﻿Washington 
  — 
  Analyses 
  of 
  Italian 
  Volcanic 
  Rocks. 
  49 
  

  

  in 
  effusive 
  magmas 
  the 
  potential 
  biotite 
  molecule 
  seems 
  to 
  tend 
  

   to 
  split 
  up, 
  yielding 
  olivine 
  and 
  orthoclase,"* 
  which 
  would 
  

   explain 
  the 
  main 
  differences 
  between 
  the 
  ciminite 
  and 
  selagite. 
  

  

  Indeed, 
  we 
  have 
  in 
  these 
  rocks 
  excellent 
  illustrations 
  of 
  this 
  

   tendency 
  of 
  the 
  complex 
  biotite 
  molecule 
  to 
  crystallize 
  as 
  

   biotite 
  in 
  intrusive 
  masses, 
  while 
  it 
  splits 
  up 
  into 
  olivine 
  and 
  

   either 
  orthoclase 
  or 
  leucite 
  in 
  extrusive 
  flows. 
  The 
  ciminites 
  

   are 
  all 
  flows, 
  so 
  far 
  as 
  have 
  been 
  observed, 
  while 
  the 
  chemically 
  

   identical 
  selagite 
  forms, 
  as 
  has 
  been 
  said, 
  a 
  stock 
  or 
  volcanic 
  

   neck. 
  Exactly 
  parallel 
  with 
  these 
  are 
  the 
  biotite-vulsinite 
  of 
  

   Eocca 
  Monflna 
  (II), 
  and 
  the 
  augite-latite 
  of 
  Table 
  Mountain 
  in 
  

   California 
  (III), 
  which 
  has 
  been 
  described 
  by 
  Turnerf 
  and 
  

   Ran 
  so 
  me.;}: 
  

  

  The 
  former, 
  rich 
  in 
  biotite 
  and 
  without 
  olivine, 
  forms 
  either 
  

   a 
  domal 
  eruption 
  which 
  took 
  place 
  in 
  the 
  already 
  eroded 
  or 
  

   exploded 
  volcano, 
  as 
  was 
  formerly 
  suggested, 
  § 
  or 
  the 
  last 
  

   up 
  welling 
  of 
  lava, 
  which 
  plugged 
  up 
  the 
  condnit 
  of 
  the 
  vol- 
  

   cano, 
  and 
  hence 
  solidified 
  under 
  conditions 
  analogous 
  to 
  those 
  

   obtaining 
  in 
  an 
  intrusive 
  mass. 
  The 
  erosion 
  of 
  this 
  volcano 
  

   has 
  been 
  so 
  extensive 
  that 
  this 
  is 
  quite 
  possible, 
  and, 
  in 
  view 
  of 
  

   later 
  observations, 
  seems 
  to 
  me 
  the 
  more 
  probable. 
  The 
  latter 
  

   rock 
  is 
  composed 
  essentially 
  of 
  labradorite, 
  augite 
  and 
  olivine, 
  

   and 
  a 
  glass, 
  which, 
  from 
  the 
  analysis, 
  would 
  have 
  crystallized 
  

   largely 
  as 
  orthoclase 
  and 
  is 
  quite 
  free 
  from 
  biotite. 
  The 
  

   extremely 
  close 
  parallelism 
  of 
  the 
  two 
  analyses 
  is 
  certainly 
  

   remarkable, 
  and 
  strongly 
  suggestive 
  of 
  this 
  idea, 
  that 
  the 
  

   diverse 
  mineralogical 
  compositions 
  were 
  determined 
  by 
  diverse 
  

   conditions 
  of 
  solidification. 
  

  

  Another 
  rock 
  of 
  interest 
  in 
  this 
  connection 
  is 
  one 
  from 
  San 
  

   Venanzo 
  in 
  Umbria, 
  recently 
  described 
  by 
  Sabatini|| 
  under 
  the 
  

   name 
  of 
  venanzite, 
  and 
  later 
  by 
  Rosenbusch.^f 
  who 
  calls 
  it 
  

   euktolite. 
  It 
  is 
  an 
  effusive, 
  olivinemelilite-leucite 
  rock, 
  with 
  

   accessory 
  biotite. 
  The 
  analysis, 
  as 
  given 
  by 
  Rosenbusch, 
  is 
  

   reproduced 
  in 
  Y. 
  Although 
  the 
  venanzite 
  is 
  mineralogically 
  

   quite 
  different, 
  and 
  chemically 
  much 
  more 
  basic, 
  yet 
  the 
  close 
  

   analogy 
  between 
  the 
  two 
  will 
  be 
  evident 
  on 
  comparing 
  the 
  

   analyses. 
  They 
  both 
  show 
  the 
  same 
  features 
  of 
  low 
  A1 
  2 
  3 
  and 
  

   Xa 
  2 
  0, 
  and 
  high 
  MgO, 
  CaO 
  and 
  K 
  2 
  ; 
  i. 
  e., 
  in 
  their 
  general 
  

   characters 
  they 
  are 
  both 
  monzonitic, 
  or 
  theralitic, 
  as 
  Rosen- 
  

   busch 
  prefers 
  to 
  call 
  it. 
  

  

  *Iddings. 
  Bull. 
  Phil. 
  Soc. 
  Wash, 
  xii, 
  pp.166, 
  172,1892; 
  Backstrom, 
  GeoL 
  

   For. 
  F6rh.,xviii, 
  p. 
  155, 
  1896; 
  Pirsson, 
  Jour. 
  Geol., 
  iv, 
  p. 
  681. 
  1896; 
  Washington, 
  

   Jour. 
  Geol 
  . 
  v, 
  359, 
  1897. 
  

  

  f 
  Turner. 
  14th 
  Ann. 
  Rep. 
  U. 
  S. 
  G. 
  S„ 
  p. 
  491, 
  1894. 
  

  

  % 
  Ransome, 
  this 
  Journal, 
  v, 
  p. 
  361, 
  1898. 
  

  

  ?i 
  Jour. 
  Geol. 
  v, 
  p. 
  244, 
  1897. 
  

  

  |f 
  Sabatini, 
  Boll. 
  R. 
  Com 
  Geol. 
  Hal., 
  1898. 
  

  

  r 
  Rosenbusch. 
  Sitz. 
  ber. 
  Berlin, 
  Akad., 
  vii. 
  p. 
  110. 
  1899. 
  

  

  Am. 
  Joue. 
  Sci.— 
  Fourth 
  Serjes 
  \^ol. 
  IX, 
  No. 
  49.— 
  January. 
  1900. 
  

   4 
  

  

  