﻿286 
  Washington 
  — 
  Analyses 
  of 
  ItalianVolcanic 
  Rocks, 
  

  

  Art. 
  XXXI. 
  — 
  Some 
  Analyses 
  of 
  Italian 
  Volcanic 
  Rocks 
  ; 
  by 
  

   Henry 
  S. 
  Washington. 
  I. 
  

  

  During 
  the 
  past 
  two 
  years 
  I 
  have 
  made 
  a 
  number 
  of 
  

   analyses 
  of 
  Italian 
  volcanic 
  rocks, 
  with 
  the 
  intention 
  of 
  incor- 
  

   porating 
  them 
  in 
  a 
  general 
  article 
  on 
  the 
  subject. 
  As, 
  how- 
  

   ever, 
  other 
  work 
  has 
  come 
  up 
  which 
  will 
  delay 
  this 
  indefinitely, 
  

   it 
  has 
  been 
  decided 
  to 
  publish 
  them. 
  Isolated 
  analyses 
  of 
  

   rocks, 
  without 
  discussion 
  of 
  their 
  relations 
  to 
  those 
  of 
  other 
  

   connected 
  types, 
  are 
  of 
  little 
  use. 
  But 
  they 
  may 
  prove 
  of 
  

   service 
  to 
  others 
  investigating 
  this 
  region, 
  and 
  personally 
  I 
  

   would 
  like 
  to 
  clear 
  out 
  this 
  pigeon-hole. 
  

  

  Trachytes 
  of 
  the 
  Phlegrcean 
  Fields. 
  — 
  The 
  material 
  compos- 
  

   ing 
  the 
  cone 
  of 
  Monte 
  I^uovo, 
  which 
  was 
  formed 
  in 
  1538, 
  is 
  

   largely 
  pumice 
  and 
  lapilli, 
  but 
  blocks 
  of 
  dense 
  lava 
  from 
  a 
  flow 
  

   are 
  found 
  in 
  a 
  quarry 
  on 
  the 
  southern 
  slope, 
  and 
  it 
  is 
  from 
  

   these 
  that 
  material 
  was 
  taken 
  for 
  analysis. 
  

  

  The 
  rock 
  is 
  compact 
  and 
  very 
  fine-grained, 
  with 
  a 
  rough 
  

   feel. 
  The 
  color 
  is 
  ash 
  gray, 
  with 
  streaks 
  of 
  lighter 
  gray. 
  A 
  

   few 
  small 
  glistening 
  phenocrysts 
  of 
  alkali-feldspar 
  are 
  visible. 
  

  

  Under 
  the 
  microscope 
  specimens 
  vary 
  considerably, 
  but 
  all 
  

   belong 
  to 
  the 
  Ponza 
  type 
  of 
  Rosenbusch. 
  ' 
  The 
  structure 
  is 
  

   hyalopilitic 
  trachytic. 
  Long 
  slender 
  prisms 
  of 
  alkali-feldspar 
  

   are 
  abundant, 
  often 
  split 
  and 
  branching 
  as 
  has 
  been 
  described 
  

   in 
  the 
  case 
  of 
  some 
  Ischian 
  trachytes."^ 
  They 
  are 
  elongated 
  

   parallel 
  to 
  the 
  axis 
  a. 
  Small 
  elongated 
  crystals 
  of 
  segirine- 
  

   augite 
  and 
  grains 
  of 
  magnetite 
  are 
  present 
  in 
  less 
  amount. 
  All 
  

   these, 
  with 
  dusty 
  grains, 
  lie 
  in 
  an 
  isotropic 
  glass-like 
  ground- 
  

   mass, 
  which 
  is 
  usually 
  colorless, 
  but 
  often 
  a 
  light 
  brown, 
  

   especially 
  in 
  patches. 
  This 
  base 
  gelatinizes 
  readily 
  with 
  HCl, 
  

   and 
  must 
  be 
  largely 
  composed 
  of 
  the 
  sodalite 
  molecule, 
  since 
  

   it 
  is 
  the 
  only 
  constituent 
  which 
  could 
  carry 
  the 
  notable 
  

   amount 
  of 
  CI 
  found. 
  I 
  could 
  find 
  no 
  trace 
  of 
  the 
  anorthite 
  

   and 
  olivine 
  mentioned 
  by 
  dell'Erba,f 
  nor 
  of 
  the 
  leucite 
  spoken 
  

   of 
  by 
  vom 
  Rathij: 
  as 
  having 
  been 
  found 
  by 
  Gr, 
  Rose. 
  

  

  The 
  specimen 
  of 
  the 
  Monte 
  Ouma 
  trachyte 
  was 
  obtained 
  

   from 
  the 
  large 
  quarries 
  at 
  the 
  west 
  foot 
  of 
  the 
  ancient 
  acrop- 
  

   olis. 
  It 
  is 
  similar 
  in 
  appearance 
  to 
  the 
  preceding, 
  showing 
  

   also 
  a 
  eutaxitic 
  structure, 
  though 
  here 
  the 
  streaks 
  are 
  darker, 
  

   and 
  minute 
  pyroxene 
  phenocrysts 
  are 
  common. 
  

  

  Microscopically 
  it 
  differs 
  considerably 
  from 
  that 
  of 
  Monte 
  

   Nuovo. 
  Prismatic 
  phenocrysts 
  of 
  alkali-feldspar, 
  elongated 
  

  

  * 
  n. 
  S. 
  Washington, 
  this 
  Journal 
  (4), 
  i, 
  p. 
  375, 
  1896. 
  

  

  \ 
  dell'Erba, 
  Boll. 
  Com. 
  Geol. 
  Ital., 
  1894, 
  p. 
  197. 
  

  

  X 
  Yom 
  Rath, 
  Zeitschr. 
  d. 
  d. 
  Gool. 
  Ges,, 
  xviii, 
  p. 
  613, 
  1866. 
  

  

  