﻿276 
  EEPOKT 
  UNITED 
  STATES 
  GEOLOGICAL 
  SURVEY. 
  

  

  strata, 
  leaving 
  the 
  sandstones 
  to 
  the 
  right 
  and 
  left 
  and 
  between 
  the 
  

   walls 
  in 
  their 
  undisturbed 
  horizontal 
  position. 
  The 
  sandstones 
  where 
  

   in 
  contact 
  with 
  the 
  dike-rock 
  seem 
  to 
  be 
  somewhat 
  metamorphosed. 
  

   On 
  the 
  outer 
  faces 
  of 
  the 
  walls, 
  which 
  are 
  in 
  places 
  from 
  30 
  to 
  40 
  feet 
  

   high, 
  are 
  the 
  impressions 
  of 
  strata 
  which 
  have 
  long 
  since 
  been 
  carried 
  

   away. 
  A 
  specimen 
  collected 
  from 
  one 
  of 
  the 
  walls 
  near 
  the 
  creek-bed 
  

   consists 
  of 
  a 
  dark 
  grayish-green 
  paste, 
  with 
  numerous 
  small 
  crystals 
  

   of 
  brownish 
  mica, 
  which 
  are 
  sometimes 
  collected 
  in 
  small 
  round 
  or 
  oval 
  

   masses. 
  A 
  specimen 
  very 
  similar 
  in 
  appearance 
  to 
  this 
  was 
  collected 
  

   from 
  a 
  little 
  conical 
  butte 
  between 
  the 
  San 
  Juan 
  Eiver 
  and 
  the 
  Mesa 
  

   Verde, 
  about 
  10 
  miles 
  west 
  of 
  the 
  Great 
  Hogback. 
  It 
  has 
  an 
  olive-green 
  

   micro-crystalline 
  paste, 
  that 
  contains 
  indistinct 
  crystals 
  of 
  some 
  triclinic 
  

   feldspar, 
  and 
  exceedingly 
  numerous 
  crystals 
  of 
  brown 
  mica 
  (biotite?) 
  

   dispersed 
  throughout 
  the 
  entire 
  mass. 
  The 
  rock 
  is 
  evidently 
  an 
  intruded 
  

   one, 
  analogous, 
  perhaps, 
  to 
  dolerite, 
  which, 
  however, 
  ha,s 
  taken 
  up 
  quartz, 
  

   various 
  silicates, 
  and 
  lime 
  from 
  the 
  strata 
  through 
  which 
  it 
  passed. 
  

  

  There 
  still 
  remain 
  undescribed 
  two 
  volcanic 
  groups, 
  the 
  Sierra 
  San 
  

   Miguel 
  and 
  the 
  Sierra 
  Abajo, 
  but 
  as 
  the 
  former 
  was 
  not 
  thoroughly 
  

   studied 
  in 
  1875, 
  and 
  the 
  latter 
  not 
  even 
  visited 
  until 
  1876, 
  I 
  prefer 
  to 
  

   leave 
  them 
  both 
  to 
  be 
  treated 
  in 
  the 
  report 
  for 
  1876, 
  which 
  will 
  shortly 
  

   be 
  published. 
  ' 
  

  

  