﻿^YasJiington 
  — 
  Analyses 
  of 
  Italian 
  Yolcanio 
  Rocks. 
  291 
  

  

  magnetite, 
  biotite, 
  aiigite, 
  hypersthene 
  and 
  apatite. 
  He 
  

   remarks 
  that 
  this 
  association, 
  with 
  the 
  chemical 
  and 
  mineralog- 
  

   ical 
  characters 
  of 
  the 
  rock, 
  points 
  conclusively 
  to 
  its 
  relation- 
  

   ship 
  with 
  the 
  monzonites 
  and 
  essexites. 
  

  

  In 
  connection 
  with 
  this 
  rock 
  it 
  is 
  interesting 
  to 
  note 
  the 
  

   composition 
  of 
  the 
  sodalite-trachyte 
  described 
  by 
  Johnston- 
  

   Lavis, 
  an 
  analysis 
  of 
  which 
  is 
  given 
  in 
  Y. 
  The 
  rock 
  occurs 
  

   west 
  of 
  Naples, 
  near 
  the 
  Phlegrgean 
  Fields. 
  It 
  carries 
  sodalite 
  

   and 
  hornblende, 
  and, 
  although 
  it 
  is 
  not 
  stated 
  to 
  contain 
  

   olivine, 
  in 
  chemical 
  composition 
  it 
  resembles 
  very 
  much 
  the 
  

   Arso 
  rock, 
  indicating 
  a 
  similarity 
  in 
  composition 
  and 
  differen- 
  

   tiation 
  of 
  the 
  magma 
  at 
  the 
  two 
  centers. 
  

  

  General 
  Relationships. 
  — 
  This 
  is 
  not 
  the 
  place 
  for 
  a 
  general 
  

   discussion, 
  but 
  a 
  few 
  words 
  may 
  be 
  devoted 
  to 
  the 
  relationship 
  

   of 
  the 
  rocks 
  whose 
  analyses 
  have 
  just 
  been 
  given 
  to 
  those 
  of 
  

   the 
  other 
  Italian 
  volcanoes. 
  The 
  apparently 
  justifiable 
  assump- 
  

   tion 
  is 
  made 
  that 
  these 
  analyses 
  fairly 
  represent 
  the 
  composi- 
  

   tion 
  of 
  the 
  rocks 
  of 
  the 
  Phlegrsean 
  Fields 
  and 
  Ischia. 
  

  

  It 
  is 
  well 
  known 
  that 
  the 
  main 
  line 
  of 
  Italian 
  volcanoes 
  has 
  

   erupted 
  rocks 
  which 
  are 
  characterized 
  chemically 
  by 
  high 
  

   potash 
  and 
  lime, 
  and 
  that 
  leucitic 
  rocks 
  are 
  abundant.^ 
  It 
  has 
  

   also 
  been 
  shown 
  that 
  the 
  rocks 
  of 
  this 
  line 
  belong 
  to 
  two 
  main 
  

   groups: 
  the 
  leucitic 
  rocks, 
  which 
  are 
  either 
  basic 
  (leucitites 
  

   and 
  leucite-tephrites) 
  or 
  intermediate 
  (leucite-trachytes), 
  and 
  

   latitesf 
  (intermediate 
  between 
  trachytes 
  and 
  andesites 
  or 
  

   basalts), 
  characterized 
  by 
  the 
  presence 
  of 
  both 
  orthoclase 
  and 
  

   plagioclase. 
  h\ 
  all 
  these 
  rocks 
  the 
  ratio 
  of 
  )Lfi 
  to 
  Na^O 
  is 
  

   constantly 
  greater 
  than 
  unity, 
  sometimes 
  much 
  so, 
  and 
  CaO 
  is 
  

   also 
  high. 
  

  

  It 
  is 
  seen 
  from 
  the 
  analyses 
  given 
  in 
  the 
  preceding 
  pages 
  

   that 
  at 
  Ischia 
  and 
  the 
  Phlegreean 
  Fields 
  the 
  rocks 
  are 
  chemi- 
  

   cally 
  quite 
  distinct, 
  JSTa^O 
  being 
  here 
  constantly 
  higher 
  than 
  

   K^O, 
  CaO 
  being 
  low, 
  and 
  leucite 
  absent, 
  or 
  at 
  least 
  very 
  rare. 
  J 
  

   They 
  also 
  tend 
  to 
  be 
  rather 
  high 
  in 
  Si02. 
  

  

  While 
  they 
  differ 
  from 
  the 
  rocks 
  of 
  the 
  peninsula, 
  they 
  show 
  

   certain 
  analogies 
  with 
  the 
  volcanic 
  rocks 
  of 
  the 
  islands 
  which 
  

   lie 
  along 
  the 
  west 
  coast 
  of 
  Italy, 
  — 
  those 
  of 
  Capraia,§ 
  the 
  Ponza 
  

   Islands,! 
  the 
  Lipari 
  Islands,*] 
  and 
  perhaps 
  Etna"^"^ 
  and 
  the 
  Yal 
  

  

  * 
  Cf. 
  Journ. 
  Geol 
  , 
  v, 
  p. 
  376, 
  1897. 
  

  

  f 
  Ransome, 
  this 
  Journal 
  (4). 
  v, 
  pp. 
  355, 
  372, 
  1898. 
  

  

  X 
  Leucitic 
  rocks 
  have 
  been 
  observed 
  as 
  blocks 
  in 
  tuff 
  at 
  Monte 
  di 
  Procida 
  and 
  

   one 
  or 
  two 
  other 
  places 
  in 
  the 
  Phlegrasan 
  Fields. 
  They 
  are 
  not 
  abundant, 
  and 
  

   their 
  relations 
  with 
  the 
  main 
  trachjtic 
  rocks 
  are 
  uncertain. 
  

  

  § 
  H. 
  P]cnmons, 
  Q. 
  J, 
  G. 
  S.. 
  xlix, 
  p. 
  129, 
  1893. 
  

  

  II 
  Doeller, 
  Denksclir. 
  k. 
  Akad. 
  Wiss. 
  Wien 
  , 
  x.xxvi, 
  p. 
  1, 
  1875. 
  

  

  ^[Cortese 
  and 
  Sabatini, 
  Descrizione 
  delle 
  Isole 
  KoUo. 
  Rome, 
  1892. 
  

  

  ** 
  V. 
  Lasaulx 
  in 
  vom 
  Waltershausen 
  der 
  .^ELnaj 
  Leipzig, 
  1880, 
  ii, 
  pp. 
  425 
  ff. 
  

   Ricciardi, 
  Gazz. 
  Chim. 
  Ital., 
  xi, 
  p. 
  149, 
  1881. 
  

  

  