﻿COSTA-MCAN 
  KOCKS. 
  337 
  

  

  compact 
  matrix, 
  full 
  of 
  small 
  crystals 
  of 
  glittering 
  felspars 
  porphyri- 
  

   tically 
  distributed. 
  This 
  rock 
  has 
  undergone 
  but 
  little 
  alteration. 
  

  

  In 
  thin 
  section 
  the 
  ground-mass 
  appears 
  black 
  with 
  a 
  greenish 
  

   grey 
  tinge. 
  It 
  is 
  opaque 
  and 
  granular, 
  but 
  relieved 
  by 
  a 
  moderate 
  

   display 
  of 
  small 
  triclinic 
  felspars. 
  On 
  the 
  whole, 
  however, 
  the 
  felsitic 
  

   texture 
  prevails 
  over 
  the 
  doleritic 
  or 
  microcrystalline. 
  

  

  The 
  large 
  felspar 
  crystals 
  are 
  numerous 
  and 
  clean 
  at 
  the 
  edges, 
  

   contrasting 
  well 
  with 
  the 
  ground-mass. 
  They 
  are 
  probably 
  all 
  tri- 
  

   clinic, 
  and 
  polarize 
  with 
  great 
  brilliancy. 
  Some 
  are 
  tolerably 
  free 
  

   from 
  inclusions 
  ; 
  others 
  contain 
  quantities 
  of 
  the 
  base, 
  which 
  fre- 
  

   quently 
  occur 
  in 
  fantastic 
  arrangements. 
  

  

  The 
  augite 
  crystals 
  are 
  not 
  numerous, 
  or 
  large, 
  but 
  are 
  well 
  de- 
  

   fined, 
  and, 
  like 
  the 
  felspars, 
  polarize 
  very 
  effectively. 
  There 
  are 
  several 
  

   specks 
  and 
  triangular 
  pieces, 
  which 
  present 
  the 
  same 
  optical 
  proper- 
  

   ties 
  and 
  probably 
  belong 
  to 
  the 
  same 
  species. 
  The 
  iron 
  mineral 
  is 
  

   mostly 
  magnetite, 
  but 
  is 
  far 
  from 
  being 
  plentiful. 
  

  

  jSTo. 
  2. 
  " 
  In 
  situ, 
  La 
  Palma, 
  N. 
  W. 
  of 
  Turrialba." 
  Specific 
  gravity 
  

   2*82. 
  A 
  somewhat 
  scanty 
  grey 
  matrix 
  full 
  of 
  crystals 
  of 
  a 
  white 
  

   felspar 
  larger 
  than 
  in 
  the 
  last 
  specimen, 
  less 
  vitreous, 
  and 
  rather 
  

   prone 
  to 
  kaolinize. 
  The 
  augite 
  crystals, 
  associated 
  with 
  some 
  iron 
  

   mineral, 
  are 
  quite 
  obvious 
  in 
  the 
  hand- 
  specimen, 
  which 
  is 
  less 
  com- 
  

   pact 
  and 
  more 
  porphyritic 
  than 
  No. 
  1. 
  

  

  In 
  thin 
  sections 
  the 
  ground-mass 
  is 
  seen 
  to 
  be 
  grey, 
  opaque 
  and 
  

   woolly 
  ; 
  so 
  that 
  in 
  this 
  case 
  the 
  felsitic 
  texture 
  predominates, 
  to 
  the 
  

   almost 
  entire 
  exclusion 
  of 
  the 
  doleritic 
  or 
  microcrystalline, 
  not- 
  

   withstanding 
  the 
  fact 
  that 
  this 
  is 
  the 
  most 
  dense 
  of 
  all 
  the 
  specimens. 
  

  

  The 
  large 
  felspars 
  are 
  not 
  so 
  clean-cut 
  as 
  in 
  No. 
  1, 
  and 
  are 
  

   more 
  contaminated 
  with 
  included 
  portions 
  of 
  the 
  ground-mass, 
  

   which, 
  in 
  one 
  or 
  two 
  cases, 
  are 
  yet 
  in 
  the 
  condition 
  of 
  glass 
  towards 
  

   the 
  centre 
  of 
  the 
  inclusions. 
  Still 
  the 
  felspars 
  polarize 
  with 
  much 
  

   brilliancy 
  and, 
  with 
  one 
  exception, 
  are 
  all 
  markedly 
  triclinic. 
  An- 
  

   desine 
  may 
  constitute 
  a 
  considerable 
  portion 
  of 
  them. 
  With 
  great 
  

   care 
  it 
  might 
  be 
  possible 
  to 
  obtain 
  a 
  sufficiency 
  for 
  an 
  analysis. 
  

  

  The 
  crystals 
  of 
  augite 
  are 
  fairly 
  numerous, 
  smaller 
  as 
  a 
  rule 
  

   than 
  the 
  felspars, 
  with 
  a 
  tendency 
  to 
  be 
  grouped 
  in 
  bunches, 
  in 
  

   which 
  case 
  they 
  are 
  much 
  intermingled 
  with 
  an 
  iron 
  mineral. 
  Some 
  

   very 
  characteristic 
  forms 
  occur, 
  such 
  as 
  it 
  is 
  impossible 
  to 
  mistake. 
  

   Pale 
  yellow 
  to 
  greenish 
  yellow 
  in 
  ordinary 
  light, 
  all 
  these 
  augites 
  

   polarize 
  very 
  effectively, 
  in 
  which 
  case 
  the 
  numerous 
  inclusions 
  of 
  the 
  

   same 
  mineral 
  are 
  very 
  conspicuous. 
  When 
  the 
  angles 
  are 
  rounded, 
  

   such 
  crystals 
  become 
  oval 
  or 
  almost 
  circular 
  in 
  section. 
  The 
  iron 
  

   mineral, 
  presumably 
  magnetite, 
  in 
  some 
  cases 
  evinces 
  a 
  tendency 
  

   towards 
  the 
  formation 
  of 
  limonite 
  at 
  the 
  edges 
  ; 
  and 
  this 
  rust-coloured 
  

   stain 
  is 
  communicated 
  to 
  some 
  of 
  the 
  crystals 
  of 
  felspar 
  and 
  augite. 
  

   There 
  are 
  appearances 
  also 
  of 
  pyrites 
  in 
  a 
  limited 
  portion 
  of 
  the 
  

   polished 
  surface 
  from 
  which 
  the 
  slice 
  has 
  been 
  cut. 
  

  

  No. 
  3. 
  " 
  San 
  Mateo 
  Boulders." 
  Not 
  examined 
  closely. 
  Near 
  to 
  

   No. 
  1, 
  but 
  with 
  a 
  more 
  felsitic 
  ground-mass. 
  

  

  No. 
  4. 
  " 
  Black 
  Hock, 
  Lower 
  San 
  Rafael 
  Level." 
  Specific 
  gravity 
  

   2'72. 
  A 
  very 
  compact 
  rock, 
  brownish-black, 
  lustreless, 
  and 
  but 
  

  

  