﻿HOLMES] 
  PRECIOUS 
  METALS, 
  LA. 
  PLATA 
  MOUNTAINS. 
  271 
  

  

  tinuatiou 
  of 
  those 
  in 
  Hesperus. 
  In 
  many 
  cases 
  the 
  interleaved 
  sheets 
  

   conform 
  very 
  closely 
  to 
  the 
  planes 
  of 
  bedding, 
  and 
  it 
  is 
  difiicalt 
  to 
  im- 
  

   agine 
  how 
  a 
  molten 
  material 
  should 
  distribute 
  itself 
  with 
  such 
  nice 
  reg- 
  

   ularity, 
  but 
  there 
  are 
  a 
  number 
  of 
  examples 
  of 
  oblique 
  intrusion, 
  and 
  in 
  

   some 
  cases 
  a 
  sheet 
  of 
  trachyte 
  changes 
  from 
  one 
  horizon 
  to 
  another 
  by 
  

   breaking 
  through 
  the 
  strata 
  in 
  the 
  manner 
  illustrated 
  in 
  Fig. 
  2, 
  Plate 
  

   XLV. 
  This 
  extensive 
  intrusion 
  of 
  sheets 
  undoubtedly 
  tends 
  to 
  arch 
  

   the 
  surrounding 
  strata 
  considerably. 
  In 
  Hesperus 
  Mountain, 
  for 
  exam- 
  

   ple, 
  one-half 
  of 
  the 
  entire 
  thickness 
  is 
  of 
  intruded 
  rock, 
  and 
  it 
  is 
  hardly 
  

   possible 
  that 
  metamorphism 
  and 
  pressure 
  would 
  be 
  sufficient 
  to 
  reduce 
  

   the 
  thickness 
  of 
  the 
  shales 
  one-half. 
  In 
  Fig. 
  1, 
  Plate 
  XLV, 
  a 
  very 
  fine 
  

   example 
  of 
  the 
  arching 
  or 
  elevating 
  of 
  strata 
  by 
  the 
  intrusion 
  of 
  a 
  wedge 
  

   of 
  trachyte 
  is 
  shown. 
  

  

  One 
  of 
  the 
  best 
  examples 
  of 
  metamorphism 
  by 
  contact 
  with 
  volcanic 
  

   rocks 
  found 
  in 
  these 
  mountains 
  is 
  on 
  the 
  west 
  face 
  of 
  the 
  first 
  mountain 
  

   south 
  from 
  Hesperus 
  (at 
  d, 
  Plate 
  XLIV). 
  A 
  mass 
  of 
  sedimentary 
  rocks, 
  

   chiefly 
  metamorphic 
  shales, 
  abuts 
  directly 
  against, 
  or, 
  perhaps, 
  more 
  

   properly, 
  is 
  welded 
  to, 
  the 
  trachytic 
  face 
  of 
  the 
  mountain. 
  The 
  exact 
  

   point 
  of 
  contact 
  cannot 
  be 
  determined, 
  as 
  the 
  metamorphism 
  has 
  been 
  

   so 
  complete 
  that 
  the 
  shales 
  seem 
  to 
  change 
  gradually 
  into 
  trachyte. 
  

   At 
  the 
  point 
  where 
  the 
  bedded 
  structure 
  is 
  last 
  detected, 
  the 
  weathering 
  

   and 
  color 
  are 
  identical 
  with 
  the 
  weathering 
  and 
  color 
  of 
  the 
  trachyte. 
  

   Farther 
  out, 
  they 
  gradually 
  assume 
  the 
  appearance 
  of 
  massive 
  grayish- 
  

   yellow 
  quartzites, 
  and 
  a 
  mile 
  or 
  more 
  from 
  the 
  place 
  of 
  contact 
  begin 
  to 
  

   assume 
  their 
  shaly 
  character 
  and 
  dark 
  color. 
  

