﻿330 
  

  

  G. 
  ATTWOOD 
  ON 
  THE 
  GEOLOGY 
  

  

  succession 
  of 
  ravines 
  and 
  gorges. 
  The 
  country 
  rock 
  is 
  the 
  same 
  as 
  

   near 
  the 
  Eio 
  Barranca 
  ; 
  and 
  on 
  the 
  monntain-slopes 
  and 
  in 
  the 
  beds 
  

   of 
  the 
  streams 
  boulders 
  of 
  the 
  same 
  black 
  rock 
  are 
  found. 
  San 
  Mateo 
  

   is 
  situated 
  about 
  1050 
  feet 
  above 
  the 
  sea-level, 
  with 
  mountain-peaks 
  

   on 
  all 
  sides. 
  The 
  black 
  boulder 
  stones 
  were 
  frequently 
  examined, 
  and 
  

   a 
  typical 
  piece 
  selected 
  for 
  examination 
  (Appendix, 
  No. 
  3, 
  p. 
  337). 
  

  

  This 
  boulder-rock 
  is 
  very 
  dark, 
  approaching 
  black, 
  with 
  a 
  compact 
  

   ground-mass; 
  it 
  is 
  extremely 
  hard, 
  and 
  in 
  places 
  presents 
  a 
  vitreous 
  

   appearance. 
  It 
  contains 
  triclinic 
  felspar, 
  augite 
  crystals, 
  magnetite, 
  

   and 
  small 
  specks 
  of 
  pyrites, 
  and 
  may 
  be 
  classed 
  as 
  a 
  variety 
  of 
  the 
  

   andesites, 
  under 
  the 
  head 
  of 
  augite-andesite. 
  

  

  Prom 
  San 
  Mateo 
  the 
  ground 
  ascends 
  rapidly 
  as 
  the 
  important 
  

   mountain- 
  chain 
  called 
  Aguacate 
  is 
  approached. 
  In 
  the 
  Aguacate 
  

   Mountains 
  are 
  found 
  the 
  best 
  gold- 
  and 
  silver-mines 
  now 
  known 
  in 
  

   Costa 
  Rica. 
  The 
  mountains 
  are 
  a 
  branch 
  of 
  the 
  Costa- 
  Rica 
  Andes; 
  

   and 
  they 
  have 
  a 
  general 
  north-westerly 
  direction 
  for 
  twenty 
  miles. 
  

   Some 
  of 
  the 
  peaks 
  are 
  over 
  1400 
  feet 
  high. 
  

  

  The 
  principal 
  mines 
  now 
  being 
  worked 
  belong 
  to 
  the 
  Aguacate, 
  

   Sacra 
  Familia, 
  and 
  Quebrada 
  Honda 
  Companies 
  ; 
  but 
  many 
  others 
  

   are 
  being 
  tried. 
  Although 
  the 
  veins 
  are 
  numerous, 
  they 
  have 
  a 
  

   general 
  strike 
  north 
  35° 
  east, 
  with 
  an 
  average 
  dip 
  of 
  80° 
  to 
  the 
  

   north-west, 
  and 
  run 
  parallel 
  to 
  a 
  great 
  extent 
  (fig. 
  3). 
  The 
  veins 
  

   can 
  be 
  traced 
  for 
  considerable 
  distances. 
  

  

  The 
  Orcamunas-San 
  Rafael-Los 
  Castros 
  lode 
  (belonging 
  to 
  the 
  

   Aguacate 
  Company) 
  has 
  been 
  followed 
  for 
  four 
  miles 
  in 
  length, 
  and 
  

   is 
  about 
  15 
  feet 
  in 
  width. 
  The 
  filling 
  of 
  the 
  fissures 
  which 
  consti- 
  

   tute 
  the 
  lodes 
  consists 
  largely 
  of 
  quartz, 
  occasionally 
  mixed 
  with 
  

   calcite 
  and 
  much 
  argillaceous 
  matter, 
  evidently 
  the 
  result 
  of 
  the 
  

   decomposition 
  of 
  the 
  felspar 
  in 
  the 
  adjacent 
  rocks. 
  

  

  The 
  gold 
  is 
  generally 
  disseminated 
  in 
  minute 
  flaky 
  or 
  filiform 
  

   particles 
  through 
  the 
  veinstone 
  ; 
  it 
  contains 
  a 
  variable 
  quantity 
  of 
  

   silver, 
  and 
  has 
  a 
  pale 
  yellow 
  to 
  yellowish-white 
  colour. 
  Occasion- 
  

   ally 
  some 
  flattened 
  and 
  rounded 
  grains 
  are 
  found. 
  The 
  average 
  

   fineness 
  of 
  the 
  gold 
  from 
  the 
  Aguacate 
  mines, 
  which 
  has 
  been 
  

   minted 
  at 
  San 
  Jose 
  (the 
  present 
  capital 
  of 
  Costa 
  Rica\ 
  according 
  to 
  

   the 
  statement 
  of 
  Mr. 
  W. 
  Whitting 
  (Director 
  of 
  the 
  Mint) 
  is 
  

  

  620 
  = 
  62 
  per 
  cent 
  gold, 
  

   380=38 
  „ 
  „ 
  silver, 
  

  

  and 
  is 
  what 
  is 
  called 
  " 
  electrum." 
  

  

  The 
  principal 
  minerals 
  in 
  the 
  vein-matter 
  with 
  which 
  the 
  gold 
  

   and 
  silver 
  are 
  more 
  or 
  less 
  associated, 
  are 
  arsenical 
  pyrites, 
  iron- 
  and 
  

   copper-pyrites, 
  galena, 
  and 
  a 
  little 
  zinc-blende. 
  The 
  arsenical 
  pyrites 
  

   predominates 
  ; 
  and 
  the 
  silica, 
  besides 
  appearing 
  as 
  hyalite 
  and 
  chal- 
  

   cedony, 
  is 
  sometimes 
  found 
  in 
  the 
  form 
  of 
  a 
  dry 
  crystalline 
  powder 
  

   resembling 
  pulverized 
  white 
  sugar. 
  The 
  latter 
  is 
  most 
  frequent 
  in 
  

   the 
  small 
  vein-feeders, 
  or 
  stringers, 
  which 
  connect 
  with 
  the 
  mother 
  

   veins. 
  

  

  A 
  large 
  amount 
  of 
  gold 
  and 
  silver 
  has 
  been 
  extracted 
  from 
  the 
  

  

  