

  * 
  Memoirs 
  California 
  Academy 
  of 
  Science, 
  186ri, 
  vol. 
  1, 
  part 
  ii. 
  

  

  