﻿288 
  Washington 
  — 
  Analyses 
  of 
  Italian 
  Volcanic 
  Rocks. 
  

  

  nation 
  does 
  not 
  substantiate 
  these 
  observations, 
  nor, 
  from 
  a 
  

   chemical 
  standpoint, 
  is 
  such 
  an 
  assumption 
  necessary, 
  as 
  will 
  

   be 
  seen 
  on 
  discussion 
  of 
  the 
  analyses. 
  Yom 
  Rath 
  also 
  

   describes 
  and 
  figures 
  a 
  fayalite-like 
  olivine 
  which 
  occurs 
  in 
  the 
  

   crevices. 
  Its 
  occurrence 
  resembles 
  that 
  of 
  fayalite 
  in 
  obsidians 
  

   of 
  Lipari, 
  described 
  by 
  Iddings 
  and 
  Penfield."^ 
  

  

  My 
  two 
  analyses 
  (I 
  and 
  II) 
  resemble 
  each 
  other 
  very 
  closely 
  

   and 
  are 
  typically 
  trachytic 
  in 
  most 
  features. 
  SiO^ 
  is 
  about 
  60, 
  

   which 
  is 
  rather 
  low 
  for 
  trachytes 
  and 
  approaches 
  the 
  silica 
  con- 
  

   tent 
  of 
  the 
  phonolites. 
  Al^Og 
  is 
  normal 
  ; 
  iron 
  oxides, 
  magnesia 
  

   and 
  lime 
  are 
  low, 
  with 
  Fe^Og 
  notably 
  higher 
  than 
  FeO 
  in 
  

   accordance 
  with 
  the 
  presence 
  of 
  the 
  segirine 
  molecule. 
  The 
  

   alkalies 
  are 
  high, 
  amounting 
  together 
  to 
  Itt 
  per 
  cent, 
  and 
  

   molecularly 
  Na^O 
  surpasses 
  K^O 
  (the 
  ratios 
  being 
  1-4:7 
  and 
  1'45), 
  

   so 
  that 
  both 
  rocks 
  must 
  be 
  held 
  to 
  belong 
  to 
  the 
  sub-group 
  of 
  

   soda-trachytes. 
  

  

  The 
  two 
  analyses 
  by 
  Abich 
  and 
  Rammelsberg 
  are 
  chiefly 
  of 
  

   historical 
  interest, 
  since 
  they 
  are 
  obviously 
  at 
  fault 
  in 
  many 
  

   particulars. 
  That 
  of 
  vom 
  Eath 
  is 
  better, 
  but 
  suffers 
  from 
  a 
  

   peculiarity 
  elsewhere 
  observed 
  in 
  his 
  rock 
  analyses, 
  viz 
  : 
  too 
  

   high 
  soda 
  and 
  too 
  low 
  potash. 
  f 
  It 
  was 
  this, 
  together 
  with 
  his 
  

   higher 
  lime, 
  which 
  led 
  him 
  to 
  infer 
  the 
  peculiar 
  composition 
  

   of 
  the 
  mineral 
  mentioned 
  above, 
  which 
  may 
  be 
  referred 
  to 
  

   sodalite, 
  or 
  possibly 
  to 
  marialite. 
  The 
  mineralogical 
  composi- 
  

   tion 
  of 
  the 
  Monte 
  Nuovo 
  rock 
  cannot 
  be 
  satisfactorily 
  deter- 
  

   mined, 
  owing 
  to 
  the 
  presence 
  of 
  glass. 
  That 
  of 
  the 
  Monte 
  

   Cuma 
  trachyte, 
  however, 
  which 
  is 
  holocrystalline, 
  calculates 
  

   out 
  very 
  nicely 
  from 
  the 
  data 
  given 
  by 
  the 
  analysis 
  and 
  the 
  

   microscope. 
  

  

  Orthoclase.__ 
  42-2 
  

  

  Albite 
  34-4 
  

  

  Sodalite 
  11*1 
  

  

  Diopside 
  4-9 
  

  

  ^girine 
  _ 
  - 
  6-5 
  

  

  Magnetite 
  0*9 
  

  

  100-0 
  

  

  Trachytes 
  of 
  Ischia. 
  — 
  Of 
  these 
  rocks 
  three 
  analyses 
  were 
  

   made. 
  They 
  are 
  all 
  so 
  well 
  known 
  that 
  a 
  detailed 
  petrograph- 
  

   ical 
  description 
  seems 
  superfluous. 
  That 
  from 
  Monte 
  Rotaro 
  

   is 
  the 
  black 
  vitreous 
  rock 
  containing 
  sheaf-like 
  (" 
  keraunoidal 
  ") 
  

   alkali-feldspars 
  already 
  mentioned. 
  The 
  Monte 
  Marecocco 
  

   lava 
  is 
  light 
  gray 
  with 
  many 
  large 
  phenocrysts 
  of 
  alkali-feld- 
  

   spar, 
  a 
  typically 
  trachytic 
  groundmass 
  of 
  feldspar 
  laths, 
  diop- 
  

  

  * 
  Iddings 
  and 
  Peufield, 
  this 
  Journal 
  (3), 
  xl, 
  p. 
  75. 
  1890. 
  

   f 
  Cf. 
  H. 
  S. 
  Washington, 
  Journ. 
  Geol., 
  v, 
  pp. 
  351, 
  357, 
  1897. 
  

  

  