  

  On 
  the 
  La 
  Plata 
  side 
  of 
  the 
  ridge 
  to 
  which 
  this 
  mountain 
  belongs, 
  

   about 
  the 
  head 
  of 
  Boren 
  Gulch, 
  the 
  trachytes 
  of 
  the 
  summits 
  are 
  in 
  con- 
  

   tact 
  with 
  Jura-Trias 
  rocks, 
  which 
  still 
  partially 
  retain 
  their 
  character 
  

   and 
  color. 
  The 
  metalliferous 
  veins 
  here 
  seem 
  to 
  pass 
  indifferently 
  

   through 
  both 
  metamorphic 
  and 
  volcanic 
  rocks. 
  Farther 
  eastward, 
  

   beyond 
  the 
  Mountain 
  Moss 
  group, 
  on 
  the 
  Animas 
  side 
  of 
  the 
  mountains, 
  

   the 
  red 
  sandstones 
  reach 
  nearly 
  to 
  the 
  summits 
  of 
  the 
  mountains, 
  the 
  

   capping 
  in 
  all 
  cases 
  being 
  of 
  trachyte. 
  The 
  typical 
  trachyte 
  is 
  gray 
  in 
  

   color, 
  moderately 
  fine-grained 
  and 
  exceedingly 
  compact. 
  It 
  is 
  composed 
  

   of 
  a 
  light 
  gray 
  crystalline 
  paste 
  which 
  contains 
  very 
  numerous 
  crystals 
  

   of 
  white 
  oligoclase. 
  There 
  are 
  also 
  acicular 
  crystals 
  of 
  hornblende 
  dis- 
  

   tributed 
  throughout 
  the 
  mass. 
  These 
  characters 
  become 
  less 
  strongly 
  

   marked 
  as 
  we 
  approach 
  the 
  outer 
  borders 
  of 
  the 
  mass, 
  and 
  the 
  rock 
  

   gradually 
  becomes 
  a 
  crystalline 
  aggregate, 
  next 
  a 
  highly 
  metamorphosed 
  

   slate, 
  and 
  finally 
  an 
  unchanged 
  shale. 
  

  

  The 
  metamorphic 
  core 
  of 
  the 
  mountains, 
  where 
  exposed 
  by 
  the 
  cutting 
  

   of 
  the 
  valley 
  of 
  the 
  La 
  Plata, 
  is 
  penetrated 
  by 
  a 
  great 
  number 
  of 
  min- 
  

   eral 
  veins, 
  many 
  of 
  which 
  carry 
  silver 
  and 
  gold. 
  So 
  far 
  as 
  I 
  could 
  

   ascertain 
  they 
  are 
  not 
  arranged 
  according 
  to 
  any 
  system, 
  but 
  seem 
  to 
  

   have 
  been 
  formed 
  in 
  the 
  irregular 
  faults 
  and 
  crevices 
  produced 
  during 
  

   the 
  period 
  of 
  uplifting 
  and 
  intrusion. 
  In 
  a 
  number 
  of 
  cases 
  they 
  were 
  

   found 
  to 
  extend 
  beyond 
  the 
  metamorphic 
  into 
  the 
  easily 
  recognizable 
  

   sedimentary 
  rocks. 
  In 
  the 
  scoring 
  out 
  of 
  the 
  valley 
  by 
  the 
  river, 
  assisted 
  

   probably 
  by 
  glaciers, 
  the 
  mineral- 
  bearing 
  rocks 
  have 
  been 
  exposed, 
  worn 
  

   down, 
  and 
  carried 
  out 
  and 
  distributed 
  over 
  the 
  low 
  country. 
  The 
  bars 
  

   of 
  loose 
  gravel 
  near 
  the 
  mouth 
  of 
  the 
  valley 
  contain 
  considerable 
  quan- 
  

   tities 
  of 
  gold. 
  A 
  portion 
  of 
  the 
  principal 
  bar 
  has 
  been 
  occupied 
  and 
  to 
  

   a 
  limited 
  extent 
  worked 
  by 
  Captain 
  Moss 
  and 
  his 
  associates. 
  The 
  water- 
  

   supply 
  is 
  insufficient 
  at 
  present 
  to 
  carry 
  on 
  extensive 
  work, 
  but 
  a 
  large 
  

   ditch 
  is 
  being 
  constructed 
  which, 
  when 
  finished, 
  will 
  afford 
  better 
  means 
  

   for 
  carrying 
  on 
  mining 
  operations. 
  

  

  