﻿COSTA-EICAN 
  ROCKS. 
  339 
  

  

  to 
  further 
  oxidation 
  at 
  the 
  edges. 
  The 
  smaller 
  ferrites 
  are 
  not 
  

   numerous. 
  

  

  There 
  is 
  an 
  anomaly 
  in 
  connexion 
  with 
  this 
  specimen 
  which, 
  un- 
  

   less 
  due 
  to 
  faulty 
  observation, 
  one 
  cannot 
  well 
  explain. 
  A 
  rock 
  

   with 
  a 
  fair 
  amount 
  of 
  triclinic 
  felspar, 
  presumably 
  oligoclase, 
  and 
  

   some 
  augite 
  might 
  be 
  expected 
  to 
  have 
  a 
  higher 
  specific 
  gravity 
  

   than 
  2-54. 
  Even 
  the 
  lightest 
  sanidine-oligoclase-trachytes 
  are 
  stated 
  

   by 
  Von 
  Cotta 
  to 
  have 
  a 
  specific 
  gravity 
  of 
  2-6. 
  It 
  is 
  just 
  possible 
  

   that 
  the 
  specimen 
  contains 
  cavities 
  not 
  accessible 
  to 
  water 
  under 
  

   ordinary 
  pressure. 
  

  

  No. 
  6. 
  " 
  Los 
  Castros 
  rock, 
  Aguacate 
  Mountains." 
  Specific 
  gravity 
  

   2'78. 
  An 
  extremely 
  close-grained 
  rock 
  of 
  a 
  dark-green 
  colour 
  and 
  

   subconchoidal 
  fracture, 
  with 
  only 
  a 
  moderate 
  quantity 
  of 
  felspar 
  crys- 
  

   tals 
  porphyritically 
  distributed. 
  Pyrites 
  in 
  small 
  crystals, 
  not 
  scarce. 
  

  

  In 
  thin 
  sections 
  the 
  ground-mass 
  appears 
  of 
  a 
  greenish-grey 
  colour, 
  

   and 
  is 
  woolly 
  and 
  thoroughly 
  dusted 
  with 
  small 
  round 
  ferrites, 
  

   opaque, 
  and 
  little 
  relieved 
  by 
  microcrystalline 
  matter. 
  Hence 
  the 
  

   texture 
  is 
  felsitic 
  rather 
  than 
  doleritic. 
  

  

  The 
  felspars 
  are 
  probably 
  all 
  triclinic. 
  Though 
  perfect 
  in 
  outline, 
  

   they 
  are 
  so 
  much 
  suffused 
  and 
  granulated 
  as 
  to 
  suggest 
  the 
  idea 
  of 
  

   partial 
  decomposition 
  ; 
  consequently 
  their 
  optical 
  properties 
  are 
  

   feeble, 
  and 
  they 
  stand 
  out 
  less 
  distinctly 
  from 
  the 
  base. 
  

  

  The 
  augites 
  polarize 
  with 
  great 
  brilliancy, 
  and 
  are 
  very 
  charac- 
  

   teristic. 
  The 
  iron 
  minerals, 
  probably 
  both 
  magnetite 
  and 
  pyrite, 
  

   seem 
  much 
  mixed 
  up 
  with 
  the 
  augite 
  crystals. 
  

  

  The 
  augites 
  constitute 
  the 
  principal 
  feature 
  in 
  this 
  rock, 
  whose 
  

   ground-mass 
  is 
  obviously 
  more 
  tinged 
  with 
  green 
  in 
  the 
  vicinity 
  of 
  

   these 
  groups 
  of 
  crystals 
  than 
  elsewhere. 
  If 
  the 
  idea 
  of 
  greenstone 
  

   were 
  not 
  so 
  much 
  associated 
  with 
  hornblende, 
  of 
  which 
  I 
  cannot 
  in 
  

   this 
  specimen 
  find 
  a 
  trace, 
  one 
  might 
  almost 
  call 
  such 
  a 
  rock 
  a 
  

   " 
  greenstone-andesite." 
  In 
  some 
  respects 
  it 
  reminds 
  me 
  forcibly 
  of 
  

   certain 
  " 
  felsi-dolerites 
  " 
  occurring 
  in 
  the 
  English 
  lake-district. 
  

  

  No. 
  7. 
  "Volcanic 
  nodule, 
  Turrialba." 
  The 
  specific 
  gravity 
  of 
  

   this 
  specimen 
  is 
  low. 
  It 
  is 
  a 
  light-grey 
  fragmental 
  rock, 
  tolerably 
  

   close 
  in 
  texture, 
  and 
  in 
  the 
  hand- 
  specimen 
  shows 
  a 
  somewhat 
  gritty 
  

   matrix 
  with 
  black 
  specks. 
  Tolerably 
  opaque 
  in 
  thin 
  sections, 
  but 
  

   slightly 
  seamed 
  with 
  translucent 
  cracks. 
  The 
  few 
  crystalline 
  frag- 
  

   ments 
  are 
  much 
  decomposed, 
  and 
  muddled 
  with 
  a 
  brownish 
  ferrite 
  ; 
  

   specks 
  of 
  augite 
  may 
  be 
  noticed. 
  

  

  The 
  rock 
  is 
  doubtless 
  pyroclastic 
  ; 
  and 
  if 
  we 
  may 
  judge 
  from 
  the 
  

   lavas 
  previously 
  described, 
  it 
  is 
  a 
  felspathic 
  tuff 
  largely 
  made 
  up 
  of 
  

   soda 
  felspars. 
  

  

  No. 
  8. 
  " 
  Near 
  Rio 
  Barranca." 
  Not 
  examined 
  closely. 
  More 
  

   coarsely 
  fragmental 
  than 
  the 
  last. 
  Largely 
  made 
  up 
  of 
  soda 
  felspars 
  

   much 
  kaolinized. 
  

  

  EXPLANATION 
  OF 
  PLATE 
  XII. 
  

  

  Geological 
  Map 
  and 
  Section 
  from 
  Punta 
  Arenas, 
  on 
  the 
  Bay 
  of 
  Nicoya, 
  to 
  

   the 
  Volcanoes 
  Irazu 
  and 
  Turrialba. 
  Costa 
  Rica. 
  

  

  